Disease and defense2.1 Flashcards
1
Q
mesosome
A
folded invaginations in the plasma membrane of bacteria that are produced by the chemical fixation techniques used to prepare samples for electron microscopy.
2
Q
Bacterial cell walls
A
in most bacteria, rigid and contains peptidoglycan, essential for resisting osmotic lysis and maintaining cell shape. The bacterial shape is determined both by cell wall and intracellular cytoskeletal elements.
3
Q
FtsZ
A
analogous to tubulin in eukaryotes. Typically located in middle for cell division.
4
Q
MreB and ParM
A
analogous to actin in eukaryotes. Can be located where out in the cell. Very dynamic. Plays role in shape polarity, and chromosome segregation.
5
Q
CreS (crescentin):
A
functions like intermediate filaments proteins. Typicaly located on cresent side of bacteria.
6
Q
Peptidoglycan
A
forms rigid mesh that surrounds cytopliasmic membrane. Consists of a polymer with repeating units of two hexose sugars (N-acteylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc)). Peptidoglycans have D configuration amino acids, unlike animals who only have L configuration.
7
Q
N-acetylmuramic acid (MurNAc):
A
one of the repeating units in peptidoglycan. They are linked to tetrapeptide chains that conatin amino acids found only in bacterial cell wall (e.g., meso-diaminopimelic acid [DAP], D-glutamic acid and D-alanine). The tetrapeptides are cross-linked from one chain to another chain via DAP in gram-negative and L-lys in gram-positive to D-ala on another chain and cross-linking in gram positive bacteria occurs via an intervening peptide such as pentaglycine in Staphylococcus aureus. The extent of the cross-linking of peptidoglycan chains is typically much greater in gram-positive bacteria than in gram- negative.
8
Q
Lysozyme
A
an enzyme present in many body secreations and which contribute to innate host defense against bacteria, hydrolyzes peptidoglycan by specific cleavage of the glycosidic bond between MurNAc and GlcNAc.
9
Q
Secretion Systems
A
Bacteria use multiple secretion systems to deliver proteins to the cell surface, assemble organelles on the cell surface, export proteins to the extracellular milieu, and inject proteins or DNA into other cells.
10
Q
Gram- positive bacteria cell wall
A
react to gram staining procedure. Osmotic pressure is 20 atm. The tetrapeptide chains of MurNAc are crosslinked of L-lys to D-ala with other intervening peptides. The cross linking is greater in gram-positive than gram-negative. They have a think, densely packed, extensively cross-linked peptidoglycan layer that also contains teichoic acids.
11
Q
Gram- negative bacteria cell wall
A
do not react to gram staining, osm pressure is about 5 atm. The tetrapeptide chains of MurNAc are crosslinked via DAP to D-ala and there is less cross-linking than gram-positive. They also have thin, sparsely cross-linked peptidoglycan layer and other major components that are located exterior to the peptidoglycan.
12
Q
Outer Membrane (OM) of gram-negative bacteria
A
is a lipid bilayer that contains Lipopolysaccharide (LPS), lipoproteins (are linked covalently to the peptidoglycan), and porins (trasmembrane chennels permitting diffusion across the membrane of hydrophilic molicules <600MW), other membrane proteins and phsopholipids. The OM is a barrier to entry of some antibiotics and also protects the cell against the action of detergents and other toxic compounds. The outer leaflet contains LPS and the inner leaflet consists of phospholipids.
13
Q
Lipopolysaccharide (LPS):
A
only in gram engative bacteria, is located exclusively in the outher leaflet of the outer membrane. LPS have three regions: lipid A (the endotoxin region), core polysaccharide, and the O side chain oligosaccharides facing the extracellular region that functions as somatic antigens (O antigen).
14
Q
Teichoic acids
A
in gram positive bacteria, have repeating polyglycerol-P or polyribitol-P backbone substituted with other molecules (sugars, aminosugars, D-alanine), and they are covalentently attached to the peptidoglycan layer.
15
Q
Lipoteichoic acids
A
are attached to the underlying cytoplasmic membrane and help to anchor the cell wall to the membrane.
16
Q
Capsules
A
are loose, gelatinous outer surface layers that usually consist of complex polysaccharides (although the capsule of Bacillus anthracis is a polymer of D- glutamic acid). Capsules often enhance virulence by enabling the encapsulated bacteria to resist phagocytosis. Most capsular polysaccharies are antigenic, and some are used as components of vaccines to prevent specific bacterial infections (e.g., in the protein-polysaccharice conjugate vaccines used to immunize against Streptococcus pneumoniae or Hemophilus influenzae type b).
17
Q
Flagella
A
are appendages originating in the cytoplasmic membrane that function as organs of motility. Bacterial chemotaxis (movement toward attractive nutrients or away from toxic substances) involves the control of flagellar rotation (counterclockwise results in swimming; clockwise results in tumbling). Motile bacteria that exhibit chemotaxis spend more time swimming and less time tumbling when attractants or repellents are present, resulting in directed motion. Most flagella are antigenic, and the H antigens used for classification of enteric bacteria are flagellar antigens.
18
Q
Common Bacterial Pathogens
A
LPS (endotoxin) is a very toxic molecule for humans. The toxic moiety, Lipid A, is embedded in the outer leaflet of the outer membrane of the Gram- negative cell wall. In many cases it is a significant component of the disease process of G- organisms. Even in minute quantities, LPS may cause fever and shock (IL-1 and TNF release). In larger doses, LPS may result in DRAMATIC life-threatening effects: Hypotension, Hemorrhagem, Intravascular coagulation (activates clotting cascade). Patients encounter LPS e.g., release of cell wall fragments following treatment with certain antibiotics, injection of contaminated materials, bacteremia.
19
Q
General rules for antimicrobial susceptibility
A
The Gram-negative outer membrane is a permeability barrier that protects the cell from many organic materials, including some antibiotics, e.g., erythromycin.
20
Q
Peritrichous flagella
A
Some bacteria have flagella distributed over their surface
21
Q
Polar flagella
A
others may have one or several flagella at one end of the cell.
22
Q
Pili
A
(also known as fimbriae) are long, slender, proteinaceous, antigenic, hair-like structures on the surface of many bacteria. Pili often play a role in bacterial adherence to surfaces and tissues, and antibodies against pili may block adherence and confer resistance to infection. Sex pili that play a role in bacterial conjugation are found in small numbers on some bacterial cells.
23
Q
Cytoplasmic membrane
A
also called the inner membrane in gram-negative bacteria) is the anatomical and physiological barrier between the inside and outside of the bacterial cell. It is a lipid bilayer made up primarily of phospholipids and proteins, but unlike plasma membranes of animal cells it usually contains no sterols and has a much higher content (60-70%) of proteins. It also has selective permeability and is impermeable to all charged substances. Only hydrophobic molecules or uncharged molecules no larger than glycerol can diffuse through it. Essential metabolites are not readily lost from the cytoplasm. The electron transport system, the principal source for generating the proton motive force during respiration in bacteria, is located in the cytoplasmic membrane. Other functions of the cytoplasmic membrane include transport of metabolites into the cytoplasm, biosynthesis of lipids and other cell envelope components, certain aspects of DNA replication, and flagellar rotation.
24
Q
Ribosomes of bacteria
A
Bacterial 70S ribosomes are closely related to the 70S ribosomes of mitochondria from eukaryotes, but they are less closely related to the 80S cytoplasmic ribosomes from eukaryotes. Protein synthesis occurs on the ribosomes. Polyribosomes are formed by the interaction of several ribosomes with a single messenger RNA. Bacterial mRNAs may by polycistronic (e.g., encode more than one protein product).
25
The Nucleoid of bacteria
The DNA of bacteria is located within a distinct region of the cytoplasm known as the nucleoid or nuclear body. The DNA is tightly packed and supercoiled, and there is no nuclear membrane surrounding the nucleoid. The older name prokaryote referred to this primitive nuclear structure. The name prokaryote is outdated as a taxonomic term because members of the bacteria and the archea, which constitute different biological kingdoms, both lack nuclear membranes. Because there is no nuclear membrane, transcription and translation can occur as coupled processes in bacteria. Several different genetic elements can contribute to the bacterial genome, including: bacterial chromosomes, plasmids, and phages
26
Bacterial chromosome
often consists of a single, double-stranded, circular DNA molecule with a contour length hundreds to thousands of times greater than the longest dimension of the bacterium. Some bacterial chromosomes are linear, and some bacteria have more than one chromosome. Cytoskeletal components appear to function as a primitive mitotic apparatus during bacterial cell division.
27
Plasmids
are extra-chromosomal, self-replicating DNA molecules, much smaller than bacterial chromosomes, and they are usually not essential for bacterial viability. Plasmids in pathogenic bacteria often encode virulence factors. Plasmids called R factors carry genes that determine resistance to antibiotics in many pathogenic bacteria.
28
Bacteriophages (phages):
are viruses that infect bacteria. The DNA genomes of temperate bacteriophages can integrate into bacterial chromosomes and replicate as part of those chromosome. Temperate bacteriophages often carry genes that encode bacterial toxins, other bacterial virulence factors or resistance to antibiotics.
29
Phage conversion
is defined as a change in the phenotype of a host bacterium as a consequence of expression of a gene that is encoded by a bacteriophage within the host bacterium (e.g., production of diphtheria toxin by isolates of Corynebacterium diphtheriae harboring a prophage that carries a gene encoding the toxin).
30
lag phase
an initial period of physiologic adjustment for the starting cells, or inoculum, involving the induction of new enzymes and the establishment of a proper intracellular environment for optimal growth in the new medium.
31
exponential (logarithmic) phase of growth
the rate of increase in cell number/cell mass is proportional to the cell number/cell mass already present. A constant interval of time (ranging from about 20 minutes up to about 1 day) is required for doubling of cell number/cell mass, and this interval is termed the generation time (doubling time). During exponential growth, the rate of cell division is maximal for the available nutritional conditions.
32
stationary phase
occurs as essential nutrients are consumed and toxic products of metabolism accumulate. Cell growth may slow dramatically or cease, and growth that occurs is balanced by cell death. Such non-growing or slow-growing cells may exhibit markedly increased resistance to antibiotics such as penicillin or other β-lactam antibiotics that act on growing cells. In nature, bacteria probably spend most of their time in stationary phase.
33
Death phase
Some bacterial species remain viable for long periods of time in stationary phase, but others are less hardy. If a death phase occurs, the number of viable bacteria will decrease over time. If spontaneous cell lysis (autolysis) occurs, the mass of intact bacteria in the culture will also decrease.
34
Minimal requirements for growth
```
Most bacteria require a nutrient medium that contains several inorganic ions (NH4+, PO4=, SO4=, K+, Mg++, Fe++, etc.) plus sources of carbon and energy. Bacteria that require an organic carbon source (including most bacterial pathogens) are heterotrophic; bacteria that obtain their carbon exclusively from CO2 are autotrophic. Many bacterial pathogens are deficient in one or more biosynthetic pathways.
Such bacteria (often called “fastidious” bacteria) require, in addition to sources of carbon and energy, a number of essential growth factors such as amino acids, vitamins, purines, pyrimidines and inorganic ions. They are typically grown in rich, complex growth media. Some bacterial pathogens are obligate intracellular bacteria that can grow within eukaryotic cells but cannot be cultivated on artificial media.
```
35
Strict aerobe
requires oxygen; cannot ferment.
36
Strict anaerobe
killed by oxygen; fermentive metabolism.
37
Indifferent (facultative anaerobe):
ferments in the presence or absence of O2.
38
Faculative anaerobe
respires with O2; ferments in absence of O2.
39
Microaerophilic
grows best at low O2 concentrations; can grow without O2.
40
Bacterial protrection against toxic oxygen metabolites
Organisms that grow in the presence of oxygen produce toxic oxygen metabolites, such as hydrogen peroxide and superoxide. Professional human phagocytes such as neutrophils and macrophages use reactive oxygen species as defense mechanisms against ingested bacterial pathogens. Bacteria that can grow in the presence of oxygen usually produce catalase (or peroxidase) and superoxide dismutase (SOD) that protect them against toxic reactive oxygen species. Anaerobes that are frequently associated with disease tend to be more aeroterant than most strict anaerobes, and they may possess small amounts of catalase or SOD.
41
Energy currency
here are two forms of “energy currency” in bacteria and higher cells: ATP and electrochemical gradients (the proton motive force). ATP drives many biosynthetic reactions, and electrochemical gradients drive other functions like flagellar rotation and certain substrate transport systems. These two types of potential energy are interconvertible by the membrane ATPase. Bacteria also require reducing power in the form of NADH and NADPH to drive various metabolic interconversions. Heterotrophic bacteria obtain both energy and reducing power by subjecting nutrients to fermentation or respiration.
42
Bacterial fermentation
organic compounds serve as both electron donors and electron acceptors, and no net oxidation of substrates occurs. Both anaerobic and facultative or indifferent bacteria grown under anaerobic conditions obtain energy by fermenting organic substrates. Indifferent organisms (aerotolerant anaerobes, see table above), obtain energy by fermentation under either anaerobic or aerobic conditions, because they are incapable of respiration.
43
Bacterial respiration
many bacterial species, like the mitochondria of higher organisms, generate ATP through electron transport and use molecular oxygen as the final electron acceptor. In anaerobic respiration, certain bacteria may use inorganic substrates such as nitrate or nitrite as terminal electron acceptors instead of O2.
44
Sporulation
a response to adverse nutritional conditions. Spores are specialized cells that are produced by certain bacteria, such as Clostridium sp. and Bacillus sp., when the nutritional supply of carbon, nitrogen or phosphorus is limited. During sporulation, these bacteria differentiate to form highly resistant, dehydrated forms (spores) that have no metabolic activity. Spores are adapted for prolonged survival under adverse conditions such as heat, drying, freezing, the presence of toxic chemicals, and radiation. When spores find themselves once again in a nutritionally satisfactory environment, they may convert back into vegetative cells through the process of germination.
45
Antimicrobial agents
Antimicrobials work on the principle of selective toxicity, namely the selective inhibition of microbial growth at drug concentrations tolerated by the host. Many aspects of microbial metabolism are very similar to those of eukaryotic organisms (including humans). However, there are some components of bacteria that are not present in eukaryotes or are sufficiently different from their counterparts in eukaryotes be effective as targets for antimicrobial agents.
46
Cell wall-active antimicrobials
Selective toxicity is due to the lack of peptidoglycan in mammalian cells. Includes β-lactams, vancomycin, and cycloserine.
47
β-lactams
(penicillins, cepalosporins, etc) inhibit the final transpeptidation reaction in cross-linking of peptidoglycan.
48
Vancomycin
inhibits utilization of lipid-linked intermediate at an intermediate step in peptidoglycan synsthesis, e.g., elongation of the peptidoglycan chain.
49
Cycloserine
inhibits alanine racemase, preventing formation of muramyl pentapeptide, an early intermediate in peptidoglycan synthesis.
50
Polymyxins
an outer and cytoplasmic membrane-active antimicrobials, are cationic surfactants that disrupt bacterial outer and cytoplasmic membranes. They are less active on mammalian cell membranes.
51
Inhibitors of protein synthesis at the ribosomal level:
Selective toxicity is due to differences between bacterial and mammalian ribosomes. Includes aminoglycosides, tetracyclines, chloramphenicol, and macrolides.
52
Aminoglycosides
(including streptomycin, kanamycin, gentamicin, neomycin, tobramycin, amikacin, etc) bind to specific target proteins in the 30S ribosomal subunit and inhibit protein synthesis.
53
Tetracyclines
reversibly bind to the 30S ribosomal subunit and inhibit binding of aminoacyl tRNA.
54
Chloramphenicol
binds reversibly to the 50S ribosomal subunit and inhibits peptidyl transferase and peptide bond formation.
55
Macrolides
(such as erythromycin) and lincomycins (such as lincomycin and clindamycin) bind to the 23S ribosomal RNA of the 50S subunit and inhibit peptidyl transferase.
56
Inhibitors of nucleic acid synthesis
includes quinolones and rifampicin.
57
Quinolones
inhibit DNA gyrase and topoisomerase and therefore interfere with DNA replication.
58
Rifampicin
inhibits RNA polymerase and interferes with the initiation of transcription.
59
Metabolic inhibitory antimicrobials
includes sulfonamides, trimethoprim, isoniazid, and metronidazole.
60
Sulfonamides
are structural analogs of p-aminobenzoic acid (PABA), which is a component of folic acid. Enzymes that use folic acid derivatives as coenzymes are needed for one-carbon transfer reactions in the synthesis of many compounds, including thymidine and purines. Sulfonamides inhibit the formation of folic acid by competing with PABA, and this in turn prevents nucleic acid synthesis. The inhibition is selective because only bacteria, and not the host, possess enzymes for making folic acid (we get ours from the bacteria), whereas bacteria, in contrast to human cells, cannot utilize pre-formed folic acid.
61
Trimethoprim
also interferes with folate metabolism by inhibiting the enzyme dihydrofolate reductase. Since both bacterial and host cells both possess this enzyme, the basis of selective toxicity lies in the 50,000-fold greater sensitivity of the bacterial enzyme to this drug.
62
Isoniazid
inhibits lipid synthesis (probably mycolic acid synthesis) in susceptible Mycobacteria.
63
Metronidazole
appears to specifically interfere with anaerobic metabolism.
64
Infection
the process where a microbe enters into a relationship with the host. It may or may not cause disease.
65
Infectious disease
a disease caused by an infection with a microbe.
66
Pathogenicity
the ability (Usually of a microbial species) to cause disease.
67
Frank pathogens
microbes that were able to cause disease readily in normal hosts.
68
Opportunistic pathogens
microbes that caused disease primarily in compromised hosts but less often in normal hosts.
69
Virulence
the degree of pathogenicity (usually of a specific strain within a species). A highly virulent microbe was likely to cause disease when it was introduced into a host in small numbers.
70
Damage- response framework
a more contemporary view that recognizes that both microbial and host factors contribute to the outcome of an infectious disease. A pathogen is a microbe capable of causing host damage, virulence is the relative capacity of a microbe to cause damage in a susceptible host, and a virulence factor is a microbial component that can damage a susceptible host.
71
Etiologic agent
a specific organism responsible for causing a specific kind of disease.
72
Kach’s postulates
Specific microbes are present regularly in characteristics lesions of the disease, the specific microbes can be isolated and grown in vitro. Injection of the cultured microbes into animals reproduces the disease seen in humans, and the specific microbes can be re-isolated from lesions of the disease in animals.
73
Limitations of Koch’s postulates
Some infectious diseases do not have a characteristic (pathognomonic) lesion. Some microbes cause specific infectious diseases but cannot be grown in vitro. Traditional concepts of pathogenicity focus primarily on properties of microbes vs. hosts. The characteristics of infectious diseases usually reflect complex interactions between microbes and their hosts.
74
Stages of infection
include encounter, entry, spread, multiplication, damage, and outcome.
75
Compare mechanisms of innate and acquired host defense against infections.
Antibody production, T cells generate lymphokines, complement- mediated lysis, opsonized phagocytosis and killing, and antibody production. Stomach acid is an example of innate defense. With cholera, its infectous dose is decrease in half when it is transmitted with bicarbonate because it cannot survive the acidic stomach acids. Shigella dysenteriase is an example of a bacteria with a very small infectious dose (10 DD50).
76
Describe the composition and importance of the microbiome of the body.
Some bacteria that are most prevalent among the (cultivatable) normal flora of skin, oropharynx, large intestine, and vagina.
77
Factors that differ among individual and affect the microbiome
diet, suppression of microbial flora by treatment with antibiotics, anatomic abnormalities, genetic differences between individuals (specific ABO and Se genotypes).
78
Compare several disease paradigms that illustrate selected mechanisms of pathogenesis.
Cholera- toxin mediated disease that alters the secretory function of the small intestine but does not cause histological damage. Pneumococcal pneumonia- acute inflammation caused by invasive bacterial pathogen that grows extracellular bacteria. Are not easily phagocytized but once opsionized (antibody mediated) the bacteria can be ingested. Tuberculosis- infection by a facultative intracellular bacterium and controlled by cell mediated immunity (T cell mediated). It can survive in macrophage. Rheumatic fever- pathology mediated by an immune response. It is as disease resulting from immunopathology triggered by the response to a prior group A streptococcal infection.
79
Regulation of transcription
Bacteria may increase or decrease (repress) transcription of certain genes in response to environmental conditions. This is typically accomplished by DNA-binding proteins that interact with the promoter regions (the region of the gene in which RNA polymerase must interact prior to initiating transcription) of regulated genes. For example, many genes are regulated in response to the concentration of free iron in the cell’s surroundings. Iron concentrations are typically high in the environment, but extremely low in an animal host. Virulence genes (e.g., diptheria toxin) are often expressed only under conditions of low-iron- conditions that are encountered in the host.
80
Control of transcription by DNA rearrangement
An entirely different kind of control involving a semistable mechanism known as phase variation can be of importance in determining the pathogenicity of a microorganism.
81
Phase variation with Salmonella typhimurium
phase variation involves a relatively rapid (10-4 - 10-5), reversible switching in the synthesis of two alternative flagellar antigens (H1 and H2). The molecular switch that determines which flagellar gene will be transcribed is a small invertible segment of DNA within which lies the promotor of the H2 gene. This allows the bacteria to evade the immune system that is targeting one of the two flaggellar antigens.
82
Phase variation Neisseria gonorrhoease
phase variation involves the successive alternation between several antigenic forms of pili expressed on the cell surface. Each strain of gonococcus possesses an expressed copy of the pilin structural gene, plus multiple, silent, non-expressed copies of variant pilin genes. Recombinational exchange between the expressed and a non-expressed copy of the pilin genes results in a new pilin gene at the expression site and production of a new antigenically distinct pili on the cell surface.
83
Spontaneous Mutation
Single base changes, deletions and insertions occur spontaneously within a population. Under appropriate selective pressure (e.g., a patient receiving streptomycin), the preferential growth of a pre-existing mutant within a population is selected. Increased resistance to antimicrobials in Pseudomonas and Mycobacterium tuberculosis, and ii) Streptococcus pyogenes strains with an increased likelihood of causing invasive disease due to a single amino acid change in pyogenic exotoxin B.
84
Mutation rates
As in higher organisms, the rate of spontaneous mutation in bacteria is very low, typically in the range from 10-6to 10- 10per cell-generation, depending on the phenotypic trait being studied.
85
Recombination
Either site-specific or homologous recombination within a particular organism, or genetic exchange and recombination between closely related organisms can contribute to the emergence of strains with new properties. Antigenic variation in Borrelia recurrentis and Neisseria gonorrhoeae are examples of how recombination between duplicated genes can give rise to new antigenic variants. In Neisseria gonorrhoeae, phase variation involves the successive alternation between several antigenic forms of pili expressed on the cell surface. Each strain of gonococcus possesses an expressed copy of the pilin structural gene, plus multiple, silent, non-expressed copies of variant pilin genes. Recombinational exchange between the expressed and a non-expressed copy of the pilin genes results in a new pilin gene at the expression site and production of a new antigenically distinct pili on the cell surface. Recombination between variant pilin genes also produces essentially hybrid genes that encode pilin with new unique antigenic properties. Standard genetic exchange between closely related but non-identical strains can produce recombinants that have a new constellation of phenotypic traits.
86
Transposable element (transposon):
a discrete segment of DNA that is capable of enzymatically moving itself (or a copy of itself) from one chromosomal location to another within the cell. They cannot self replicate by them selves. These elements typically encode one or more proteins mediates transposition (transposase). Transposition is not dependent upon regions of extensive homology and does not require host recombination machinery. A transposon that is introduced into a cell, (e.g., as a component of a bacteriophage or plasmid) may transpose and become stably and permanently integrated into the bacterial chromosome. The structure of a transposon is flanked by interstion sequence and contains the gene for transposase, which allows movement for the entire element. They often encode virulence factors and antibiotic resistance gene.
87
Insertion Sequences (IS elements)
are transposons that encode transposase. Play a role in genome evolution by inactivating genes into which they transpose, or turning on expression of adjacent genes.
88
Complex transposons
carry additional genes such as those encoding antibiotic resistance, toxins, adhesins and other virulence factors.
89
Bacteriophage conversion
Certain virulence genes (including those encoding diphtheria toxin, cholera toxin, streptococcal pyrogenic toxins, botulism toxins and certain LPS antigens) are carried on bacteriophage and are not a "normal" component of the respective bacterial genome. Therefore, the respective virulence factor is only carried and expressed by bacterial strains that have become lysogenized and the bacteriophage genome is stably maintained by the bacterium.
90
Acquisition of Plasmids
Bacterial plasmids are autonomously replicating, usually circular, extrachromosomal DNA's ranging in size from a couple genes to a few percent the size of the bacterial chromosome. Often they can be transferred from one bacterium to another by conjugation or transduction. Plasmids can carry virulence genes, genes conferring antibiotic resistance and heavy metals, metabolic functions, and self transmission (some can promote their transfer from one cell to another).
91
Acquisition of Pathogenicity Islands
Pathogenicity Islands are generally relatively large segments of DNA present in the chromosome of some, but not all strains of a particular bacterial species. PIs encode genes that contribute to the virulence of these isolates. Bacterial isolates that lack the PI may be avirulent, or have a somewhat different disease-causing potential. It is not certain where PIs originated, or how they were first transferred to the ancestral cell. Acquisition from a heterologous organism is often implicated. In some cases, PIs contain site-specific recombination sequences similar to those of bacterial viruses, suggesting that they may have been transmitted by viruses.
92
Pathogenicity Islands and the evolution of virulence in Salmonella
Sequential evolutionary acquisition of two “pathogenicity islands”. SPI 1: Invasion of intestinal epithelial cells. SPI 2: Survival in macrophages. More recent acquisition of genes, which alter the host, range specificity (eg chicken or human).
93
E. coli
is the most abundant aerobic normal flora of the gut but can become infectious with plasmid mediated genes for enterotoxin (travellers diarrehea), epithelia cell destruction. Bacteriophage encoded toxins can also transmit virulence factors.
94
Transformation
the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material (exogenous DNA) from its surroundings and taken up through the cell membrane(s). This is dependent on homologous recombination. The active component in transformation is naked DNA, probably DNA released from lysing cells. Transforming DNA can be either chromosome fragments or plasmids from the donor cell. Many transformable species become competent for DNA uptake at only certain points in the growth cycle, and competence requires synthesis of specialized proteins to mediate the uptake. Some species of bacteria that are not normally transformed (e.g., E. coli) can be induced to become competent for transformation by treatment with calcium chloride and low temperature. In nature, transformation probably occurs most frequently between members of the same species, even though competent cells readily take up heterologous DNA fragments, which may contribute to acquisition of new genes and genetic potential of a given species.
95
Transduction
is gene transfer mediated by a bacteriophage. In transduction, bacterial viruses (bacteriophages) transfer segments of DNA (a couple genes up to a couple hundred genes) from one cell to another.
96
Bacterial conjugation
is a form of genetic transfer that is dependent upon physical contact between the donor and recipient cells, and is usually mediated by certain types of bacterial plasmids (also conjugative transposons).
97
Bacterial plasmids
Bacterial plasmids are autonomously replicating, usually circular, extrachromosomal elements ranging in size from a couple genes to a few percent the size of the bacterial chromosome. Plasmids are generally considered to be dispensible for the viability of the organism, although they often confer a selective advantage under certain environmental conditions. Among medically important bacteria, plasmids often encode antibiotic resistance and virulence factors. Many plasmids can be transferred between bacteria of the same or different species. Plasmids may acquire new genetic material, especially by transposition.
98
Conjugative plasmids
such as transmissible drug resistance plasmids, are self-transmissible. These plasmids mediate their own transfer, and may occasionally mediate transfer of chromosomal genes from donor to recipient cells. Of the plasmids which cannot mediate their own conjugal transfer, some can be mobilized during conjugation mediated by another plasmid in the same cell. Plasmids may also be transferred between cells by generalized transduction, particularly among the Gram positive bacteria.
99
The F plasmid
is the prototype self-transmissible plasmid that was discovered in a laboratory strain of E. coli. The F plasmid DNA is about 2% the size of the E. coli chromosome. The F plasmid contains genetic information encoding the following traits: Autonomous replication of the plasmid DNA. Synthesis of sex pili (F pili) which are essential for mediating pair
formation between donor and recipient cells. Conjugative transfer of F DNA to recipient (F- ) cells. Ability to integrate into the bacterial chromosome. Transfer of F DNA is coupled to a special round of DNA synthesis which is initiated at a site called oriT (the origin of transfer). The DNA is transferred as a single strand through a conjugation bridge.
100
Conjugative transposons
are mobile elements which mediate conjugation between pairs of cells, in which the transferred DNA is the conjugative transposon itself. Once transferred, the element transposes to the chromosome of the recipient cell. Therefore these elements encode both transfer (tra) genes and transposition genes. Conjugative transposons may encode antibiotic resistance, especially resistance to tetracycline (tetM), and are possibly responsible for the widespread dissemination of the tetM resistance gene.
101
Growth of virulent bacteriophage
Virus adsorbs to the bacterial cell surface and injects its nucleic acid into the cell. The viral genome is replicated, and the viral genes are transcribed and translated. This is termed the "latent period", during which viral components are being synthesized, but no assembled, infectious virus particles are present within the infected cell. Once the components are synthesized, progeny virus are assembled, and are subsequently released (usually) upon lysis of the infected cell.
102
Temperate phages and lysogenic bacteria
do not invariably kill their susceptible host cells. Infection by temperate phages may instead elicit either a lytic response, leading to phage multiplication and host cell lysis, or a lysogenic response, in which the host cell remains viable and the infecting phage DNA is maintained by the host cell in a noninfectious state known as prophage.
103
prophage
often consists of phage DNA which is linearly inserted into the host cell genome where it becomes passively replicated as part of the bacterial chromosome. A prophage can be induced enter the lytic state, resulting in viral replication, production of progeny virus and lysis of the infected cell. The lysogenic state is maintained by a prophage-encoded repressor protein that blocks expression of the phage genes necessary for viral DNA replication and lytic development. Under certain conditions, usually stress conditions, the prophage is induced to initiate lytic development with the production and release of new viral particles.
104
Lysogen or lysogenic bacterium
is a bacterial cell in which a phage exists as DNA in its dormant state (prophage).
105
generalized transduction
a process in which any segment of the donor cell genome (chromosome or resident plasmids) may be passed into another cell. Generalized transducing phages are formed as a consequence of errors in DNA packaging during phage assembly. Occasionally, the phage's packaging system will insert a "headful"-sized piece of bacterial DNA into a maturing phage capsid in place of a normal phage DNA molecule. These errors occur at a frequency ca 10-3, and the amount of DNA incorporated may represent 1-2% of the bacterial genome. These transducing particles contain no viral genetic information, but they are still able to attach to other host cells and inject the bacterial DNA which they contain. The injected DNA may then recombine with homologous segments in the recipient genome to produce a genetic recombinant, or transductant. Bacterial plasmids may also be transferred by generalized transduction. This mechanism of antibiotic resistance transfer is particularly prevalent among the Gram positive bacteria.
106
Bacteriophage conversion (lysogenic conversion).
When certain temperate bacteriophage encodes gene(s) which may be expressed during the lysogenic state and cause the appearance of a new phenotypic trait (e.g. toxin production in C. diptheriae) in the lysogenic host. In lysogenic conversion the genes controlling the new phenotypic trait are found only as a component of the phage genome. Diphtheria toxin, scarlet fever toxin, cholera toxin and certain types of botulism toxin are all bacteriophage- encoded toxins and produced by strains of bacteria that have become lysogenized by the respective bacteriophage.
107
Gram + Cocci
The genus Staphylococcus, The genus Streptococcus (and relatives)
108
Genus Staphylococcus
Staphylococcus. aureus and SSNA (“staph species, not aureus” e.g. S. epidermidis), Gram + Cocci
109
Staphylococcus aureus
Primary pathogenic species of the genus. Gram-positive cocci in clusters. Asymptomatic carriage in ~30% of healthy individuals (depending on the population). The sites of carriage are primarily in the anterior nares and perineum. One’s endogenous flora may be the source of endogenous (self) infection or may be the source of bacteria that infects other individuals. Case in point would be a health-care worker who carry S. aureus in their anterior nares, and transmits the bacteria to a patient. Members of the species are responsible for a very wide spectrum of disease- depending on the strain. Some of the typical diseases caused by this organism include: cutaneous infection, toxin-mediated disease, pneumonia, foreign-body associated infection, bacteremia/endocarditis. Gram + Cocci
110
Cutaneous infection with staph
e.g., folliculitis, boils, wound infections. The characteristic lesion is a localized abscess. Both the bacterium and the host contribute to the formation of a fibrinous capsule, which tends to wall off the infection and limit spread to adjacent tissues. The fibrinous capsule also restricts access of phagocytic cells, antibodies, and antimicrobials, etc to the site of infection. Effective treatment typically includes drainage of the abscess. Enzymes and toxins to wall off infection and interfere with host defenses (phagocytosis and killing). Cutaneous S. aureus infections are often associated with the presence of a “foreign body” at the site, e.g., a suture or splinter. The presence of such an object interferes with bacterial clearance by phagocytes and provides a surface for the bacterial to colonize. Spread from the initial lesion occurs, and may result in bacteremia, sepsis or metastatic lesions (endocarditis, bone, etc).
111
Coagulase
an essential virulence factor that is associated with formation of the fibrin capsule and deposition of fibrin on the cell surface, which interfers with phagocytosis.
112
Alpha-toxin
is the major cytotoxic agent released by bacterium Staphylococcus aureus and the first identified member of the pore forming beta-barrel toxin family. Knockout strains show reductions in invasiveness and virulence.
113
Toxin-mediated diseases with staph
Certain strains of S. aureus possess the genes for one or more protein superantigen exotoxins.
114
Superantigen toxins
are a class of antigens that cause non-specific activation of T-cells, resulting in polyclonal T cell activation and massive cytokine release. SAgs can be produced by pathogenic microbes (including viruses, mycoplasma, and bacteria) as a defense mechanism against the immune system. Compared to a normal antigen-induced T-cell response where .0001-.001% of the body’s T-cells are activated, these SAgs are capable of activating up to 25% of the body’s T- cells.
115
Toxic shock syndrome
Patients generally have a localized infection by a toxinogenic S. aureus strain; circulating toxin produced locally by bacteria at the site of infection results in severe systemic manifestations. TSS resulting from infection with the bacterium Staphylococcus aureus typically manifests in otherwise healthy individuals with high fever, accompanied by low blood pressure, malaise and confusion, which can rapidly progress to stupor, coma, and multiple organ failure. The characteristic rash, often seen early in the course of illness, resembles a sunburn, and can involve any region of the body, including the lips, mouth, eyes, palms and soles. In patients who survive the initial phase of the infection, the rash desquamates, or peels off, after 10–14 days. are examples of staphylococcal toxin-mediated diseases.
116
Staphylococcal food poisoning
is ingestion of preformed toxin in contaminated and improperly stored food.
are examples of staphylococcal toxin-mediated diseases.
117
Pneumonia with staph a
especially in patients with impaired host defenses (particularly high mortality 50%). Although it is not a very common cause of pneumonia in patients who present to the clinic (outpatient), it is common isolate in patients who develop pneumonia once they are in the hospital or are recently or chronically associated with health care (health care-associated pneumonia (HCAP); hospital acquired-pneumonia (HAP); and ventilator-associated pneumonia (VAP).
118
Foreign-body associated infections with staph a
vascular catheter-related infections, prosthetic joint infections, hardware infections (cardiac pacemakers, vascular grafts).
119
Bacteremia/endocarditis with staph a
One of the most common isolates from blood cultures (associated with concurrent foreign-body infections or skin/soft tissue infection) and a common cause of heart valve infection (endocarditis). Compare endocarditis and rheumatic heart disease (Streptococcus pyogenes)
120
Antibiotic resistance with staph a
is of particular concern in this organism. For example, there has been a progressive acquisition of genes which confer resistance to penicillins, followed by resistance to methicillin, and the more recently emerging resistance to vancomycin . The most important of these is methicillin resistance (“methicillin- resistant Staph aureus” MRSA).
121
Staphylococcus epidermidis
This is the prototype of the group of staphylococcal species collectively termed SSNA (“staph species, not aureus”), or CNS (“coagulase negative staphylococcus”). These are generally considered to be normal skin flora and relatively non-pathogenic. However in certain circumstances, it is associated with various sorts of localized infection. Infections are typically associated with various sorts of foreign bodies, e.g., catheters, shunts, hip prostheses, artificial (or damaged) heart valves. Staphylococcus aureus followed by Streptococci of the viridans group and coagulase negative Staphylococci are the three most common organisms responsible for infective bacterial endocarditis]. The members of this group that are most often associated with disease are those that produce “slime”, an extracellular glycocalyx that allows the organisms to adhere very tenaciously to the various implanted devices, and allows them to grow in a protected biofilm on the surface of the device. Infections are quite difficult to treat and often require removal of the device. Antibiotic resistance (including methicillin) and
limited accessibility of the drug to the bacteria within the biofilm. Gram + Cocci
122
The genus Steptococcus and relatives
gram-positive cocci often in chains or pairs. Catalase negative (staphylococci are catalase positive). Includes Streptococcus pyogenes
(Group A strep), Streptococcus pneumoniae
, “Viridans” streptococci, Enterococcus faecalis/Enterococcus faecium
123
Pharyngeal infection with Streptococcus pyogenes
Strep throat. Streptococcus pyogenes is the causative agent of common “strep throat”. Untreated, this infection of the pharynx generally self-limiting and resolves in a couple weeks. The primary virulence factor is the “M-protein”, a surface exposed protein that inhibits phagocytosis and killing by PMNs, and enhances adherence to epithelial cells. Bacteria that produce M-protein resist phagocytic killing. Production of M-protein-specific antibody by the host makes the bacterial cells susceptible to killing and is associated with recovery from disease and with immunity to subsequent disease caused by an antigenically related strain. So why don’t we develop immunity after the first course of disease? Among the Group A strep that cause human disease, there are over 70 serotypes based on antigenic differences in the M-protein. Antibody against the M-protein is protective against disease caused by the same streptococcal serotype, but individuals remain susceptible to infection by isolates with serologically distinct M-protein. Transmission is generally by contact with nasal secretions of an infected individual, or by droplets produced by coughing, etc. Curiously, this organism can be isolated from the pharynx of several percent of healthy individuals.
124
Skin and wound infections with Streptococcus pyogenes
Group A Strep are associated with infections of the skin and wounds. The typical lesion is that of a spreading infection of the cutaneous and subcutaneous tissues (cellulitis). [Contrast with typical S. aureus infections-which are more prone to produce focal abscesses, which may be accompanied by a surrounding cellulitis]. Bacteremia and sepsis are possible. GAS produce a variety of hydrolytic enzymes that act in concert to break down tissue and damage or kill phagocytic cells, thereby facilitating spread of the organism through the tissues.
125
Glomerulonephritis
a post- streptococcal disease. an immune complex disease that may follow skin or pharyngeal infection by Group A strep. Streptococcal antigen-antibody complexes are deposited in the kidney and accumulate at the basement membrane. Self-limiting, complement- mediated damage to the kidney results.
126
Rheumatic fever
a post- streptococcal disease. This is an autoimmune inflammatory disease that may follow group A streptococcal pharyngitis. The possibility that an individual may develop RF is one of the principle justifications for aggressively treating streptococcal pharyngitis. It is characterized by fever and inflammation of the heart, joints, and other tissues. RF is generally thought to result from the production of self-reactive antibodies produced in response to pharyngeal infection by Group A Strep. During a bout of Strep throat, individuals produce antibodies that recognize, and bind to, a variety of Streptococcal antigens. This response is necessary for recovery from disease. However, for some strains of Group A Strep (“rheumatogenic strains”) antibody produced in response to certain bacterial antigen(s) has the unfortunate property of also recognizing and binding to specific host antigens of the myocardium and heart valves.. This leads to progressive antibody-mediated damage to these tissues. In RF, the heart tissue itself is NOT colonized by the infecting Streptococcal organisms, making the disease distinct from infective (bacterial) endocarditis- a true bacterial infection of the heart valves (e.g., Staphylococcus epidermidis).
127
Streptococcus pneumonia
G+ cocci in pairs (diplococci; “pneumococcus”). Normal flora in UR tract of up to 40% of healthy people. Diseases include non-invasive and invasive disease:
Non-invasive: Pneumonia (one of the most frequent causes of bacterial pneumonia in all age groups, world-wide). Sinusitis, otitis media, bronchitis. Invasive disease: Meningitis, bacteremia/septicemia, pneumonia with septicemia. Pathogenesis is most closely related to ability to grow and evade host defenses:
Antiphagocytic polysaccharide CAPSULE (at least 91 distinct antigenic types)
and recovery/immunity due to anticapsular antibody. Predisposing factors include
young or old, alcoholism (e.g., mucocillary defect), and respiratory viral infection.
128
Adults vaccine for pneumonia
(pneumovax, PPSV23) Commonly refered to as the “pneumonia vaccine”. Provides measurable (but by no means complete) protection against INVASIVE disease in elderly and immunocompromised adults. Ironically, does NOT provide protection against pneumonia.
129
Children vaccine for pneumonia
Hepta-valent (Prevnar) and newer 13-valent (Prevnar 13) vaccines in kids are remarkably successful at reducing disease in vaccinated. Confers a degree of “herd immunity” on unvaccinated individuals. Rather unexpectedly, widespread vaccination of kids also reduced vaccine-type pneumococcal carriage across all age groups. Some studies suggest efficacy is threatened by “serotype replacement”.
130
Antibiotic resistance vaccine for pneumonia
(Emerging pencillin resistance due to alterations of penicillin-binding proteins-
importance of which will be discussed in antibiotic resistance lecture.
131
Viridans streptococci
one of the important causes of infective (bacterial) endocarditis.
The viridans streptococci are a large group of commensal streptococcal species that are either α- hemolytic, producing a green partial hemolysis on blood agar plates, or nonhemolytic.
Particularly abundant in mouth where some are associated with dental caries. The oral viridans streptococci may also gain access to the bloodstream following dental extractions or other dental manipulations, in which case they have the potential to cause infective (bacterial) endocarditis. These bacteria produce dextrans, which are important for adherence to teeth and other oral tissues in their “normal” habitat. The production of dextrans also allows them to serendipitously adhere to the fibrin and platelet deposits on damaged heart valves. The growth of bacteria on the valves leads to progressive tissue damage and impaired function.
132
Enterococcus faecalis and Enterococcus faecium
The primary causes of enterococcal infections; common commensal organisms (normal flora) in the
intestines of healthy humans. Important to distinguish from streptococci because of its intrinsic and emerging acquired antibiotic resistance (antimicrobial arsenal is becoming depleted, including vancomycin). Biggest problem here is vancomycin-resistant enterococci (i.e., VRE). Mechanism and importance to be covered in resistance lecture. Common sites of infection: Urinary tract, surgical wounds, biliary tract, endocarditis. Frequent cause of nosocomial infections. Normal flora in GI tract of some healthy individuals. Selected by therapy with cephalosporins (cell-wall active). Patient to patient transfer on hands of hospital personnel or medical devices. Often seen as a mixed infection of several different organisms, including anaerobes. This is particularly important in cases in which there is a perforation of the colon and release of contents to the peritoneal cavity.
133
Gram-positive rods
Most common G+ rods encountered in human disease are members of the genus Clostridium.
134
The Clostridia are characterized
Strict anaerobes- growing cells are KILLED by molecular oxygen, Endospore-formers- Endospores are extremely resilient, metabolically inactive developmental
stage of these bacteria. The spores are difficult to kill with sanitizing agents and, unlike the growing forms of the bacteria, are largely insensitive to oxygen. As such they are critical to the persistence of these bacteria in the environment. Diseases and organisms considered in this section:
Hospital-acquired diarrhea and colitis (C. difficile), tetanus (C. tetani),
botulism (C. botulinum),
gangrene and other tissue infections (C. perfringens and others)
135
Clostridium difficile
typically a hospital-acquired (nosocomial) infection, and the Clostridia that is most likely to be
encountered in the hospital.
diarrhea
and Pseudomembranous colitis. Constituent of the normal flora of the GI tract in ~10% of healthy individuals. Relatively resistant to most common antibiotics. Most frequently associated with diarrhea and colitis following antibiotic treatment for unrelated conditions. Most antibiotics have been implicated, although first described following treatment with clindamycin. Disease is believed to result from depletion of the intestinal flora by antibiotic treatment, and resulting overgrowth of C. difficile. The source of the C. difficile may be the patient’s own intestinal flora, or may be transmitted to the patient from another infected patient or from hospital staff. Spores not killed by alcohol-based hand sanitizers. The result is often epidemics of antibiotic associated diarrhea on a hospital ward, and isolation of symptomatic patients is often appropriate to reduce transmission. This organism produces two discrete toxins , one an entertoxin and the other a potent cytotoxin, which together are responsible for the observed pathology and symptoms. Can be diagnosed on the basis ELISA test for toxin in patient stool. Alternatively, the organism can be cultured from stool, although positive culture must be confirmed by demonstration of toxin production by the patient’s isolate. Many hospitals including UCH are moving to PCR-based detection of the organism for diagnosis.
136
Clostridium tetani
Common organism in soil and GI tract of animals (importance of spores). Local infection (anaerobic) and toxin production. Retrograde axonal transport of toxin to CNS. Toxin blocks inhibitory interneurons in CNS resulting in “spastic paralysis”. Vaccine (inactive toxoid) used to induce production of protective anti-toxin antibody. Compare to passive immunization with antitoxin (tetanus-immune human gamma globulin) to treat non-immune individuals with clinical signs of tetanus
137
Clostridium botulinum
Common organism in soil and GI tract of animals (importance of spores). Bacteria grow in contaminated food under anaerobic conditions (e.g., home canned foods). During growth, bacteria produce botulinum toxin, which is secreted into the food. People ingest the preformed toxin by eating the under-cooked, contaminated food. (Compare and contrast this scenario with Staphylococcal food poisoning and ETEC).Toxin blocks acetylcholine transmission at neuro-muscular junctions, resulting in “flaccid paralysis”
138
Clostridium perfringens
Wound infections and food poisoning.
139
Wound infections with Clostridium
especially crushing-type injuries, and others that lead to compromised blood
flow to tissues and creation of a low-oxygen environment in the devitalized tissue conducive to growth of this anaerobic bacterium. While growing in the wound, the bacterium produces “alpha toxin” (phospholypase enzyme), and several other damaging enzymes and toxins. Alpha toxin kills phagocytic cells and muscle tissue. Disease can range from cellulitis to myonecrosis to gas gangrene.
140
Clostridial food poisoning
Some strains of C. perfringens produce enterotoxin. Large numbers are ingested with contaminated food; the enterotoxin is produced in vivo when the bacteria sporulate in the gut. The action of the toxin disrupts tight junctions between endothelial cells in the ilium resulting in a dysregulation of fluid transport.
141
Aerobic and facultative) Gram-Negative Rods
Organisms considered in this section: E. coli (Escherichia coli) and Pseudomona aeruginosa
142
E. coli
the “prototypical” Gram-negative bacterium
: Normal flora in large intestine. Disease may be caused by endogenous organisms or acquisition (ingestion). Although normally sensitive to most antibiotics, this is one of the most common organisms to acquire resistance thru acquisition of drug-resistant plasmids
• Many different strains of E. coli with varying pathogenic potential. Three types of E. coli disease are considered here: gastrointestinal disease, urinary tract infections and abdominal infections.
143
E. coli GI disease
Many strains which vary in type and severity of disease. Typically from drinking contaminated food and water. ETEC- typical traveler’s diarrhea- (Enterotoxigenic E. coli). Disease is self-limiting and patient management may only require fluid replacement. Two properties of the bacteria are essential for disease: Adherence to the intestinal mucosa (pili). Toxin(s) that disrupts the electrolyte balance in gut (Refer to the bacterial toxins lecture for molecular details of the toxins).
144
E. coli Urinary tract infections
Isolates are typically endogenous from GI tract. Access the UT via urethra -> bladder -> kidney. “Special” strains getting into the “wrong” place. The typical urinary tract isolate has the following properties, which are rather uncommon in the bulk of the intestinal population: Adherence to bladder epithelium. Specific interactions with bladder epithelial cells. UTI strains are typically β-hemolytic
145
E. coli Abdominal infections
Release/escape of contents of colon to peritoneal cavity and adjacent tissues, e.g., Surgical wounds, traumatic wounds, etc
Colon cancer. Such infections are often bacteriologically mixed cultures. Often associated with anaerobic bacteria to form anaerobic abscess.
146
Pseudomonas aeruginosa
Very common environmental bacterium, to which most individuals are highly resistant to infection. Three types of disease are considered here: Infections of traumatic injuries, surgical wounds, and especially BURNS. Opportunistic
pathogen often affecting immunocompromised patients. Chronic lung infection of patients with Cystic fibrosis. Hospital-acquired infections (UTIs, pneumonia, less frequently associated with intravascular
catheter-related infections).
147
P. aeruginosa and CF
CF patients produce copious, viscous bronchial secretions. This tends to result in stasis in the lungs and predisposes the patient to infection. Early in life, pulmonary infections with S. aureus are relatively common, and are usually controlled with antimicrobials. However, nearly all CF patients become chronically infected with P. aeruginosa by age 15 - 20. This is facilitated in part by its intrinsic resistance to many anti-staphylococcal drugs. Within the lungs of the chronically infected patient, the bacteria are relatively protected from phagocytosis by the patient’s viscous lung secretions, the mucoid exopolysaccharide made by the bacterium and the production of bacterial toxins. There is progressive damage to the lungs due to the action of the toxins and the host immune response. Treatment of the chronic infection is difficult due to the intrinsic resistance of P. aeruginosa to many antimicrobials, and to the difficulty in delivering the drug to the site of bacterial replication. The consequences of chronic lung infection with P. aeruginosa is very frequently the cause of death in CF patients.
148
P. aeruginosa and hospital-acquired infections
Hospital-acquired infections by P. aeruginosa include UTIs, pneumonia, and less frequently intravascular catheter-related infections. This is a big player to think about for most all hospital- acquired infections, (especially UTIs, pneumonias). The point not only is that it is relatively common, but given its high intrinsic antibiotic resistance, many empiric regimens won’t cover it. If a hospitalized patient is suspected of having a new infection, one should consider using a specialized antibiotic regimen that covers this organism.
149
Gram-negative (diplo)cocci
Neisseria gonorrhoeae (gonococcus)
150
Neisseria gonorrhoeae (gonococcus):
Causative agent of gonorrhea and of conjunctivitis leading to blindness in infants born to
infected mothers. Key to infectivity is the pilus. This structure is required for adherence and interferes with bacterial killing by neutrophils. The pilus is required for virulence, and antibody against the pilus is protective. However, different strains may express antigenically distinct forms of pili. In addition, during infection, these bacteria undergo a process of antigenic variation, in which individual cells switch producing one antigenic type of pillus to another antigenically distinct type. Therefore, patients may be repeatedly infected with strains of N. gonorrhoeae strains that have antigenically distinct pili. Growth on mucosal surface incites a robust inflammatory response, resulting in a purulent discharge and local tissue invasion. Prolonged infection may lead to scarring and fibrosis. Males range from asymptomatic to urethritis. Females: infection of cervix, urethra. More often asymptomatic than in males. Ascending infection including uterine tubes may result in fibrosis and infertility
151
Antibiotic resistance
| to N. gonorrhoeae
isolates are almost always resistant to penicillin and strains with reduced sensitivity to cephalosporins are emerging.
Fluroquinolone resistance is also highly prevalent and use of these agents to treat N. gonorrhoeae is no longer recommended.
152
Anaerobic Bacteria (other than the clostridia):
The most common anaerobic bacteria (other than the Clostridia) that are implicated in human disease are members of the normal flora that inhabit the many anaerobic nitches in/on the human body. These anaerobic sites include Colon, Mouth (e.g., gums, tongue), Female genital tract and Skin. Anaerobic bacteria are killed by the presence of molecular oxygen in their environment. Special precautions must be taken in obtaining, transporting and culturing these organisms. Traditional “swabs” do not support the recovery of anaerobic pathogens and are not recommended for use in obtaining clinical specimens. Instead, use of Eswabs, tissue samples, capped syringes with fluid are all reasonable methods for obtaining clinical samples that will support the recovery of anaerobes from clinical specimens. Bacteriologically, these bacteria are very diverse in terms of Gram reaction (i.e. cell wall structure) and morphology. They are often considered as a group they share certain common properties of the disease. Infective organisms are generally of endogenous origin- normal flora getting into the wrong place. For example, lung infection due to aspiration of oral bacteria, abdominal infection due to leakage of colon contents to the peritoneum. The typical lesion is an abscess. A hallmark is that of a mixed infection containing both aerobic and anaerobic bacteria. Aerobes
may play a role in metabolizing the local oxygen, making the site more conducive to growth of the anaerobes.
Because of their anaerobic metabolism, certain drugs are more effective (metronidazole), or less effective (aminoglycosides) against anaerobes. This is one of the important pharmacologic properties of the classes of antimicrobials that guides their proper use.
153
Bacteroides fragilis
Despite the abundance and diversity of anaerobic bacteria in the human normal flora, most endogenous anaerobic abscesses are caused by relatively anaerobic few species. Members of the Bacteroides group are among the most frequently encountered; Bacteroides fragilis is considered to be the prototypical endogenous anaerobic pathogen. Although B. fragilis is a minor component of the normal human gastrointestinal flora, making up only 1- 2% of the total bacteria of the colon, it is associated with greater than 80% of intra-abdominal infections. [Bacteroides species common to the oral cavity are commonly associated with anaerobic infections above the diaphram (e.g., lung, brain)]. Relatively aerotolerant. Virulence factors. B. fragilis produces a variety of tissue-destructive enzymes, an anti-
phagocytic capsule and production of superoxide dismutase.
154
Obligate Intracellular Bacteria
The Rickettsia (e.g., the etiologic agent of Rocky mountain spotted fever) and the Chlamydia are important examples of obligate intracellular bacteria. These are organisms that grow only within an infected eukaryotic cell. They cannot be cultured in standard bacteriological media, but can generally be cultured by infecting tissue culture cells. The degree to which the obligate intracellular bacteria have adapted to an intracellular life is exemplified by the Rickettsia, which have lost the capacity to synthesize their own ATP and rely on the infected host cell to supply this essential compound. In any differential diagnosis, the possibility of obligate intracellular bacterial etiologic agent poses special considerations for identification of the infecting organism (how do you do it if you can’t culture it?) and for the appropriate choice of antimicrobial for treatment (some drugs, e.g., the aminoglycosides, do not enter eukaryotic cells efficiently).
155
Chlamydia trachomatis
An obligate, intracellular bacterium Diseases: Trachoma: chronic infection of conjunctiva, leading to scarring and blindness. Trachoma is endemic in parts of Asia and Africa where there is relatively poor standards of personal hygiene. Genital infections. In this country, C. trachomatis is a relatively common sexually transmitted disease. It is often found in coinfections with N. gonorrhoeae. As an STD, it is the causative agent of “non-gonococcal urethritis” (men); and urethritis, cervicitis and PID (women). Neonatal infections. Infants born to mothers with C. trachomatis genital infection may become infected at birth, resulting in neonatal conjunctivitis and neonatal pneumonia. Systematic screening and treatment of chlamydial infection in pregnant women has resulted in a dramatic decrease in perinatal chlamydial infections in the United States.
156
Bacteria Without Cell Walls
“Mycoplasmas” (family designation) includes two genera of bacteria lacking cell walls and containing sterols in the plasma membrane: Mycoplasma and Ureaplasma. The most important member of this group is Mycoplasma pneumoniae
157
Mycoplasma pneumonia
One of the common causes of pneumoniae, especially in ages 5-20. Mycoplasma pneumonia is generally mild. Person to person transmission by direct contact with infected respiratory secretions. Fever, headache, sore throat, non-productive cough, chest and body aches, fatigue. Resolution and recovery occurs slowly over 1-4 weeks. Because the bacteria lack a rigid cell wall, shape is highly pleomorphic and penicillins are not effective. Organism can be grown in specialized cell-free bacteriologic medium. However, culture is difficult and growth is slow. Laboratory diagnosis is more often by serological tests or PCR. Does not stain with common Gram-stain, which would be used primarily to help rule-out
other bacterial causes
Mycoplasma pneumoniae adheres to respiratory epithelial cells. Bacterial growth remains extracellular. Bacteria produce hydrogen peroxide and superoxide radicals, which damages host tissue.
158
Adults vaccine for pneumonia
(pneumovax, PPSV23) Commonly refered to as the “pneumonia vaccine”. Provides measurable (but by no means complete) protection against INVASIVE disease in elderly and immunocompromised adults. Ironically, does NOT provide protection against pneumonia.
159
Children vaccine for pneumonia
Hepta-valent (Prevnar) and newer 13-valent (Prevnar 13) vaccines in kids are remarkably successful at reducing disease in vaccinated. Confers a degree of “herd immunity” on unvaccinated individuals. Rather unexpectedly, widespread vaccination of kids also reduced vaccine-type pneumococcal carriage across all age groups. Some studies suggest efficacy is threatened by “serotype replacement”.
