Bacteria, Toxins, & Virulence Flashcards
Koch’s Postulates
- Specific microbe is present regularly in characteristic lesions of the disease
- This microbe can be isolated and grown in vitro
- Injection of the cultured microbe into animals reproduces the disease in animals
- The specific microbe can be re-isolated from lesions of the disease in animals
Pathogen
A microbe capable of causing host or damage; includes classical and opportunistic pathogens; damage can result from direct microbial action or host immune response
Pathogenicity
The capacity of a microbe to damage a host
Virulence
Relative capacity of a microbe to damage a host
Compared by ID50 (minimally infective dose in 50%), DD50 (disease producing dose in 50%), and LD50 (lethal dose in 50%)
Peptidoglycan
Forms a rigid mesh surrounding the cytoplasmic membrane of bacteria
Basic subunit consists of two alternating sugars (GlcNAc and MurNAc) and a 5 amino acid tail hanging off of MurNAc terminating in D-ala-D-ala
Transglycosylation joins the MurNAc and GlcNAc residues of adjacent subunits
Transpeptidation cleaves the terminal D-ala and joins the amino acid tails, allowing cross-linking
Lysozyme
Enzyme present in many body secretions; contributes to innate host defenses against bacteria by hydrolyzing peptidoglycan at the glycosidic bond between MurNAc and GlcNAc
Gram Negative
Has an outer membrane containing LPS, lipoproteins, and porins; LPS is located exclusively in the outer leaflet of the OM and phospholipids are located in the inner leaflet
Gram Positive
Contains a single cytoplasmic membrane; cell wall contains greater degree of peptidoglycan cross-linking; cell wall also contains Teichoic acids, which form a repeating polyglycerol-P or polyribitol-P backbone; lipoteichoic acids help anchor the cell wall to the membrane
Lipopolysaccharide (LPS)
Contains lipid A, the toxic component of endotixin, a core polysaccharide, and an oligosaccharide that functions as an antigen (O antigen)
Bacterial ribosomes
30S + 50S = 70S
DNA is located in the cytoplasm so transcription and translation are coupled; polyribosomes transcribe on one mRNA molecule simultaneously and mRNA may be “polycistronic,” coding for multiple proteins
Bacterial cytoplasmic membrane
AKA “inner membrane” in gram-negative bacteria; lipid bilayer made up of phospholipids and proteins but no sterols; exhibits selective permeability, essentially impermeable to all charged substances including protons; only hydrophobic molecules smaller than glycerol can diffuse
Phases of the Bacterial growth curve
- Lag phase - physiological adjustment to new medium
- Exponential (logarithmic) phase of growth - rate of increase in cell mass is proportional to the cell mass already present; a constant interval of time is required for doubling cell mass (generation time)
- Stationary phase - growth plateaus as essential nutrients are consumed and toxic metabolites accumulate
- The number of viable bacteria decreases as autolysis occurs
Heterotrophic Bacteria
Bacteria that require an organic carbon source, includes most bacterial pathogens
Autotrophic Bacteria
Bacteria that obtain their carbon exclusively from CO2
Fastidious Bacteria
Bacteria that are deficient in one or more biosynthetic pathways and require, in addition to sources of carbon and energy, a number of essential growth factors (AAs, purines/pyrimidines, and inorganic ions)
Obligate Intracellular Bacteria
Bacteria that can only live within eukaryotic cells
Strict Aerobe
Requires oxygen, cannot ferment
Ex: Mycobacterium tuberculosis
Strict Anaerobe
Killed by oxygen; fermentative metabolism
Ex: Clostridium
Indifferent (Aerotolerant anaerobe)
Ferments in presence or absence of O2
Ex: Strptococcus pyogenes
Facultative
Respires with O2, ferments in absence of O2
Ex: E. coli, Staph aureus
Microaerophilic
Grows best at low O2 but can also grow without O2
Fermentation
Organic compounds serve as both electron donors and electron acceptors; no net oxidation of substrates occurs; undertaken by:
Strict anaerobes
Facultative & indifferent bacteria - grown under anaerobic conditions
Indifferent organisms (aerotolerant anaerobes)
Ex: Glucose –> Ethanol
Bacterial respiration
Carbon compounds are oxidized by passage through the respiratory chain located in the cytoplasmic membrane; molecular oxygen (aerobic) or inorganic substrates (anaerobic) are final electron acceptor
Glucose –> CO2
Sporulation
A response to adverse nutritional conditions; spores are highly resistant, dehydrated, and have no metabolic activity; they can survive for long periods of time under adverse conditions and germinate when conditions improve to convert back to vegetative cells
Gram positive cocci
Streptococci (pneumoniae, pyogenes)
Staphylococci (aureus - MSSA, MRSA)
Enterococci (faecium)
Gram negative cocci
Neisseria (meningitidis, gonorrheae)
Atypical bacterial pathogens
Chlamydia, Mycoplasma
Gram positive rods - anaerobic
Clostridium (difficile, tetani, botulinum)
H. pylori
Gram negative rod - anaerobic
Bacteroides fragilis
Toxin Class - Spreading factors
Toxic enzymes secreted by microbial pathogens that break down the ECM, allowing the microbe to spread through tissues and cause disseminated infection
Ex: Collagenase, elastase, hyaluronidase
Toxin Class - Cytotoxins
Generally, toxins that act on the plasma membrane to cause cellular damage
Often toxins that insert into membranes and assemble into multimeric complexes that form pores, causing lysis of target cells
Ex: a-Hemolysin of Staph aureus
Toxin Class - Superantigens
Potent T-cell activators; bi-functional molecules that bind simultaneously to conserved region of MHC-II on antigen presenting cells AND to a conserved region of the TCR, generating a cytokine response that is independent of specific antigenic effects
Ex: Toxic Shock Syndrome Toxin (TSST-1) of Staph. aureus
Pyrogenic exotoxins of Strep. pyogenes
Cholera Toxin
Toxin Class: Intracellular signal disruption; produced by Vibrio Cholerae
B subunit binds the GM-1 receptor initiating endocytosis of the A subunit; in the cytosol, subunit A ADP-ribosylates the alpha subunit of the Gs protein, leading to its constitutive activation; Gs activates Adenylate Cyclase, which increases production of cAMP; cAMP binds the CFTR channel causing efflux of Cl- and water into the gut
Pertussis Toxin
Toxin Class: Intracellular signal disruption
ADP-ribosylates and inactivates the alpha subunit of inhibitory Gi protein; decreased inactivation of Gi leads to increased activity of Adenylate Cyclase and increased production of cAMP
Heat Stable Enterotoxin of E. coli
Toxin Class: Intracellular signal disruption
Activate guanylate cyclase on the surface of gut epithelial cells, leading to increased production of cGMP, causing secretory diarrhea
Anthrax Toxin
Toxin class: Protein synthesis inhibitor, made by Bacillus anthracis
Edema Factor (EF) is an Adenylate Cyclase that enters target cells and produces cAMP (dependent on Ca2+ and Calmodulin within cells); leads to water efflux and edema
Lethal Factor (LF) is an endopeptidase that cleaves MAP kinase kinase, inactivating its function in signal transduction
Diptheria Toxin
Toxin class: Protein synthesis inhibitor; made by C. diptheriae
Binds to a specific cell surface receptor via the B (binding) subunit; A (active) subunit is endocytosed and forms a pore in the endosome to enter the cytosol; A subunit inactivates elongation factor 2 (EF-2) by ADP-ribosylation, preventing protein synthesis by the ribosome and leading to cell death
Shiga Toxin
Toxin Class: Protein synthesis inhibitor; made by Shigella dysenteriae and Enterohemorrhagic E. coli (EHEC)
RNA glycosidase that removes one adenine residue from the 28S RNA of the 60S ribosomal subunit
C. diff Enteric Disease
Toxin Class: Protein synthesis inhibitor, produced by Clostridium difficile
C. diff toxin alters the actin cytoskeleton of target cells by transferring glucose to several Rho family GTPases, inactivating them
Botulinum Toxin
Toxin Class: Neurotransmitter inhibition; produced by Clostridium botulinum
Inhibits release of ACh at neuromuscular junctions by cleavage of certain SNARE proteins; prevents muscle contraction, causing flaccid paralysis
Tetanus Toxin
Toxin Class: Neurotransmitter inhibition; produced by Clostridium tetani
Inhibits the release of neurotransmitter from inhibitory interneurons by cleavage of certain SNARE proteins; causes sustained muscular contraction (tetany) of skeletal muscles
Conjugation
Exchange of plasmid DNA between two physically linked bacteria, usually via conjugative plasmids (i.e. the “F” plasmid) that are self-transmissible; the F plasmid produces a special sex pilus that forms a direct connection with a recognized mating pair, allowing DNA replication of the plasmid across this cytoplasmic bridge
Generalized transduction
Occasionally during the process of bacteriophage replication a piece of bacterial DNA is incorporated into the viral capsid; this viral particle is called a “transducing phage” and can inject this bacterial DNA into a new host cell, forming a stable recombinant
Transformation
The ability of competent bacteria to take up naked DNA from the environment and incorporate it into their own genome
Plasmid
Self-replicating, extra-chramosomal DNA element that is not essential for pathogen viability
May encode a variety of functions, including: r=Resistance to antibiotics Virulence factors (toxins, adherence pili, capsule) Metabolic functions (sugar utiization) Self transmission
Pathogenicity Island
Insertions of one or more genes when comparing the genomes of two strains of the same pathogenic species; often carry genes that encode virulence factors
Strains may transform from non-virulent to virulent via sequential acquisition of multiple pathogenicity islands
Staphylococcus aureus
Gram + cocci carried asymptomatically by 30% of people in naris and perineum; produces alpha-hemolysin (alpha toxin) which forms a transmembrane pore in the membranes of infected cells, causing lysis; characteristic infections:
Cutaneous infection characterized by local abscess, often associated with the presence of a foreign body
Toxic Shock Syndrome
Pneumonia - hospital acquired
Bacteremia
Endocarditis
Staphylococcus epidermis
Gram positive cocci, component of normal skin flora
Commonly found in localized skin infections, especially in the presence of foreign bodies; also common in infective bacterial endocarditis
Streptococcus pyogenes
Gram + cocci
Causal pathogen of Group A Strep (pharyngeal infection) and post-strep disease (Rheumatic Heart Disease)
Skin and wound infections - diffuse cellulitis
Streptococcus pneumonia
Gram + cocci, normal flora in upper respiratory tract
Common causal pathogen in many respiratory infections: pneumonia, sinusitis, bronchitis
Otitis media
Streptococci viridians
Gram + cocci abundant in mouth; associated with dental caries
Oral viridians may gain access to blood strem following dental surgery, causing infective endocarditis
Enterococcus faecalis
Gram + cocci, part of the normal intestinal flora
Frequent cause of nosocomial (hospital-acquired) infections of urinary tract, surgical wound, and biliary tract
Vancomysin-resistant enterococci (VRE) is an emerging problem
Clostridium difficile
Anaerobic, gram + rod, part of the normal gut flora
Overgrowth of C. dif following antibiotic treatment in hospitals OR transmission by hospital staff leads to infective colitis, diagnosed by ELISA for toxin in stool
Clostridium tetani
Anaerobic, gram + rod common in soil and GI tract of animals
Produces tetanus toxin which blocks inhibitory interneurons in the CNS resulting in tetanic paralysis of skeletal muscles
Infection may be prevented by vaccine (inactive toxoid) or treated by antitoxin (tetanus-immune IgG)
Clostridium botulinum
Anaerobic, gram + rod found in soil & GI tract of animals; often grows in contaminated food under anaerobic conditions
Produces botulinum toxin which blocks ACh transmission at neuromuscular junctions, resulting in flaccid paralysis
Escherichia coli
Gram negative rod, part of the normal gut flora; common infections include:
Traveler’s Diarrhea (via LT and ST of Enterotoxigenic E. coli [ETEC])
UTIs - caused by transplantation of E. coli from the GI tract
Abdominal infections - caused by release of colon contents into peritoneal cavity
Pseudomonas aeruginosa
Gram negative rod, commonly found in the environment; often opportunistic, infecting immunocompromised patients; common infections include:
Chronic lung infections in patients with CF
Hospital-acquired infections - especially UTIs, pneumonias
Neisseria gonorrhea
Gram negative cocci
Causative agent of gonorrhea; growth on mucosal surface incites an inflammatory response, resulting in purulent disgharge and local tissue invasion; prolonged infection may lead to scarring and fibrosis
Generally penicillin-resistant; often fluroquinolone resistant
Bacteroides fragilis
Anaerobic, gram negative rod; makes up 1-2% of normal flora of the colon but associated with over 80% of intra-abdominal infections, generally abscesses
Chlamydia trachomatis
Gram negative, obligate intracellular pathogen; common infections include:
Trachoma - chronic infection of conjunctiva leading to scarring and blindness
Genital infections - causative agent of non-gonococcal urethritis (in men)
Neonatal infections of infants born to mothers with genital infections, resulting in neonatal conjunctivitis and pneumonia
Treated with macrolides or tetracyclines
Mycoplasma pneumoniae
Atypical bacteria lacking cell wall and containing sterols in the plasma membrane
Common cause of pneumonia in children and young adults; infection is generally mild and resolves over 1-4 weeks
Treated with macrolides or tetracyclines
Spreading Factors
Toxic enzymes that facilitate the breakdown of ECM and the spreading of microbes throughout a tissue
Ex: hyaluronidase, elastase, collagenase
alpha-Hemolysin
Staphylococcus aureus toxin; assembles into multimeric complexes that can form a pore in host cell membrane, causing lysis
Superantigens
The most potent known T-cell activators; they act by binding to MHC-II molecules on antigen presenting cells and simultaneously to T cell receptors at a site that is different from the normal antigen binding site. This interaction stimulates excessive production of cytokines, causing pathological, systemic effects
Diptheria Toxin - Mechanism
Diptheria toxin exists as an alpha-beta dimer; the beta (binding) unit facilitates endocytosis of the alpha (active) subunit; once intracellular, the alpha subunit inactivates elongation factor-2 (EF-2), preventing protein synthesis by the ribosome and leading to cell death
Shiga Toxin
Produced by Shigella dysenteriae and enterohemorrhagic e. coli (EHEC)
Shiga toxin is an RNA glycosidase that removes one particular adenine residue from the large ribosomal subunit thereby inactivating it and preventing protein synthesis, which causes cell death
Vibrio cholera
& heat labile E. coli enterotoxin
Binds go the ganglioside receptor on cell surfaces; once intracellular, the alpha subunit ADP ribosylates the alpha subunit of the stimulatory Gs G protein, which activates it; this leads to increased adenylate cyclase activity and higher concentrations of cAMP, which binds the CFTR channel causing active chloride secretion leading to secretory diarrhea
Pertussis Toxin
ADP ribosylates the alpha subunit of the inhibitory Gi G protein, which inactivates it; this causes increased adenylate cyclase activity and increased cAMP levels
Anthrax toxin
Edema factor (EF) - an adenylate cyclase; Ca2+ and Calmodulin dependent production of elevated cAMP within the target cell leading to water efflux and edema
Lethal factor (LF) - an endopeptidase that cleaves MAP kinase kinase protein, inactivating its function in cell signaling and leading to cell death
Botulinum toxin
A toxic endopeptidase that inactivates specific SNARE proteins required for neuroexocytosis; causes flaccid paralysis of skeletal muscles by inhibiting release of ACh at neuromuscular junctions
Tetanus toxin
A toxic endopeptidase that inactivates specific SNARE proteins required for neuroexocytosis; causes sustained muscular contraction of skeletal muscles by inhibiting the release of neurotransmitter from inhibitory interneurons
Immunotoxins
Hybrid molecules which contain the active (A) fragment of a toxin chemically conjugated to a ligand for a specific receptor that differs from the receptor for the native toxin; often designed to kill tumor cells that display a tumor-specific receptor but not normal cells that lack that receptor
Transposons
Discreet segments of DNA that are capable of moving themselves from one carrier of genetic information (genome or plasmid) to another; they are not capable of self-replication except as part of another self-replicating molecule
Conjugative transposons are capable of relocating from one cell to another
Lysogenic conversion
The process by which a bacteriophage infects a host bacterial cell but, rather than enter the lytic cycle, the viral DNA (prophage) is integrated within the bacterial genome; this viral DNA often encodes for virulence factors that alter the phenotype of the newly infected lysogen
Gram Negative Rods - Enteric
E. coli
Enterobacter
Klebsiella
Gram Negative Rods - Non Enteric
Pseudomonas aeruginosa
ETEC
Enterotoxigenic E. coli produces heat labile exotoxin, which activates Adenylate Cyclase to produce cAMP, leading to secretory diarrhea; it also produces heat stable exotoxin, which activates guanylyl cyclase in enteric epithelial cells, triggering fluid secretion
Catalase
Reduces hydrogen peroxide to oxygen and water
Staph is catalase positive; strep and entero are gram negative
Coagulase
Coagulase is an agent that inhibits spread of microbes by promoting the deposition of fibrin and helping to “wall off” and localize infection
Expression of coagluase by Staph aureus partially explains the formation of abscesses by staph infection
Prevnar
Pediatric flu vaccine comprised of carbohydrate flu antigens conjugated with capsule proteins
Which organisms produce glycocalyx?
Coagulase negative Staph
Which organism produces an anti-phagocytic capsule?
Strep pneumo
Which organism produces Dextrans?
Strep viridians; enhances adherence to damaged heart valves and teeth
Which organism produces M protein?
Strep pyogenes
Which organism produces antigenically distinct pili?
N. gonorrhea
Mechanisms of resistance - E. faecium vs. E. faecalis
E. faecium - PBP5, VRE
E. fecalis - plasma encoded NSBL
Narrow spectrum beta lactamases - Genes & Bugs
bla - Staph aureus (plasmid, all)
TEM1 - E. coli (plasmid, some)
SHV1 - Kleb. pneumo (chromosome, all)
