MICROBIOLOGY USMLE PART 1 Flashcards
What is the chemical composition and function of pepidoglycan?
Gives rigid support, protects against osmotic pressure.
Sugar backbone with peptide side chains crosslinked by transpeptidase.
Funcion & composition of cell wall/membrane of gram positives?
Major surface antigen.
Peptidoglycan for support. Lipoteichoic acid induces TNF and IL-1.
Function and composition of outer membrane of gram negatives?
Site of endotoxin (lipopolysaccharide [LPS]); major surface antigen.
Lipid A induces TNF and IL-1;
O polysaccharide is the antigen.
Function and composiion of plasma membrane?
F: Site of oxidative and transport enzymes.
C: Phospholipid bilayer.
Function and composition of Ribosomes?
F: Protein synthesis
C: 50S and 30S subunits
Function and chemical composition of Periplasm?
Space between the cytoplasmic membrane and outer membrane in gram-negative bacteria.
Contains many hydrolytic enzymes, including β-lactamases.
Function and composition of capsule?
Protects against phagocytosis.
Polysaccharide (except Bacillus anthracis, which contains d-glutamate).
Function and composition of pilus/fimbria?
Mediate adherence of bacteria to cell surface; sex pilus forms attachment between 2 bacteria during conjugation.
C: Glycoprotein.
function and composition of flagellum?
F: Motility
C: Protein
function and composition of spores?
Resistant to dehydration, heat, and chemicals.
Comp: Keratin-like coat; dipicolinic acid; peptidoglycan.
Function and composition of plasmid?
Contains a variety of genes for antibiotic resistance, enzymes, and toxins.
Comp: DNA
Function and composition of glycocalyx
Mediates adherence to surfaces, especially foreign surfaces (e.g., indwelling catheters).
Compostion: Polysaccharide.
Difference between cell walls of gram + vs. gram -
Unique to G+: Lipoeicoic acid (combinaion of lipids and teichoic acids).
hick peptidoglycan
Unique to gram negative organisms:
Endotoxin/LPS (outer membrane)
Periplasm
thin peptidoglycan
Properties of cell wall common to both Gram negative and gram positive
Flagellum
Pilus
Capsule
Peptidoglycan
Cytoplasmic membrane
Circular coccus examples
G+:
Staphylococcus Streptococcus
G-:
Neisseria
Rod (bacillus) Gram + examples
G+: Clostridium Corynebacterium Bacillus Listeria Mycobacterium (acid fast) Gardnerella (gram variable)
Branching filamentous
Actinomyces
Nocardia (weakly acid fast)
Rod (bacillus) Gram negative enterics examples
Gram Negative:
Enterics: E. coli Shigella Salmonella Yersinia Klebsiella Proteus Enterobacter Serratia Vibrio Campylobacter Helicobacter Pseudomonas Bacteroides
Rod (bacillus) gram negative respiratory examples
Respiratory: Haemophilus (pleomorphic) Legionella (silver) Bordetella
Rod (bacillus gram negative zoonotic examples
Zoonotic: Francisella Brucella Pasteurella Bartonella
Pleomorphic gram negative examples
Rickettsiae (Giemsa)
Chlamydiae (Giemsa)
Spiral gram negative examples
Spirochetes: Borrelia (Giemsa) Leptospira Treponema
How do mycoplasma have unsual cell membranes/walls
Contain sterols and have no cell wall.
does not gram stain
How do mycobacteria have unsual cell membranes/walls
Contain mycolic acid. High lipid content.
Bugs that do not gram stain well and why?
These Microbes May Lack Real Color
Treponema (too thin to be visualized).
Mycobacteria (high lipid content in cell wall detected by carbolfuchsin in acid-fast stain).
Mycoplasma (no cell wall).
Legionella pneumophila (primarily intracellular).
Rickettsia (intracellular parasite).
Chlamydia (intracellular parasite; lacks muramic acid in cell wall).
How to visualize Treponemes?
dark-field microscopy and fluorescent antibody staining.
What stain is used to visualize Legionella?
Silver stain.
Giemsa stain is used for which bugs?
Chlamydia, Borrelia, Rickettsiae, Trypanosomes, Plasmodium.
“Certain Bugs Really Try my Patience.”
PAS (periodic acid-Schiff) is used for?
Stains glycogen, mucopolysaccharides; used to diagnose Whipple disease (Tropheryma whipplei).
“PASs the sugar.”
Ziehl-Neelsen (carbol fuchsin) stain is used for which organisms?
Acid-fast organisms (Nocardia, Mycobacterium
India ink is used for?
Cryptococcus neoformans (mucicarmine can also be used to stain thick polysaccharide capsule red).
Silver stain is used for/?
Fungi (e.g., Pneumocystis), Legionella, Helicobacter pylori.
culure for H. influenzae
Chocolate agar with factors V (NAD+) and X (hematin)
Culture used for neisseria?
Thayer Martin
Thayer Martin (VPN) culture inhibits which organisms?
Thayer-Martin (or VPN) media—Vancomycin (inhibits gram-positive organisms),
Polymyxin (inhibits gram-negative organisms except Neisseria), and
Nystatin (inhibits fungi); “to connect to Neisseria, please use your VPN client”
Culture for B. pertussis?
Bordet-Gengou (potato) agar (Bordet for Bordetella)
Culture for C. diphtheriae?
Tellurite agar, Löffler medium
Culture for M. tuberculosis?
Löwenstein-Jensen agar
Culture for M. pneumoniae
Eaton agar, requires cholesterol
Culture for lactose-fermenting enterics?
Pink colonies on MacConkey agar (fermentation produces acid, turning colony pink);
E. coli is also grown on eosin–methylene blue (EMB) agar as colonies with green metallic sheen
Legionella culture?
Charcoal yeast extract agar buffered with cysteine and iron
Fungi culture?
Sabouraud agar.
“Sab’s a fun guy!”
Obligate aerobes, what are they and examples?
Use an O2-dependent system to generate ATP.
Examples include Nocardia, Pseudomonas aeruginosa, and MycoBacterium tuberculosis.
“Nasty Pests Must Breathe”
P. aeruginosa is an aerobe seen in burn wounds, complications of diabetes, nosocomial pneumonia, and pneumonias in cystic fibrosis patients.
After immune compromise or TNF-alfa inhibitor use reactivates and causes what…?
Reactivation of M. tuberculosis (e.g., after immune compromise or TNF-α inhibitor use) has a predilection for the apices of the lung, which have the highest Po2.
Obligate anaerobes lack what? and what are some examples?
Examples include Clostridium, Bacteroides, and Actinomyces.
“Can Breathe Air”
They lack catalase and/or superoxide dismutase and are thus susceptible to oxidative damage. Generally foul smelling (short-chain fatty acids), are difficult to culture, and produce gas in tissue (CO2 and H2).
Anaerobes are normal flora of where?
Anaerobes are normal flora in GI tract, pathogenic elsewhere
How well do aminoglycosides work against anerobes?
AminO2glycosides are ineffective against anaerobes because these antibiotics require O2 to enter into bacterial cell.
Obligate intracellular bugs?
Rickettsia, Chlamydia.
Can’t make own ATP.
‘Stay inside (cells) when it is Really Cold. “
Facultative intracellular bugs?
Salmonella, Neisseria, Brucella, Mycobacterium, Listeria, Francisella, Legionella, Yersinia pestis.
“Some Nasty Bugs May Live FacultativeLY.”
Encapsulated bacteria?
Examples are Streptococcus pneumoniae, Haemophilus influenzae type B, Neisseria meningitidis, Escherichia coli, Salmonella, Klebsiella pneumoniae, and group B Strep.
Their capsules serve as an antiphagocytic virulence factor. Capsule + protein conjugate serves as an antigen in vaccines.
“SHiNE SKiS.”
What role does the spleen play against encapsulated bacteria?
Are opsonized, and then cleared by spleen. Asplenics have opsonizing ability and are at risk for severe infections.
Give S. pneumoniae, H. influenzae, N. meningitidis vaccines.
Catalase-positive organisms, what happens and examples?
Catalase degrades H2O2 before it can be converted to microbicidal products by the enzyme myeloperoxidase.
People with chronic granulomatous disease (NADPH oxidase deficiency) have recurrent infections with catalase organisms
. Examples: Pseudomonas, Listeria, Aspergillus, Candida, E. coli, S. aureus, Serratia.
You
“you need PLACESS for your cats”
Bacterial virulence factors
These promote evasion of host immune response.
Protein A, IgA protease, M protein
Protein A
Binds Fc region of IgG. Prevents opsonization and phagocytosis. Expressed by S. aureus.
IgA protease
Enzyme that cleaves IgA. Secreted by S. pneumoniae, H. influenzae type B, and Neisseria (SHiN) in order to colonize respiratory mucosa.
M protein
Helps prevent phagocytosis. Expressed by group A streptococci.
Pigment-producing bacteria
Actinomyces israelii—yellow “sulfur” granules, which are composed of filaments of bacteria.
“Israel has yellow sand.”
S. aureus—yellow pigment. aureus (Latin) = gold.
Pseudomonas aeruginosa—blue-green pigment.” Aerugula is green”.
Serratia marcescens—red pigment. “think red maraschino cherries.”
Urease-positive bugs
Cryptococcus, H. pylori, Proteus, Ureaplasma, Nocardia, Klebsiella, S. epidermidis, S. saprophyticus.
“CHuck Norris hates PUNKSS”
How do encapsulated bacteria vaccines work?
Some vaccines containing polysaccharide capsule antigens are conjugated to a carrier protein, enhancing immunogenicity by promoting T-cell activation and subsequent class switching. A polysaccharide antigen alone cannot be presented to T cells.
Pneumococcal vaccine
PCV (pneumococcal conjugate vaccine, i.e., Prevnar); PPSV (pneumococcal polysaccharide vaccine with no conjugated protein, i.e., Pneumovax)
H. influenzae type B vaccine
conjugate cvaccine
Meningococcal vaccine
conjugate vaccine
Exotoxins:
Source, secretion, chemistry, genes, toxicity, antigenicity , vaccines, heat stability?
Source: Certain species of some gram-positive and gram-negative bacteria
Secreted From cell: Yes
CHemistry: Polypeptide
LocatioN oF geNes: Plasmid or bacteriophage
Toxicity: High (fatal dose on the order of 1 µg)
Antigenicity: Induces high-titer antibodies called antitoxins
VacciNes: Toxoids used as vaccines
Heat stability: Destroyed rapidly at 60°C (except staphylococcal enterotoxin)
Endotoxins
Source, chemistry, location of genes, toxicity, clinical effects, mode of action, antigenicity, vaccines
Source: Outer cell membrane of most gram-negative bacteria
NOT SECRETED FROM CELL
Chemistry: Lipopolysaccharide (structural part of bacteria; released when lysed)
locatioN oF geNes: Bacterial chromosome
toXicity: Low (fatal dose on the order of hundreds of micrograms)
cliNical eFFects: Fever, shock (hypotension), DIC
moDe oF actioN: Induces TNF, IL-1, and IL-6
aNtigeNicity: Poorly antigenic
No toxoids formed and no vaccine available
Heat stability: Stable at 100°C for 1 hr
Typical exotoxin diseases
tetanus, botulism, diphtheria
typical endotoxin
meningococcemia; espsis by gram-negative rods
Transformation
Ability to take up naked DNA (i.e., from cell lysis) from environment (also known as “competence”).
A feature of many bacteria, especially S. pneumoniae, H. influenzae type B, and Neisseria (SHiN). Any DNA can be used.
Conjugation F+ x F-
F+ plasmid contains genes required for sex pilus and conjugation.
Bacteria without this plasmid are termed F–.
Plasmid (dsDNA) is replicated and transferred through pilus from F+ cell.
No transfer of chromosomal genes.
Conjugation Hfr x F -
F+ plasmid can become incorporated into bacterial chromosomal DNA, termed high-frequency recombination (Hfr) cell.
Replication of incorporated plasmid DNA may include some flanking chromosomal DNA.
Transfer of plasmid and chromosomal genes.
Transposition
Segment of DNA (e.g., transposon) that can “jump” (excision and reintegration) from one location to another, can transfer genes from plasmid to chromosome and vice versa.
When excision occurs, may include some flanking chromosomal DNA, which can be incorporated into a plasmid and transferred to another bacterium.
Examples include antibiotic resistance genes on R plasmid.
Generalized Transduction
A “packaging” event. Lytic phage infects bacterium, leading to cleavage of bacterial DNA. Parts of bacterial chromosomal DNA may become packaged in viral capsid. Phage infects another bacterium, transferring these genes.
Specialized Transduction
An “excision” event. Lysogenic phage infects bacterium; viral DNA incorporates into bacterial chromosome. When phage DNA is excised, flanking bacterial genes may be excised with it. DNA is packaged into phage viral capsid and can infect another bacterium.
5 bacterial toxins genes encoded in a lysogenic phage
ABCDE
Genes for the following 5 bacterial toxins are encoded in a lysogenic phage (ABCDE): ShigA-like toxin Botulinum toxin (certain strains) Cholera toxin Diphtheria toxin Erythrogenic toxin of Streptococcus pyogenes
Endotoxin causes what…
An LPS found in outer membrane of gramnegative bacteria (both cocci and rods).
"ENDOTOXIN": Edema Nitric oxide DIC/Death Outer membrane TNF-α O-antigen eXtremely heat stable IL-1 Neutrophil chemotaxis
what happens when lipid A (endotoxin) activates macrophages?
IL-1 –> fever
TNF –> fever and hypotension
Nitric oxide –> hypotension
What happens when lipid A (endotoxin) activates complement?
C3a –> hypotension, edema
C5a –> neutrophil chemotaxis
what happens when lipid A endo toxin activates tissue factor?
coagulation cascade –> DIC
Corynebacterium diphtheriae toxin, mechanism, and manifestation
Diphtheria toxin (exotoxin that inhibits protein synthesis)
inactivates elongation factor 2
pharyngitis with pseudomembranes in throat and sever lymphaadenopathy
Psedomonas aeruginosa toxin, mechanism, and manifestation
Exotoxin A that inhibits protein synthesis
inactivates EF-2
causes host cell death
Shigella toxin , mechanism, and manifestation
Shiga toxin ST exotoxin that inhibits protein synthesis
inactivates 60S ribosome by removing adenine from rRNA
causes GI mucosal damage –> dysentery
enhances cytokine release, causing hemolytic uremic syndrome
Enterohemorragic Ecoli (EHEC) including 0157:H7 strain
toxin, mechanism, and manifestation
Shiga-like toxin exotoxin that inhibits protein synthesis
inactivates 60S ribosome by removing adenine from rRNA
SLT enhances cytokine releasing causing HUS
unlike Shigella, EHEC does not invade host cells
Enterotoxigenic E. coli (ETEC)
toxings, mechanisms and manifestations
exotoxins that increase fluid secretion:
Heat-labile toxin (LT)
Overactivates adenylate cyclase (increases cAMP) –> increases Cl- secretion in gut and H2O efflux
Heat-stable toxin (ST)
Overactivates guanylate cyclase (increases cGMP) –> decreases resorption of NaCl and H2O in gut
Watery diarrhea: “labile in the Air (Adenylate cyclase), stable on the Ground (Guanylate cyclase)”
Bacillus anthracis toxin, mechanism, and manifestation
Edema factor: exotoxin that increases fluid secretion
Mimics the adenylate cyclase enzyme ( increases cAMP)
Likely responsible for characteristic edematous borders of black eschar in cutaneous anthrax
Vibriio cholerae toxin, mechanism, and manifestation
exotoxin that increases fluid secretion
Cholera toxina
Overactivates adenylate cyclase ( increases cAMP) by permanently activating Gs –> increase Cl- secretion in gut and H2O efflux
Voluminous “rice-water” diarrhea
Bordetella pertussis toxin, mechanims, manifestation
Exotoxin that inhibits phagocytic ability
Pertussis toxina
Overactivates adenylate cyclase ( increases cAMP) by disabling Gi, impairing phagocytosis to permit survival of microbe
Whooping cough: child coughs on expiration and “whoops” on inspiration (toxin may not actually be a cause of cough; can cause “100-day cough” in adults)
Clostridium tetani toxin mechanism and manifestation
Tetanospasmin: exotoxin that inhibits release of neurotransmitter
Both are proteases that cleave SNARE proteins required for neurotransmitter release
Spasticity, risus sardonicus, and “lockjaw”; toxin prevents release of inhibitory (GABA and glycine) neurotransmitters from Renshaw cells in spinal cord
Clostriduim botulinum
Botulinum toxin : exotoxin that inhibits release of neurotranmitter
Both are proteases that cleave SNARE proteins required for neurotransmitter release
Flaccid paralysis, floppy baby; toxin prevents release of stimulatory (ACh) signals at neuromuscular junctions flaccid paralyss
aToxins
ADP ribosylating A-B toxin:
B (binding) component binds to host cell surface receptor, enabling endocytosis;
A (active) component attaches ADP-ribosyl to disrupt host cell proteins.
(Diphtheria, Exotoxin A, ST, SLT, LT, Cholera toxin, pertussis toxin)
Clostriduim perfringens toxin, mechanism and manifestations
Alpha toxin: exotoxin that lyses cell membranes
Phospholipase (lecithinase) that degrades tissue and cell membranes
Degradation of phospholipids myonecrosis (“gas gangrene”) and hemolysis (“double zone” of hemolysis on blood agar)
Streptococcus pyogens toxin, mechanism , and manifestation
Streptolysin O : exotoxin that lyses cell membrane
Protein that degrades cell membrane
Lyses RBCs; contributes to β-hemolysis; host antibodies against toxin (ASO) used to diagnose rheumatic fever (do not confuse with immune complexes of poststreptococcal glomerulonephritis)
Exotoxin A: superantigen causing shock
Bring MHC II and TCR in proximity to outside of antigen binding site to cause overwhelming release of IFN-γ and IL-2 shock
Toxic shock syndrome: fever, rash, shock
Staphylococcus aureus toxin, mechanism, manifestation
Toxic shock syndrome toxin (TSST-1): exotoxin / superantigen that cuases shock
Bring MHC II and TCR in proximity to outside of antigen binding site to cause overwhelming release of IFN-γ and IL-2 shock
Toxic shock syndrome: fever, rash, shock; other toxins cause scalded skin syndrome (exfoliative toxin) and food poisoning
