USMLE STEP 1 Microbiology
LATEST EXAM QUESTIONS ANDLATEST EXAM QUESTIONS AND
REVIEWED ANSWERS
What is the exotoxin? Mechanisms of action? - ANSWER Exotoxins:
protein toxins secreted by G+/G- - can be produced into toxoids - A-B ('two') component protein toxins
- A = active (...
USMLE STEP 1 Microbiology
LATEST EXAM QUESTIONS AND
REVIEWED ANSWERS
What is the exotoxin? Mechanisms of action? - ANSWER ✔ Exotoxins:
protein toxins secreted by G+/G-
- can be produced into toxoids
- A-B ('two') component protein toxins
- A = active (toxic) component; ex: ADP-ribosyl transferase
- B = binds; binds to specific cell receptors to facilitate internalization
- Cytolysins: lyse cells from outside by damaging membrane
- C. perfringens alpha toxin is a lecithinase
- S. aureus alpha toxins insert itself to form pores in the membrane
What are some exotoxins subclasses? - ANSWER ✔ Enterotoxins
Neurotoxins
Cytotoxins
What are some bacteria with cytolysin-function? - ANSWER ✔ C.
perfringens: alpha toxin is a lecithinase; damages cell membranes;
myonecrosis
S. aureus: alpha toxins form pores in membrane, becomes leaky
What are some exotoxins that inhibit protein synthesis? - ANSWER ✔ "A
VERy SHITty Dip"
A: exotoxin A
VER: Verotoxin
SHIT: Shiga-Toxin
What are some superantigens? - ANSWER ✔ 1. TSST-1 (Toxic shock
syndrome toxin-1; S. aureus+; increased susceptibility to LPS, rash, shock,
capillary leakage)
2. Exotoxin A (erythrogenic/pyrogenic toxin; S. pyogenes; similar to TSST-
1)
What are some exotoxic cAMP inducers? - ANSWER ✔ kj: "cAMP"
c: Cholera
A: Anthrax
M: Montezuma's revenge (ETEC)
P: pertussis
These exotoxins induce cAMP increase resulting in the stimulation of 2nd
messenger cascade resulting in activation of the cellular function
,1. LT (heat labile toxin; E. coli ETEC; stimulates *adenylate cyclase* by
ADP ribosylation of GTP binding protein, resulting in ↑secretion of fluid and
electrolytes from intestinal epithelium; results in *watery diarrhea*)
2. Cholera toxin (vibrio cholera; similar mechanism to E. coli LT; watery
diarrhea)
What type of normal flora is found on cutaneous surfaces? Urethra? Outer
ear? - ANSWER ✔ Staphylococcus epidermidis
What is bacteremia? - ANSWER ✔ Bacteria in bloodstream without overt
clinical signs
What is septicemia? - ANSWER ✔ Bacteria in bloodstream (multiplying)
with clinical symptoms
What is the important mechanism in which bacteria can attach to
catheters, shunts and prosthetic devices coated with Teflon? Enamel of
teeth? - ANSWER ✔ Biofilm production
, Changes surface tension of the liquid around them and thereby "glue"
themselves to the material
Which group of microorganisms posses a complex carbohydrate cell wall?
- ANSWER ✔ Chitin, glucan or mannan
*Fungus*
Gram positive and negative stain colors? - ANSWER ✔ G+: Blue
(Positively BLUE over you!)
G-: Red (Negative RED commies!)
Endotoxin vs. Exotoxin - ANSWER ✔ Endotoxins are not protein excreted
but part of cellular membrane that sheds off
What are the presentations of toxic shock syndrome? - ANSWER ✔ Fever
Rash
Desquamation (flaking)
Diarrhea
Hypotension (shock)
What is the toxic portion of LPS? - ANSWER ✔ Lipid A
What are the 2 G+ rods that produce spores? - ANSWER ✔ Spores are
spheres that protect a dormant bacterium form the harsh environment
Bacillus
Clostridium
What are the 4 G+ rods? - ANSWER ✔ Corynebacterium
Listeria
No spores formation
What is the only group of G- cocci? - ANSWER ✔ Diplococcus
Neisseria
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