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USMLE STEP 1 Microbiology A Exam questions with answers
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USMLE STEP 1 Microbiology A Exam
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USMLE STEP 1 MicrobiologyWhat is a special cellular component of fungal membrane targeted in anti-fungal
medication? - answerErgosterol
A major sterol
What is the special component of bacterial cell wall? - answerPeptidoglycan
Where are the normal flora found? Contributions to health? Causes of infection? -
answerLocation:
- found on body surfaces contiguous with outside environment (GI, etc.)
- Semipermanent (dependent on life-changes)
Contributions to health:
- Protective functions by maintaining a certain pH other microorganisms cannot
proliferate
- Nutritional: synthesizing vitamin K + B
Causes of infection:
- "Getting lost" (E. coli to urethra causing UTI)
- Overgrowth due to immunocompromised state
Are there normal flora in the blood or organs? - answerNo
Sterile environment
What is a medically important normal flora found in the nose? - answerStaphylococcus
aureus
What are the pathogenicity mechanisms? (8) - answer1. Colonization (adherence)
2. Avoiding immediate destruction by host defense sys.
3. "Hunting + Gathering" needed nutrients
4. Antigenic variation
5. Ability to survive intracellularly
,6. Type III secretion systems
7. Inflammation of Immune-mediated damage
8. Physical damage
What are the types of colonization? - answer1. Adherence to cell surface
- *Pili*/fimbriae: primary mechanism in most G- bacteria (E. coli)
- Teichoic acids: G+ bacteria (acts as an antigenic determinant)
- Adhesins
- IgA proteases
2. Partial adherence
- biofilms (S. epidermidis, S. mutans)
(Biofilms are densely packed microorganisms forming a protective 'film' by doing so;
"cops standing outside of general admission; cannot get to people within general
admission area doing illegal things")
What are the types of mechanism used in avoiding immediate destruction by host
defense system? - answer1. Anti-phagocytic surface components (inhibit phagocytosis):
- *Capsules*/slim layers: "bar of soap slippery"
kj: "Some Killers Have Pretty Nice Capsules Bro"
S: Streptococcus pneumoniae
K: Klebsiella penumoniae
H: Haemophilus influenzae type B
P: Pseudomonas aeruginosa
N: Neisseria meningitidis
C: Cryptococcus neoformans
B: Bordetella pertussis
- Streptococcus pyogenes: M protein
- Neisseria gonorrhoeae: Pili
- Staphylococcus aureus: A protein (binds IgG-Fc portion)
"MAP"
2. IgA proteases
3. Siderophores: steal (chelate) iron
What are antigenic variation mechanisms and microorganisms? - answer- Changing
surface antigens to avoid immune destruction
- N. gonorrhoeae: pili + outer membrane proteins
- Trypanosoma brucei rhodesiense + T. b. gambiense: phase variation
- Enterobacteria: capsular and flagellar antigens may or may not be expressed
,What are bacteria with abilities to survive intracellularly (obligate/facultative)? -
answerkj: "Listen Sally Yer Friend Bruce Must Leave" (Facultative Intracellular bugs)
List: Listeria
Sal: Salmonella
Yer: Yersinia
Fr: Francisella
Bruce: Brucella
M: Mycobacterium tuberculosis
Le: Legionella
Obligate intracellular: "stay inside cells when its Really Chilly and Cold" (Can't make
own ATP)
R: Rickettsia
C: Chlamydia
C:Coxiella
1. Evading intracellular killing by professional phagocytic cells: allows intracellular
growth
- M. tuberculosis: inhibiting phagosome-lysosome fusion (slime layer)
- Listeria: escapes phagosome into the cytoplasm before phagosome-lysosome fusion
(rocket-shooting cell to cell)
2. Invasins: ability to invade non-phagocytic cells
- Yersinia pseudotuberculosis (causing diarrhea)
What happens when intracellular organisms escape macrophage phagocytosis? -
answerMacrophages "block" in the microorganism forming *granulomas*
What is the type III secretion system? - answerAbility found in many pathogens
Capable of secreting toxins into host cell (macrophages) without entering it; tunnel
between bacteria to host cell
What are immune-mediated damage mechanisms? - answer- Cross-reaction of
bacteria-induced antibodies with tissue antigens (Rheumatic fever; type II
hypersensitivity)
- Delayed hypersensitivity and the granulomatous response (type IV hypersensitivity)
- Immune complexes (type III; poststrep acute glomerulonephritis)
- Peptidoglycan-*teichoic acid of G+ cells* (chemotactic neutrophils; structural toxin
released at cell death)
Infection: Physical damage mechanisms? - answerSwelling from infection
Large size of organism may cause blockage
, Ulcerations
What is LPS? G- or G+? - answerLipopolysaccharide = Endotoxin
- *Gram NEGATIVE* (part of outer membrane)
- Toxic portion is *lipid A* (generally released at cell death; *exception*: N. meningitidis;
over produces outer membrane fragments)
- LPS is heat stable; cannot be converted to toxoid
What is the endotoxin mechanism? - answer- LPS activates macrophages (release
TNF-⍺, IL-1, IL-6)
- Macrophages activation and products lead to tissue damage
- Damage to endothelium from bradykinin-induced vasodilation leads to shock
- Coagulation (DIC) is mediated through the activation of Hageman factor
What is the exotoxin? Mechanisms of action? - answerExotoxins: 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? - answerEnterotoxins
Neurotoxins
Cytotoxins
What are some bacteria with cytolysin-function? - answerC. 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
Dip: Diphtheria toxin
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