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USMLE Step 1 > Microbiology > Flashcards

Microbiology Flashcards

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USMLE® Step 1 flashcards
1
Q

Poor gram staining

A

These Rascals May Microscopically Lack Color
Treponema (darkfield)
Rickettsia
Mycobacteria
Mycoplasma (no cell wall)
Legionella (intracellular - silver stain)
Chlamydia (intracellular, lacks muramic acid)

Remember: GS detects peptidoglycan in cell wall

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2
Q

Wright-Giemsa stain

A

Borrelia
Chlamydia
Plasmodium
Trypanosomes (T. brucei - sleeping sickness, T. cruzi - Chagas)

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3
Q

PAS (periodic acid-Schiff)

A

Stains glycogen, mucopolysaccharides

  • Tropheryma whippelii (PAS-pos foamy macrophages)
  • GSDs
  • apha1 anti-trypsin
  • adenocardinomas (mucin secreting)
  • mycosis fungiodes and Sezary sydrome (cutaneous T cell lymphoma, incr mucopolysaccharides in affected T cells)
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4
Q

Ziehl-Neelson (carbolfuchsin, methylene blue counter-stain)

A

Acid-fast organisms (lots of mycolic fatty acid, stains red, resist decoloration with acid)

  • Mycobacterium: TB, leprae, kansasii, avium-intracelliulare
  • Nocardia: contains mycolic acids, too! weakly acid-fast
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5
Q

India ink

A

Cryptococcus neoformans

- mucicarmine stains thick polysaccharide capsule red

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6
Q

Silver stain

A
  • Fungi (e.g. Pneumocystis: dark disk-shaped yeast)
  • Legionella: facultative intracellular GNR, charcoal yeast extract
  • Bartonella: Cat scratch disease
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7
Q

Chocolate agar with V and X

A

H. flu

V = NAD+, X = hematin

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8
Q

Thayer-Martin (VPN/VCN) media

A

Neisseria gonorrhoeae (and meningitidis, but don’t really need to abx becuase usually growing from sterile fluid)

Chocolate agar with abx: V = vancomycin (inh GPs), P = polymyxin/C = colistin (inh other GNs), N = nystatin (inh fungi)

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9
Q

Chocolate agar, plain

A

N. meningitidis

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10
Q

Tellurite plate, Loffler’s media

A

Corynebacterium diphtheriae

  • Non-spore forming gram-pos aerobic rod with red/blue granules
  • Growth like “chinese letters”
  • ABCDEFG: ADP ribsyl, beta-prophage, Coryne, Dip, EF-2, Elek test for toxin, Granules
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11
Q

Lowenstein-Jensen agar

A

Mycobacterium tuberculosis

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12
Q

Eaton’s agar

A

Mycoplasma pneumoniae

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13
Q

Pink colonies on MacConkey’s agar

A

Lactose-fermenting organisms

  • Citrobacter (slow)
  • Klebsiella (fast)
  • E. coli (fast)
  • Enterobacter (fast)
  • Serratia (slow)
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14
Q

Green colonies on eosin-methylene blue agar

A

E. coli

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15
Q

Charcoal yeast agar buffered with cysteine iron

A

Legionella

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16
Q

Sabouraud’s agar

A

Fungi (e.g. dermatophytes)

17
Q

Obligate aerobes

A

Nagging Pests Must Breathe

  • Nocardia (branching partially acid fast GP)
  • Pseudomonas (GNR, lactose-, oxidase+)
  • Myobacterium tuberculosis
  • Bacillus (GPR)
18
Q

Obligate anaerobes

A

Can’t Breathe Air

  • Clostridium (GPR, spore-forming)
  • Bacteriodes (GN non-spore forming)
  • Actinomyces (branching, non-acid fast GP)
19
Q

Bordet-Gengou agar

A

Potato agar

- Bordetella pertussis (GN “cocciod” rod)

20
Q

Obligate intracellular

A

Stay inside bc it’s Really Cold. Can’t make own ATP

  • Rickettsia
  • Chlamydia
21
Q

Falcultative intracellular

A

Some Nasty Bugs May Live FacultativeLY

  • Salmonella
  • Neisseria
  • Brucella
  • Mycobacterium
  • Listeria
  • Francisella
  • Legionella
  • Yersinia pestis
22
Q

Encapsulated bacteria

A

SHiN SKiS

  • Strep pneumo
  • H. flu
  • Niesseria meningitidis
  • Salmonella
  • Klebsiella
  • Strep agalactiae (GBS)

Caution:

  • Asplenic
  • B cell immune deficiency
23
Q

Catalase positive

A

SSPACEL for your “cats”

  • S. aureus
  • Serratia (+lactose, slow)
  • Pseudomonas (-lactose, +oxidase)
  • Aspergillus
  • Candida
  • E. coli
  • Listeria
24
Q

Urease positive

A

PUNCH-KSS

  • Proteus
  • Ureaplasma
  • Nocardia
  • Cryptococcus
  • H. pylori
  • Klebsiella
  • Staph sap
  • Staph epi
25
Q

Pigment producers

  1. yellow “sulfur” granules
  2. yellow/honey-colored pigment
  3. blue-green pigment
  4. reg pigment
A
  1. Actinomyces israelii
  2. S. aureus
  3. Pseudomonas aeruginosa
  4. Serratia marcescens
26
Q

Select virulence factors:

  1. Protein A
  2. IgA protease
  3. M protein
A
  1. S. aureus
  2. SHiN (S. pneumo, H. flu, Neisseria)
  3. GAS/S. agalactiae
27
Q

Exotoxins that inhibit protein synthesis

A
  1. Diptheria toxin (C. diptheriae): inhibits protein synthesis; A subunit ADP ribosylates EF-2 inhibiting mRNA translation; B subunit binds to heart and neural tissue
  2. Exotoxin A (Pseudomonas): similar to (1) but targets liver, ADP ribosylates and inhibits EF-2
  3. Shiga toxin (Shigella): inactivates 60S ribosome, cleaving rRNA; leads to dysentery, cytokine release, HUS
  4. Shiga-like toxin (EHEC–O157:H7, EIEC): same as Shigella, inactivates 60S ribosome; causes HUS but does NOT invade
28
Q

Exotoxins that increase fluid secretion

A
  1. Heat-stable toxin (Yersinia): incr cGMP, decr NaCl and H20 reabs; Yersinia invades and causes watery diarrhea
  2. Heat-stable toxin (ETEC): incr cGMP, decr NaCl and H20 reabs
  3. Heat-labile toxin (ETEC): similary to choleragen; incr cAMP, incr Cl- secretion and H20 efflux
  4. Cholera toxin (Vibrio cholerae): incr cAMP via permanent Gs activation, incr NaCl secretion; “rice water diarrhea,” death from dehydration!
  5. B. cereus toxin: also similar to choleragen
  6. Edema factor (Bacillus anthracis): mimics adenylate cyclas, incr cAMP, which impairs neutrophil/macrophage phagocytic function and disrupts water homeostasis.
29
Q

Inhibit phagocytic ability

A 
Pertussis toxin (Bordetella pertussis)
- A subunit activates membrane G protein to activate adenylate cyclase, incr cAMP
30
Q

Bordetella pertussis toxins

A
  1. Pertussis toxin: B subunit Binds, A subunit incr cAMP and inhibits phagocytosis
  2. Extracytoplasmic adenylate cyclase: similar to anthrax EF, impairs phagocyte chemotaxis and phagocytosis
  3. Filmentous hemagglutinin: allows binding to ciliated epithelial cells
  4. Tracheal cytotoxin: damages respiratory epithelial cells
31
Q

Neurotoxins

A
  1. Tetanus toxin (C. tetani): cleaves SNARE protein, preventing inhibitory NT (GABA, Gly) release
  2. Botulinum toxin (C. botulinum): cleaves SNARE, inhibits stimulatory ACh release from NMJ leading to flaccid paralysis
32
Q

Lyse cell membranes

A
  1. Alpha toxin/lecithinase (Clostridum perfringens): hydrolyses lecithin in cell membranes causing cell death –> tissue destruction and gas gangrene
  2. Streptolysin O (S. pyogenes): Lyses RBCs [think beta hemolysis!]
33
Q

Pyrogenic toxins/superantigens causing shock

A
  1. Exotoxin A (Strep pyogenes/GAS)
  2. Toxic shock syndrome toxin-1 (S. aureus)

Both activate endogenous mediators of shock. Bring MHC II and TCR closer together to cause overwhelming release of IFN-gamma and IL-2. IFN-gamma from Th1 cells activate macrophages to release TNF-alpha and IL-1

34
Q

Functions of S. pyogenes toxins

  1. Hemolysins/streptolysin O and S
  2. Streptokinase
  3. DNAases
  4. Hyaluronidase
  5. NADase
A
  1. Lyses RBCs
  2. Activates plasminogen to lyse fibrin clots
  3. Hydrolyzes DNA
  4. Breaks down proteoglycans
  5. Hydrolyzes NAD
35
Q

Most common causes of aseptic meningitis

A

Coxsackie
Echovirus
Mumps virus

35
Q

Functions of S. aureus toxins

  1. Lipases
  2. PCNases
  3. Staphylokinase
  4. Leukocidin
  5. Exfoliatin
  6. Complement-binding factors
A
  1. Hydrolyzes lipids
  2. Destroys PCNs
  3. Activates plasminogen to lyse fibrin clots
  4. Lyses WBCs
  5. Epithelial cell lysis
  6. Cripples host complement defense

USMLE Step 1 (13 decks)

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