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USMLE Step 1 Micro Exam questions and answers
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USMLE Step 1 MicroCell Wall Overall Picture - Compare *Gram (+) to Gram (-)* - answer
*Flagellum* on bacteria is composed of what? What is the function? -
answerComposition: Proteins
Function: motility
*Pilus/Fimbria* on bacteria is composed of what? What is the function? -
answerFunction: Mediate *adherence to cell surfaces*, conjugation *sex pilus*.
Composition: Glycoprotein
*Spore* formation is ONLY possible is what type of bacteria? Why is the formation of
spores important/beneficial?
What is the composition of a spore? - answer*Gram⨁ only*
Function: Resist *dehydration, heat, chemicals*
Composition: Keratin-like coat; dipicolinic acid; peptidoglycan.
*Capsule* will be composed of what on bacteria? What is the benefit of a capsule? -
answerFunction: *⨂ phagocytosis*
Composition: Organized, discrete *polysaccharide* layer (except Bacillus anthracis,
which contains d-glutamate)
Glycocalyx - answerFunction: Mediates *adherance to foreign surfaces* (e.g., indwelling
catheters)
Composition: Loose network of *polysaccharides*
Periplasm - answer*Gram⊖ only*
Space between membranes
(peptidoglycan in the middle)
,Accumulates components exiting cells, including hydrolytic enzymes (β-lactamases)
Outer membrane (gram negatives) - answer*Gram⊖ only*
Function:
1. Endotoxin: Lipid A induces TNF and IL-1; O polysaccharide is antigenic
2. Most OMPs are antigenic
Porins: transport across outer membrane
Composition:
1. Outer Leaflet: endotoxin lipopolysaccharide [LPS/LOS]
2. Embedded proteins: porins and other outer membrane proteins (OMPs)
3. Inner Leaflet: phospholipids
Cell wall and Gram Stain - answerComposition: Peptidoglycan is a sugar backbone with
peptide chains cross-linked by transpeptidase
Function: Net-like structure give rigid support, protects against osmotic pressure
damage.
Gram Stain:
Thick peptidoglycan layer retains crystal violet = gram +
Thin peptidoglycan layer turns red or pink with counterstain = gram -
Cytoplasmic Membrane - answerComposition: Phospholipid bilayer sac with embedded
proteins (PBP's) and other enzymes.
Lipoteichoic acids (gram⊕ only) extend from membrane exterior.
Function: Site of *oxidative and transport enzymes*
PBPs involved in cell wall synthesis
Lipoteichoic acids induce TNF and IL-1
Plasmid - answer-Contains a variety of genes for antibiotic resistance, enzymes, and
toxins
-Composed of DNA
Gram⊕ cocci - answer-Staphylococcus (clusters)
-Streptococcus (chains, pairs)
Gram⊕ bacilli - answer-Bacillus
-Clostridium
-Corynebacterium
-Gardnerella (gram variable)
-Lactobacillus
-Listeria
,-Mycobacterium (acid fast)
-Propionibacterium
Gram⊕ branching filamentous bacteria - answer-Actinomyces
-Nocardia (weakly acid-fast)
Gram⊖ cocci - answer-Moraxella catarrhalis
-Neisseria
Gram⊖ bacilli
(Enterics) - answerEnterics:
1. Bacteroides
2. Campylobacter
3. E. coli
4. Enterobacter
5. Helicobacter
6. Klebsiella
7. Proteus
8. Pseudomonas
9. Salmonella
10. Serrate
11. Shigella
12. Vibrio
13. Yersinia
Gram⊖ bacilli
(Respiratory and Zoonotic) - answerRespiratory:
1. Bordetella
2. Haemophilus (pleomorphic)
3. Legionella (silver stain)
Zoonotic:
1. Bartonella
2. Brucella
3. Francisella
4. Pasteurella
Gram⊖ pleomorphic bacteria - answer-Chlamydiae (Giemsa)
-Rickettsiae (Giemsa)
Gram⊖ spirochetes - answer-Borrelia (Giemsa)
-Leptospira
-Treponema
Bacteria with *no cell wall* - answerMycoplasma
Ureaplasma (contains sterols, which do not gram stain)
, These bugs *do not Gram stain well*:
- no cell wall
- too thin
- mycolic acid
- intracellular - answer*No cell wall*
1. Mycoplasma
2. Ureaplasma
*Too thin to be visualized*
1. Treponema
2. Leptosipra
Cell wall has high lipid content
*Mycolic acid* in cell wall detected by acid-fast stain
1. Mycobacteria
*Primarily Intracellular*
1. Legionella pneumophila (silver stain).
2. Rickettsia
3. Chlamydia (lacks normal peptidoglycan because of low muramic acid)
4. Bartonella
5. Ehrlichia
6. Anaplasma
What is seen with dark-field microscopy and fluorescent antibody staining? -
answerTreponemes are seen with FTA-ABS (fluorescent treponemal Ab absorption)
*Giemsa stain* - answerRickettsia
Chlamydia
Trypanosomes
Plasmodium
Borrelia
H. pylori
"*Rick*y got *Chlamydia* *Try*ing to *Pl*ow a *Bor*ing *H*ot *Giesha*"
*PAS* (periodic acid-Schiff) - answer→ stains glycogen, mucopolysaccharides
Whipple disease (*Tropheryma whipplei*).
"*PAS*s the *sugar* so I can *whip* up some waffles"
*Ziehl-Neelsen* (carbol fuchsin) - answer*Acid-fast bacteria*
1. Mycobacteria
2. Nocardia
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