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First Aid USMLE STEP 1 Exam questions with solutions
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First Aid USMLE STEP 1Cell 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? -
answerComposition: Glycoprotein
Function: Mediate adherence of bacteria to cell surface, sex plus during conjugation.
*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? - answerGram Positive Only
Function: Resistant to dehydration, heat, and chemicals.
Composition: Keratin-like coat; dipicolinic acid; peptidoglycan.
*Capsule* will be composed of what on bacteria? What is the benefit of a capsule? -
answerFunction: Protects against phagocytosis
Composition: Organized, discrete polysaccharide layer (except Bacillus anthracis, which
contains d-glutamate)
Glycocalyx - answerFunction: Mediates adherance to surfaces, especially foreign
surfaces (e.g., indwelling catheters)
Composition:Loose network of polysaccharides
Periplasm - answerSpace between the cytoplasmic membrane and outer membrane in
gram-negative bacteria
(peptidoglycan in the middle)
Gram Neg only
Accumulates components exiting gram neg cells, including hydrolytic enzymes (β-
lactamases)
Outer membrane (gram negatives) - answerGram Neg 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 Pos+ 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-positive spherical (coccus) bacteria - answer-Staphylococcus (clusters)
-Streptococcus (chains, pairs)
Gram-positive rod (bacillus) bacteria - answer-Bacillus
-Clostridium
-Corynebacterium
-Gardnerella (gram variable)
-Lactobacillus
-Listeria
-Mycobacterium (acid fast)
-Propionibacterium
Gram-positive branching filamentous bacteria - answer-Actinomyces
,-Nocardia (weakly acid-fast)
Gram-negative spherical (coccus) bacteria - answer-Moraxella catarrhalis
-Neisseria
Gram-negative rod (bacillus) bacteria 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-negative rod (bacillus) bacteria Respiratory and Zoonotic - answerRespiratory:
1. Bordetella
2. Haemophilus (pleomorphic)
3. Legionella (silver stain)
Zoonotic:
1. Bartonella
2. Brucella
3. Francisella
4. Pasteurella
Pleomorphic gram-negative bacteria - answer-Chlamydiae (Giemsa)
-Rickettsiae (Giemsa)
Spiral gram-negative bacteria - answerSpirochetes:
-Borrelia (Giemsa)
-Leptospira
-Treponema
No cell wall bacteria - answerMycoplasma, Ureaplasma (contains sterols, which do not
gram stain)
These bugs do not Gram stain well: - answerToo thin to be visualized.
1. Treponema
2. Leptosipra
, Cell wall has high lipid content; mycolic acids in cell wall
detected by carbolfuchsin in acid- fast stain).
1. Mycobacteria
No Cell Wall
1. Mycoplasma
2. Ureaplasma
Primarily Intracellular
1. Legionella pneumophila (silver stain).
2. Rickettsia
3. Chlamydia ( lacks classic 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
Giemsa staining - answerChlamydia, Borrelia, Rickettsia, Trypanosomes, Plasmodium
PAS (periodic acid-Schiff) - answerStains glycogen, mucopolysaccharides
Used to diagnose Whipple disease (Tropheryma whipplei).
Ziehl-Neelsen (carbol fuchsin) - answerAcid-fast bacteria
1. Nocardia
2. Mycobacteria - stains mycelia acid in cell wall
Protozoa
1. Cryptosporidium oocysts
-Alternative is auramine-rhodamine stain for screening (inexpensive, more sensitive but
less specific).
India ink - answerCryptococcus neoformans (mucicarmine can also be used to stain
thick polysaccharide capsule red).
Silver stain - answerFungi (e.g., Coccidioides, Pneumocystis jirovecii)
Legionella
Helicobacter pylori
H. influenzae special culture requirements - answerChocolate agar: Factors V (NAD+)
and X (hematin)
N. gonorrhoeae, N. meningitidis special culture requirements - answerThayer-Martin
agar:
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