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First Aid USMLE STEP 1: bacteria Questions with
Detailed Verified Answers (100% Correct Answers)
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*Flagellum* on bacteria is composed of what? What is the function?
Ans: Composition: Proteins
Function: motility
*Pilus/Fimbria* on bacteria is composed of what? What is the function?
Ans: Composition: 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?
Ans: Gram 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?
Ans: Function: Protects against phagocytosis
Composition: Organized, discrete polysaccharide layer (except Bacillus
anthracis, which contains d-glutamate)
Glycocalyx
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Exampromax – Stuvia US 2025/2026
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Ans: Function: Mediates adherance to surfaces, especially foreign surfaces
(e.g., indwelling catheters)
Composition:Loose network of polysaccharides
Periplasm
Ans: Space 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)
Ans: Gram 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
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Exampromax – Stuvia US 2025/2026
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Exampromax – Stuvia US
Ans: Composition: 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
Ans: Composition: 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
Ans: -Contains a variety of genes for antibiotic resistance, enzymes, and
toxins
-Composed of DNA
Gram-positive spherical (coccus) bacteria
Ans: -Staphylococcus (clusters)
-Streptococcus (chains, pairs)
Gram-positive rod (bacillus) bacteria
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Ans: -Bacillus
-Clostridium
-Corynebacterium
-Gardnerella (gram variable)
-Lactobacillus
-Listeria
-Mycobacterium (acid fast)
-Propionibacterium
Gram-positive branching filamentous bacteria
Ans: -Actinomyces
-Nocardia (weakly acid-fast)
Gram-negative spherical (coccus) bacteria
Ans: -Moraxella catarrhalis
–Neisseria
Gram-negative rod (bacillus) bacteria Enterics
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Ans: Enterics:
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
Ans: Respiratory:
1. Bordetella
2. Haemophilus (pleomorphic)
3. Legionella (silver stain)
Zoonotic:
1. Bartonella
2. Brucella
3. Francisella
4. Pasteurella
Pleomorphic gram-negative bacteria