First Aid USMLE Step 1 Microbiology – Complete Exam Study Guide 2026/2027 with Verified Answers | Newest Version. A+

First Aid USMLE STEP 1
Microbiology
Cell Wall Overall Picture - Compare *Gram (+) to Gram (-)* ------- ✔ CORRECT ANSWER ✓✓



*Flagellum* on bacteria is composed of what? What is the function? ------- ✔ CORRECT
ANSWER ✓✓Composition: Proteins

Function: motility



*Pilus/Fimbria* on bacteria is composed of what? What is the function? ------- ✔ CORRECT
ANSWER ✓✓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? ------- ✔ CORRECT ANSWER ✓✓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? ------- ✔
CORRECT ANSWER ✓✓Function: Protects against phagocytosis

,Composition: Organized, discrete polysaccharide layer (except Bacillus anthracis, which contains
d-glutamate)



Glycocalyx ------- ✔ CORRECT ANSWER ✓✓Function: Mediates adherance to surfaces, especially
foreign surfaces (e.g., indwelling catheters)



Composition:Loose network of polysaccharides



Periplasm ------- ✔ CORRECT ANSWER ✓✓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) ------- ✔ CORRECT ANSWER ✓✓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 ------- ✔ CORRECT ANSWER ✓✓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 ------- ✔ CORRECT ANSWER ✓✓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 ------- ✔ CORRECT ANSWER ✓✓-Contains a variety of genes for antibiotic resistance,
enzymes, and toxins

-Composed of DNA



Gram-positive spherical (coccus) bacteria ------- ✔ CORRECT ANSWER ✓✓-Staphylococcus
(clusters)

-Streptococcus (chains, pairs)



Gram-positive rod (bacillus) bacteria ------- ✔ CORRECT ANSWER ✓✓-Bacillus

, -Clostridium

-Corynebacterium

-Gardnerella (gram variable)

-Lactobacillus

-Listeria

-Mycobacterium (acid fast)

-Propionibacterium



Gram-positive branching filamentous bacteria ------- ✔ CORRECT ANSWER ✓✓-Actinomyces

-Nocardia (weakly acid-fast)



Gram-negative spherical (coccus) bacteria ------- ✔ CORRECT ANSWER ✓✓-Moraxella catarrhalis

-Neisseria



Gram-negative rod (bacillus) bacteria Enterics ------- ✔ CORRECT ANSWER ✓✓Enterics:

1. Bacteroides

2. Campylobacter

3. E. coli

4. Enterobacter

5. Helicobacter

6. Klebsiella

7. Proteus

8. Pseudomonas

9. Salmonella

10. Serrate

11. Shigella