An 18-year-old college student seeks evaluation at an emergency department with complaints of fevers with chills, fatigue, diarrhea, and loss of appetite, which have lasted for 1 week. He says that his symptoms are progressively getting worse. He was taking over-the-counter acetaminophen, but it was ineffective. The past medical history is insignificant. His temperature is 38.8°C (101.9°F) and his blood pressure is 100/65 mm Hg. The physical examination is within normal limits, except that the patient appears ill. Eventually, a diagnosis of typhoid fever was established and he is started on appropriate antibiotics. Which of the following cellular components is most likely to be responsible for the toxic symptoms in this patient?

Lipid A - a toxic component present in the bacterial cell wall

Pili on the bacterial cell surface

Toxins secreted by the bacteria

While testing various strains of Streptococcus pneumoniae, a researcher discovers that a certain strain of this bacteria is unable to cause disease in mice when deposited in their lungs. What physiological test would most likely deviate from normal in this strain of bacteria as opposed to a typical strain?

Hemolytic reaction when grown on sheep blood agar

A group of scientists studied the effects of cytokines on effector cells, including leukocytes. They observed that interleukin-12 (IL-12) is secreted by antigen-presenting cells (APCs) in response to bacterial lipopolysaccharide. When a CD4+ T cell is exposed to this interleukin, which of the following responses will it have?

Responds to extracellular pathogens

A scientist is studying the process of thymus-dependent B cell activation in humans. He observes that, after bacterial infections, the germinal centers of secondary lymphoid organs become highly metabolically active. After subsequent reinfection with the same pathogen, the organism is able to produce immunoglobulins at a much faster pace. Which of the following processes is likely taking place in the germinal centers at the beginning of an infection?

Development of early pro-B cells

Development of immature B cells

A 3-year-old boy is brought to the physician for evaluation of a generalized, pruritic rash. The rash began during infancy and did not resolve despite initiating treatment with topical corticosteroids. Three months ago, he was treated for several asymptomatic soft tissue abscesses on his legs. He has been admitted to the hospital three times during the past two years for pneumonia. Physical examination shows a prominent forehead and a wide nasal bridge. Examination of the skin shows a diffuse eczematous rash and white plaques on the face, scalp, and shoulders. Laboratory studies show a leukocyte count of 6,000/mm3 with 25% eosinophils and a serum IgE concentration of 2,300 IU/mL (N = 0–380). Flow cytometry shows a deficiency of T helper 17 cells. The patient’s increased susceptibility to infection is most likely due to which of the following?

Impaired IL-17 secretion by Th17 cells

Impaired Ig class-switching in lymphocytes

Impaired DNA repair in lymphocytes

Impaired actin assembly in lymphocytes

Impaired chemotaxis of neutrophils

Bacterial cell structure and physiology for USMLE Step 1: High-Yield Microbiology Lessons

Bacterial Cell Wall - The Great Wall

Maintains cell shape and prevents osmotic lysis. The primary component is peptidoglycan (murein).

Gram-positive: Thick peptidoglycan layer. Contains teichoic & lipoteichoic acids, which are major surface antigens.
Gram-negative: Thin peptidoglycan layer in the periplasmic space. Has an outer membrane containing lipopolysaccharide (LPS).
Atypical: Mycobacterium contains a waxy mycolic acid layer (target of acid-fast stain). Mycoplasma has no cell wall.

Gram-positive vs. Gram-negative bacterial cell walls

⭐ The Lipid A component of LPS is the endotoxin responsible for fever and septic shock.

Membranes & Internals - The Guts Inside

Cytoplasmic Membrane: Fluid phospholipid bilayer lacking sterols (exception: Mycoplasma). Site of electron transport chain (energy generation), transport, and synthesis of cell wall precursors.
Ribosomes: 70S type (composed of 50S & 30S subunits), the site of protein synthesis.

⭐ Antibiotic targets: Aminoglycosides & Tetracyclines bind the 30S subunit; Macrolides, Clindamycin, & Linezolid bind the 50S subunit.
Nucleoid: A non-membrane-bound region containing a single, circular chromosome.
Plasmids: Small, circular, extrachromosomal DNA carrying non-essential genes like those for antibiotic resistance.

Bacterial Cell Structure Diagram

External Structures - Whips, Hairs & Slime

Flagella: For motility (chemotaxis). Composed of filament, hook, and basal body. Powered by proton motive force. The filament's flagellin protein is the H antigen.
Pili (Fimbriae): Hair-like appendages made of pilin protein.
- Fimbriae: Adherence to host cells; a key virulence factor.
- Sex Pilus: Facilitates DNA transfer (conjugation).
Glycocalyx: "Slime layer" or "Capsule" made of polysaccharide.
- Function: Adherence, biofilm formation, protection from phagocytosis.
- Capsule: Organized, firmly attached. Visualized with India ink or Quellung reaction.

⭐ Biofilms are crucial for infections on prosthetic devices (e.g., catheters) and in chronic diseases like cystic fibrosis (P. aeruginosa).

Bacterial Cell Structure Diagram

Spores & Growth - Survival & Multiplication

Bacterial Spores (Endospores): Metabolically dormant, dehydrated structures for survival in harsh conditions (not for reproduction).
- High resistance to heat, chemicals, & radiation due to a keratin-like coat and core calcium dipicolinate.
- Formed by Gram-positive rods: Bacillus, Clostridium.

Bacterial Endospore Structure

Bacterial Growth Curve: Phases in a closed system.

- **Lag**: ↑Metabolic activity, no division.
- **Log**: Rapid binary fission.
- **Stationary**: Growth rate = death rate.
- **Death**: Logarithmic decline.

⭐ Spore resistance is the benchmark for sterilization. Geobacillus stearothermophilus spores are used to validate autoclave cycles.

High‑Yield Points - ⚡ Biggest Takeaways

Gram-positive bacteria possess a thick peptidoglycan layer, staining purple; Gram-negatives have a thin layer and an outer membrane containing LPS.

The Lipid A component of LPS acts as an endotoxin, triggering fever and septic shock.

Peptidoglycan synthesis is the primary target for β-lactam antibiotics like penicillins.

Spores (e.g., Bacillus, Clostridium) are dormant, highly resistant structures that survive harsh conditions.

Capsules are key virulence factors that inhibit phagocytosis.

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