[MICRO-01] Bacterial Ultrastructure, Gram vs Ziehl–Neelsen Staining, Growth Kinetics & Selective Media

MICRO-01 • 7. Microbiology

Learning Objectives

Compare Gram-positive (thick 20–80 nm peptidoglycan + teichoic acids) vs Gram-negative (thin 2–7 nm peptidoglycan + outer LPS endotoxin membrane + porins) vs Acid-Fast (mycolic acid) cell walls.; Trace the 4 phases of the bacterial growth curve (Lag, Log/Exponential — maximum β-lactam susceptibility, Stationary — sporulation & exotoxin/antibiotic production, Decline).; Match selective and differential culture media (MacConkey, Mannitol Salt, Lowenstein–Jensen, TCBS, Thayer–Martin, Sabouraud Dextrose Agar) to pathogens.

Chapter Overview & Core Explanation

Bacteria are single-celled prokaryotes whose outer cell wall architecture determines both how they stain under a microscope and which antibiotics can penetrate and kill them.

Key Exam & Clinical Takeaways

• Penicillins and Cephalosporins exhibit maximum bactericidal activity during the **Log (Exponential) Phase** of bacterial growth when peptidoglycan transpeptidation is actively occurring.
• [8.48] Vancomycin is inactive against Gram-negative bacteria because its bulky glycopeptide molecule (~1,450 Da) cannot pass through Gram-negative outer membrane porins.

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