Bacteria

Structure and Gram Classification

• Key structures: peptidoglycan wall, cholesterol-free membrane, nucleoid with circular DNA, 70S ribosomes, plasmids (often carrying resistance genes), flagella for chemotaxis, pili for attachment and conjugation, and an anti-phagocytic capsule.
• Gram-positive: thick peptidoglycan, stains purple. Gram-negative: thin peptidoglycan plus an outer membrane containing LPS (endotoxin), stains pink; released LPS on lysis can cause fever, septic shock, and DIC, and the outer membrane blocks many antibiotics.
• Selective toxicity targets what humans lack: peptidoglycan, the 70S ribosome, DNA gyrase, and folate synthesis.

Growth and Metabolism

• Growth curve: lag (enzyme synthesis) to log (exponential division, peak antibiotic sensitivity) to stationary (division equals death) to death phase.
• Binary fission: $N = N_0 \times 2^n, \quad n = \frac{\text{elapsed time}}{\text{generation time}}$
• Metabolic classes pair an energy source (photo- or chemo-) with a carbon source (auto- or heterotroph). Oxygen classes: obligate aerobes, obligate anaerobes (killed by $O_2$ — no catalase or superoxide dismutase), facultative anaerobes, and aerotolerant anaerobes.

Key Takeaways

• Gram stain color encodes wall architecture and endotoxin risk.
• LPS is the Gram-negative danger signal on lysis.
• Exponential growth math only holds during log phase.
• Uniquely prokaryotic structures explain antibiotic selectivity.