Microbial Cell Structure
Domain 2 tests whether you can relate each microbial structure to function, staining behaviour and chemotherapy. Questions commonly compare prokaryotic and eukaryotic cells and ask which wall component explains Gram reaction or endotoxin.
Core rules and concepts
- Prokaryotes have a nucleoid without a nuclear membrane, 70S ribosomes, and usually a peptidoglycan wall. Gram-positive walls have thick peptidoglycan and teichoic acids; Gram-negative walls have thin peptidoglycan plus an outer membrane containing lipopolysaccharide (LPS).
- The capsule is a polysaccharide or polypeptide layer that resists phagocytosis; flagella provide motility; pili mediate attachment and, in some cases, conjugation; endospores are dormant survival structures, not reproductive bodies.
- Mycoplasmas lack a cell wall and are pleomorphic; they are not killed by penicillin. Eukaryotic microbes have 80S ribosomes and membrane-bound organelles such as mitochondria and endoplasmic reticulum.
- LPS is an endotoxin: its lipid A portion triggers fever and shock when released from dying Gram-negative cells.
Worked example
A rod-shaped isolate stains purple after crystal violet, iodine, alcohol decolorization and safranin. Reasoning: the thick peptidoglycan layer retained the crystal violet-iodine complex despite alcohol, so the safranin counterstain could not colour it pink. The organism is Gram-positive. If it had been Gram-negative, the thin wall would lose the complex and appear pink/red.
Mark-losing traps
- Do not say all bacteria have capsules, flagella or endospores; these are species-specific.
- 70S ribosomes are prokaryotic; 80S are eukaryotic. Antibiotics such as streptomycin target 70S ribosomes, which is why selectivity matters.
- Pili are not motility organelles; flagella are.
- Endospores are formed by Bacillus and Clostridium, not by all Gram-positive bacteria.