Morphology, Physiology and Growth of Microorganisms
Introduction
This domain links bacterial structure to staining, physiology, and growth kinetics. The exam tests Gram reaction, cell wall differences, growth curve phases, generation time, and the effect of oxygen and temperature.
Core rules
- Gram-positive bacteria have thick peptidoglycan and teichoic acids; Gram-negative bacteria have thin peptidoglycan and an outer membrane containing lipopolysaccharide.
- Common shapes include cocci, bacilli, vibrios, spirilla, and spirochaetes; arrangements include chains, clusters, and pairs.
- Endospores are formed by Bacillus and Clostridium; capsules resist phagocytosis; flagella provide motility.
- The growth curve has lag, exponential, stationary, and death phases. In exponential phase, N = N0 × 2^n, where n is the number of generations.
- Generation time = time interval / number of generations. Temperature, pH, water activity, nutrients, and oxygen affect growth.
Worked example
E. coli has a generation time of 20 minutes. Starting with 10^3 cells, 2 hours contains 120/20 = 6 generations. Final cell number = 10^3 × 2^6 = 10^3 × 64 = 6.4 × 10^4 cells. This is a prediction for exponential growth; nutrient depletion or waste accumulation may stop it earlier.
Traps
A colony count measures viable cells, not total cells. Optical density measures total biomass and cannot distinguish live from dead cells. Stationary phase is not the same as death phase. In Gram staining, over-decolourisation can make Gram-positive cells appear pink.