Microbial Nutrition, Growth and Culture Techniques
Domain 4 tests nutritional categories, the bacterial growth curve, generation-time calculations and the choice of culture media. Expect questions that ask why exponential phase is used for susceptibility testing and how selective media work.
Core rules and concepts
- Nutrition is described by carbon source (autotroph/heterotroph), energy source (phototroph/chemotroph) and electron source (lithotroph/organotroph). Macronutrients include C, N, P, S, K, Mg; micronutrients include Fe, Zn, Mn.
- Growth curve phases: lag (adaptation), log/exponential (balanced rapid division), stationary (nutrient depletion and waste accumulation), decline/death.
- Generation time is the time for one doubling. In exponential phase, N = N0 × 2^n, where n is the number of generations.
- Media: defined (known composition), complex (peptone, yeast extract), selective (inhibits unwanted microbes), differential (distinguishes colonies), enrichment (favours a target group). Streak, pour and spread plates are used for isolation and counting.
Worked example
A culture starts with 1,000 cells and reaches 8,000 cells in 90 minutes. Reasoning: 8,000/1,000 = 8 = 2^3, so n = 3 generations. Generation time = 90 min ÷ 3 = 30 minutes. If the same culture were in stationary phase, this calculation would be invalid because growth and death are balanced.
Mark-losing traps
- Do not use the lag phase for generation-time calculations.
- Stationary phase is not simply "no growth"; it is a dynamic balance, and secondary metabolites often form there.
- Selective and differential are not synonyms: MacConkey agar is both, but a medium can be one without the other.
- Obligate anaerobes need reducing agents and anaerobic jars; they will not grow on an ordinary bench plate.