Microbial Nutrition, Growth and Culture Techniques
Domain 3 tests nutrient requirements, transport, growth mathematics and how media and plating methods select or recover microbes.
Nutrition
Macronutrients include C, H, O, N, S, P, K, Ca, Mg and Fe; micronutrients are trace metals. Growth factors include vitamins, amino acids and bases. Autotrophs fix carbon dioxide; heterotrophs use organic carbon. Phototrophs use light; chemotrophs oxidise chemicals. Auxotrophs need a specific growth factor; prototrophs do not. Transport occurs by passive diffusion, facilitated diffusion, active transport and group translocation.
Growth
Binary fission produces two daughter cells. Generation or doubling time varies with species and conditions. The batch growth curve has lag (adaptation, no net increase), log or exponential (fastest, constant rate), stationary (nutrient depletion and waste accumulation balance growth and death) and decline phases. Continuous culture in a chemostat maintains log phase. Total counts include direct microscopy and turbidity; viable counts include plate count and most probable number.
Culture
Defined media have known ingredients; complex media use digests and extracts. Selective media suppress unwanted microbes; differential media reveal a biochemical property; enrichment favours a target. The streak plate dilutes a sample to single colonies; pour and spread plates quantify. Anaerobes need reducing agents or gas packs; capnophiles need added carbon dioxide.
Worked example
E. coli has a 20-minute generation time. Starting with 1,000 cells, how many after 2 hours? Reasoning: 120 minutes ÷ 20 = 6 generations, so 1,000 × 2^6 = 64,000 cells. This assumes log phase; nutrient exhaustion would move the culture into stationary phase.
Traps
Lag phase involves synthesis but no rise in number; stationary phase is not zero growth. Generation time is not incubation time. A selective medium can also be differential, so read the question. Autoclaving destroys heat-labile vitamins and antibiotics; filter-sterilise those instead.