Growth Requirements — temperature, pH and osmotic pressure
Microbes are picky about their physical environment. Temperature decides who can grow where — from cold-loving psychrophiles to heat-loving thermophiles — and each organism has an optimum growth temperature at which it grows its best. pH matters too (most bacteria prefer 6.5–7.5; fungi like it more acidic), and osmotic pressure can make — or break — a cell.
Chemical Requirements & the Oxygen Spectrum
Besides water, cells need carbon sources (heterotrophs use organics; autotrophs fix CO₂), nitrogen and sulfur for amino acids and vitamins, phosphorus for ATP, and trace elements as enzyme cofactors. Oxygen is stranger than it looks: it can be essential, optional, tolerated — or lethal. The difference lies in a handful of protective enzymes.
Culture Media — how we grow microbes in the lab
To study a microbe you have to feed it. Media come in physical forms (agar plates, slants and deeps, or liquid broth), with recipes that are either chemically defined (exact composition known — needed by picky, fastidious organisms) or complex (from extracts; varies batch to batch). Some microbes need special tricks: oxygen-free anaerobic systems or extra CO₂.
Selective, Differential & Enrichment Media
Special-purpose media turn a plate into a diagnostic tool. Selective media suppress unwanted bacteria and encourage the desired one; differential media make it easy to distinguish colonies; combined media do both — and enrichment media boost rare microbes until they’re detectable.
Culturing & Preservation — isolating and keeping microbes
A pure culture starts with a colony — which arises from a single cell or groups of cells. The workhorse is the streak plate method, diluting the sample by streaking and sterilizing the loop between series until single colonies appear. Once you have the isolate, how do you keep it? Deep-freezing and lyophilization (freeze drying) both preserve cultures for years.
Binary Fission & Generation Time
Bacteria grow by binary fission: the cell elongates, its DNA is replicated, the wall and membrane divide, and two daughter cells separate. Each division takes one generation time — and because every generation doubles the population, bacterial growth is exponential.
The Growth Curve — lag, log, stationary, death
When you track a population over time, bacterial growth follows a signature shape: a slow start (lag phase), explosive exponential growth (log phase), a plateau where births equal deaths (stationary phase), and finally a decline (death phase). Click each phase to explore why.
Measuring Growth — counting the invisible
How many bacteria are there? Plate counts count only the viable cells (as CFU — colony forming units) using serial dilutions. Filtration concentrates dilute samples, MPN estimates statically, and direct microscopic counts count everything on a grid. Indirect methods — dry weight, urine thresholds, turbidity — read the crowd without counting it.
Chapter Quiz — Microbial Growth
Thirty-nine questions covering the whole chapter: physical and chemical growth requirements, oxygen classes, culture media, selective/differential/enrichment techniques, culturing and preservation, binary fission, the growth curve, and methods of measuring growth.