A nutrient medium is a substance that provides the essential nutrients for microbial growth, in either a liquid form (broth) or a solid form (agar).

Aseptic techniques must be used when culturing microorganisms to prevent contamination with unwanted microorganisms.

This ensures that the nutrient medium remains sterile until use and that high yields of the desired product are made.

1. Sterilise all equipment before use, e.g. by holding a wire inoculating loop in a Bunsen flame.
2. Dip the sterilised wire inoculating loop into a starter culture.
3. Transfer the microbes to a Petri dish containing a sterile nutrient medium.
4. Close the plates and lightly tape them so they are not completely sealed.
5. Label the plates, and incubate upside down under the required conditions.
6. Repeat steps one to five for a control agar dish with no bacteria.
7. Assess microbial growth by observing colony formation on the agar.

1. Lag phase - cells show slow initial growth as they adapt and produce essential enzymes.
2. Log phase (exponential phase) - rapid doubling of cell numbers occurs under ideal conditions.
3. Stationary phase - growth rate plateaus as nutrients diminish and waste accumulates, and cell growth is equal to cell death.
4. Death phase - cell death rate exceeds cell growth rate due to resource limitation and build-up of toxins.

1. Nutrients - as the population increases, the nutrient demand exceeds the nutrient supply
2. Temperature - too low and the bacteria cannot grow, too high and the bacterial enzymes denature
3. pH - carbon dioxide may reduce the pH, which can inhibit effective enzyme activity
4. Oxygen - as the population increases, the oxygen demand exceeds the oxygen supply
5. Contamination and waste - unwanted microbial contamination or build up of toxic waste may kill the culture

Primary metabolites are substances that are produced in processes that are essential for normal microbial functioning, like ethanol from anaerobic respiration in yeast.

Secondary metabolites are substances produced in non-essential processes, like antibiotics or plant defence chemicals.

Batch fermentation is a process where microorganisms grow in a fixed volume until nutrients deplete and waste accumulates.

Continuous culture is a method where nutrients are added and culture broth is removed continuously to maintain growth.

A mixing mechanism is important in bioreactors to ensure even distribution of food, oxygen, and temperature throughout the culture.