Nutrients and Moisture
Different micro-organisms have different nutrient requirements but all require an energy source, moisture(water), vitamins and minerals. The presence of suitable nutrients determine if micro-organisms can grow in the food source or not. eg milk is rich in nutrients so is a good environment.
Water is essential for all living things as a substance in which chemical reactions take place and materials are transported. If there is no water, micro-organisms can't grow and reproduce.
Chemicals
- pH -- Different micro-organisms have different optimum (best) pH and a range of pH they can live in. Outiside the optimum pH, rate of growth slows as enzymes are sensitive to the pH. Most bacteria prefer pH between 6-8, but lactic acid bacteria can grow in a much lower pH (Acidic places) so they can outcompete other bacteria in those places.
- Toxins -- Poisonous wastes produced by micro-organisms which can kill them in high concentrations.
- Antibiotics -- chemical that kill bacteria or slow there growth. eg penecillin made by a certain bacteria.
- Disinfectants -- chemicals that kill or break down micro-organisms. eg bleach, peroxide. These join onto the cell wall and enter the cell, disrupting the cell membrane or DNA and stop life processes such as the transport of material.
- Antiseptics -- disinfectants used in skin eg hydrogen peroxide. This joins to enzymes and destroy their structure (denaturing).
Competition
Micro-organisms can make chemicals that help them out-compete others. eg fungi produce antibiotics that kill bacteria, reducing competition for space and nutrients. Lactic acid bacteria also produce chemicals that kill other bateria.
Host species
Viruses and pathogenic bacteria/fungi require a host cell to grow. Few host cells mean growth and reproduction of pathogen is reduced.