*also act as buffers
1)To facilitate the solubilisation or retention of iron in solution, complexing agents such as EDTA or citrate may be added to the medium.

Physical / Environmental Factors

Temperature

Most microorganisms grow well at the normal temperatures favoured by man, higher plants and animals. However, certain bacteria grow at temperatures (extreme heat or cold) at which few higher organisms can survive. Depending on their preferred temperature range, bacteria are divided into three groups: Psychrophiles (cold-loving microorganisms) found mostly in the depths of the oceans, in ice and snow and in the arctic regions, have an optimum growth temperature between 0°C and 15°C and a maximum growth temperature of not more than 20°C. Mesophiles (moderate-temperature-loving bacteria) found in water, soil and in higher organisms, are the most common type of microbe studied. Their optimum growth temperature ranges between 25°C and 40°C. The optimum temperature for many pathogenic bacteria is 37°C, thus the mesophiles constitute most of our common spoilage and disease microbes. Thermophiles (heat-loving microbes) are capable of growth at high temperatures with an optimum above 60°C. Some organisms grow at temperatures near the boiling point of water and even above 100°C when under pressure. Most thermophiles cannot grow below 45°C.

pH

Most bacteria grow best in an environment with a narrow pH range near neutrality between pH 6.5 and 7.5. Those that grow at extremes of pH are classed as acidophiles (acid-loving) or alkalinophiles (base-loving). Acidophiles grow at pH values below 4 with some bacteria still active at a pH of 1. Alkalinophilic bacteria prefer pH values of 9-10 and most cannot grow in solutions with a pH at or below neutral. Often during bacterial growth, organic acids are released into the medium, which lower its pH and so interfere with or totally inhibit further growth. Although common media ingredients such as peptones and amino acids have a small buffering effect, an external buffer is needed in most bacteriological media to neutralise the acids and maintain the correct pH. Phosphate salts are the most commonly used buffers because they buffer in the growth range of most bacteria, are non-toxic and provide a source of phosphorus, an essential nutrient element. High phosphate concentration has the disadvantage, however, that it can result in a severe nutrient limitation caused by the precipitation of insoluble metal phosphates (such as iron) in the medium.

Osmotic Pressure

Microbes contain approximately 80-90% water and if placed in a solution with a higher solute concentration will lose water which causes shrinkage of the cell (plasmolysis). However, some bacteria have adapted so well to high salt concentrations that they actually require them for growth. These bacteria are called halophiles (salt-loving) and are found in salterns or in areas such as the Dead Sea.