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Temperature
Specific bacteria will only grow within a certain range of temperatures. Pathogenic bacteria, which can infect humans, are mesophiles, growing optimally at moderate temperatures. These bacteria grow optimally at body temperature (approximately 37 degrees Celsius) but their rate of growth decreases above 63 degrees Celsius, and below 5 degrees Celsius. This is why refrigeration is a good method to reduce the growth of harmful bacteria.
pH Level
pH is a measure of the acidity or alkalinity of a substance. Human blood has an approximate pH level of 7.4. Pathogenic bacteria grow optimally in neutral pH conditions (pH of 7) and will not grow in acidic conditions with pH 4.5 or below. This makes the human body ideal for their growth conditions. Bacteria that favor neutral pH values are neutrophiles.
Moisture
All bacteria require water in order to survive and multiply. This is because water is the medium within the cell membrane used to transport essential molecules such as nutrients, and enzymes (biological catalysts) around the cell. Low water concentration (moisture) results in slower growth, impaired nutrient transport and loss of membrane fluidity. How much water a bacterium needs is quantified by water activity. This is defined as the amount of water that is present within a substance relative to the solutes. For example, pure water has a water activity of 1. Bacteria can live in a medium with a water activity between 0.7 to 1.
Food
In addition to these three environmental conditions, bacteria also require food in order to survive and grow. The type and amount of food needed is diverse and depends upon the specific bacterium. The vast majority of bacteria metabolize carbon and so can feed on the food eaten by humans. For example, when bacteria are cultured within a lab they are placed in an agar gel that contains simple sugars such as sucrose.