The earth has a great variety of microbes with a great variety of food habits. Microbes like E.coli are about 70% water; and 97% of the dry weight is composed of organic molecules. For survival, growth, and reproduction all microbes must get its energy from some source. Microbes that are able to use carbon dioxide as a carbon source and energy from the sun or inorganic materials are called autotrophs. Many autotrophs are photosynthetic. Algae like Chlorella and Vaucheria, cyanobacteria, protozoans like Euglena and Volvox, and plants are autotrophs. Heterotrophs are organisms that use organic molecules (usually carbohydrates) as a carbon source in the production of energy. Parasites (use tissues and fluids from a living host) and saprobes (use protoplasm from a dead organism) are two types of heterotrophs. Most bacteria, protozoans, fungi, and animals are heterotrophs.

Enzyme activity is dependent on temperature. Microbes assume the temperature of the environment; and microbes adapt to cold, moderate, and hot environments. Organisms that grow best in cold temperatures are called psychrophiles. These microbes live in polar snow fields and ocean bottoms. Chlamydomonas (a green aglae) will grow well in early spring snowbanks. Most microbes grow best at moderate temperatures such as soil, water, air, and body temperatures; these microbes are called mesophiles. Because the human body has a lower surface temperature and higher core temperature, many microbes are found as human pathogens. The microbes causing leprosy and the rhinovirus that causes the common cold will grow only in certain cooler areas of the body; some pathogens that cause gonorrhea and tuberculosis are restricted to to certain body organs with constant temperatures; other microbes like Staphylococcus and Enterococcus grow well in a wide range of temperatures. Microbes that grow well in these moderate temperatures are called mesosphiles. Some bacteria grow well in the high temperatures of hot springs. These organisms like Thiobacillus, Thermoplasma, and some cyanobacteria are called thermophiles and grow best at temperatures above 40 degrees Centigrade. Thermus aquaticus has been cultured from hot water heaters with water temperatures between 55-80 degrees Centigrade. Why do boiled eggs turn black in the refrigerator? Does milk sour in the refrigerator? Some organisms like Proteus and Streptococcus do grow in refrigerated foods. However, the primary purpose of pasteurization is to destroy the pathogens and most psychrophiles so that milk has a long shelf-life when kept in the refrigerator.