Different microorganisms often require distinct environments, with varied temperature, levels of oxygen, light and acidity or pH level. Some microbes grow faster in environments with extremely low pH values. These are called acidophiles, because of their preference for acidic environments. Although most microorganisms requires neutral pH values to have optimum growth, alkaliphilic microorganisms prefer low-acidity or high pH environment.

Microorganisms which optimum growth at pH levels lower than 5 are called acidophiles. These microbes are found in a variety of environments, including geysers and sulfuric pools, as well as in the human stomach. Examples of acidophiles include the microscopic algae Cyanidium caldarium and Dunaliella acidophila. The microscopic fungi, Acontium cylatium, Cephalosporium and Trichosporon cerebriae can grow near pH 0. A primitive microorganism called Picrophilaceae have optimum pH values close to zero and can also grow at negative pH values.

Alkaliphilic microorganisms have optimum growth at pH values between 9 and 12. These microorganisms thrive in alkaline lakes, soils and other high pH environments. In slag dumps of Lake Calumet, southeast Chicago, the water can reach a pH of 12.8, which is similar to caustic soda. Some bacteria related to the Clostridium and Bacillus live in that extremely alkaline environment. Mono Lake, in California, and Octopus Spring in Yellowstone Park are examples of environments were alkaliphilic microorganisms are found.

Neutral pH values, laying between 6 and 8, are more commonly found in nature. Along their evolution, most microorganisms have adapted to have optimum growths in acidity-neutral environments. These microorganisms are called neutrophiles, and include most species of microalgae and other organisms that form the phytoplankton, as well as some soil-dwelling bacteria and yeasts.

Most microorganisms associated with human, animals and plant diseases are neutrophiles, such as Escherichia coli, which causes intestinal infections; Erwinia caratovora, a plant parasite; Pseudomonas aeruginosa, which causes a series of infection in humans and animals; and Streptococcus pneumoniae, which causes pneumonia. However, pathogens are also found among acidophiles and alkaliphilics. The bacterium Lactobacillus acidophilus have optimum growth in low pH levels, causing vaginal infections. The alkaliphilic bacterium Vibrio cholerae causes cholera in humans.