Neutrophilic and acidophilic heterotrophic bacteria make up the majority of species of bacteria. The terms "neutrophilic" and "acidophilic" refer bacterial species' optimum level of pH--a measure of a substance's acidity or basicity. For example, vinegar measures as acidic, and baking soda as a base. The pH scale ranges from 0 to 14, with 7, the pH of pure water, right in the middle.

The majority of bacteria, neutrophiles, live in soil or water and grow best at a neutral pH between 6 and 8. If the pH varies too far outside of this range, neutrophilic bacteria cannot survive. Most bacteria that cause diseases in humans are also neutrophilic heterotrophs, well-suited to survive inside a human body.

Acidophilic bacteria grow better at lower pH levels, usually under a pH of 6, as they have biological mechanisms that enable them to keep their internal pH near neutral. Acid mine drainage--the contaminated, highly acidic runoff from mining areas--contains a high population of acidophiles that oxidize the sulfide found in metal ores. According to the Science Education Resource Center at Carleton College, the acidophile Ferroplasma found in acid mine drainage exhibited pH levels as low as zero.

Obligate acidophiles require a low pH, below 4 or 5, to survive. The cell membrane of obligate acidophiles actually dissolves at neutral pH levels, causing cell death. Many obligate acidophiles are also thermophiles--organisms that grow best at high temperatures--and commonly found in volcanic soils. Thiobacillus ferrooxidans probably ranks as the most often studied iron-oxidizing acidophilic bacterium.