The pH scale measures the concentration of hydrogen ions in a solution, ranging from 0-14 with lower numbers being more acidic. The pH of biological systems determines which microorganisms can survive, as most prefer a pH close to 7 like distilled water. Some bacteria thrive in more acidic environments. The pH of growth media is important for microbes and must be monitored to keep it stable for their survival. Microorganisms can tolerate a range of pHs through membrane buffering but some are able to modify their environment's pH through metabolic processes like fermentation.