5.2.1: pH and Microbial Growth

Microorganisms can be found growing at all pH ranges. However, each specific microorganism will have range at which it can grow. The range consists of a minimum that allows growth, a maximum pH for growth, and an optimal pH between these two extremes where growth is most prolific.

pH is a measurement of the concentration of hydrogen ions in a sample or environment. pH is measured on a scale of 0 to 14. As hydrogen ion concentrations increase, the measurement moves toward 0 on the pH scale and is said to be acidic. On the other hand, when hydrogen ions decrease, the measurement moves toward 14 and is alkaline. pH 7 is considered neutral. This information is important to note when attempting to understand the classification of microorganism based on their pH preferences. Microorganisms that prefer a neutral pH range near pH 7 are called neutrophiles. Most microorganisms belong to the classification.

Microorganisms whose optimal pH range is less than pH 5.5 are referred to as acidophile. A clinically important example of acidophiles are the Lactobacillus species that are part of the normal flora of the vagina. Lactobacillus species require a pH less than 5.5 and produce products that help maintain that pH within the vagina. In doing so the Lactobacillus bacteria inhibit the growth of microorganisms that require a higher pH, i.e. yeast. There also microorganisms that prefer alkaline conditions, with a pH of 8 or greater.

Such microorganisms are referred to as alkaliphiles. Vibrio cholerae is an example of clinically significant alkaliphile. Vibrio cholerae grows best at a pH slightly above pH 8.

The pH of the microorganism’s environment is important because changes in pH can detrimentally affect cellular function. When the hydrogen concentration is too high (acidic) or too low (alkaline) hydrogen bonding in the cell may be impacted. The is problematic for both DNA and proteins. The double helix of DNA is held together by hydrogen bonds. Proteins, many of which act as enzymes within the cell to facilitate cellular metabolism are also dependent on hydrogen bonding for their secondary and tertiary structure.

Interrupting the hydrogen bonds due changes in pH can cause proteins to denature permanently. This can halt metabolism and thus kill the cell. In addition to affecting protein and DNA structure, alterations in pH can affect the process of cellular respiration. Some bacteria depend on cellular respiration to produce their needed ATP. The process requires that a gradient of hydrogen ions be present across the cell membrane so that they can enable ATP synthase to function in producing ATP.

In this exercise you will examine three different species of bacteria in different pH environments to determine each microorganism’s optimal pH for growth.

---