Factors Influencing bacterial growth 301

FIG. 10-13 Drawing showing oxygen growth relationships for aerobic, anaerobic, and fac­ultative bacteria in tubes of nutrient media. The growth of aerotolerant anaerobes (obli-gately fermentative) would be the same as that of facultative anaerobes (organisms capable of fermentation and respiration).

Psychrophiles have optimal growth temperatures of under 20° C; mesophiles grow best between 20° and 45° C; and thermophiles grow best at higher temperatures above 45° C, with 55° to 60° С often being optimal.

Oxygen

Another factor that greatly influences bacterial growth rates is the concentration of molecular oxy­gen. Bacteria are classified as aerobes, anaerobes, fac­ultative anaerobes, or microaerophiles, based on their oxygen requirements and tolerances (FIG. 10-13). Aerobic bacteria (obligate aerobes) grow only when oxygen is available to support their respiratory metabolism. In laboratory cultures and industrial batch cultures, oxygen is often supplied by forced aeration or mixing (for example, on a rotary shaker) to support the growth of aerobes. Anaerobic bacteria (obligate anaerobes) grow in the absence of molecu­lar oxygen. Anaerobic bacteria may carry out fer­mentation or anaerobic respiration to generate ATP. Some anaerobes have very high death rates in the presence of oxygen, and such organisms are termed strict anaerobes. Even the briefest exposure to air can kill strict anaerobes. Other obligately anaerobic bac­teria, although unable to grow, have low death rates in the presence of oxygen.

While obligate anaerobes grow only in the absence of molecular oxygen, facultative anaerobes such as I coli can grow with or without oxygen. Many facul­tative anaerobes are capable of both fermentative and respiratory metabolism. Some are capable of both aerobic and anaerobic respiration.

Aerobes need oxygen to support their respiratory metabolism and anaerobes grow in the absence of molecular oxygen, carrying out fermentation or anaerobic respiration; facultative anaerobes grow aerobically or anaerobically.

Although oxygen is required for the growth of many microorganisms, it can also be toxic. Some mi­croorganisms grow only over a very narrow range of oxygen concentrations. Such microorganisms are known as microaerophiles. Microaerophiles require oxygen but exhibit maximal growth rates at reduced oxygen concentrations because higher oxygen con­centrations are toxic to these organisms.

Oxygen can exist in several energetic states, some of which are more toxic than others. One of these en­ergetic states, called singlet oxygen, is a chemically reactive form that is extremely toxic to living organ­isms. Phospholipids in bacterial plasma membranes can be oxidized by singlet oxygen, leading to a dis­ruption of membrane function and the death of bac­terial cells. Peroxidases in saliva and phagocyte cells (blood cells involved in the defense mechanism of the human body against invading microorganisms) generate singlet oxygen, accounting in part for the antibacterial activity of saliva and the ability of phagocytic blood cells to kill invading microorgan­isms.

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