Factors Influencing bacterial growth 303

FIG. 10-14 The action of catalase is readily visualized when hydrogen peroxide is added to cells that have pro­duced this enzyme.

intermediary form of oxygen known as the superox­ide anion (0₂"), in addition to forming singlet oxy­gen. The superoxide anion is converted to hydrogen peroxide and oxygen by the action of the enzyme su­peroxide dismutase, which is produced by most aer­obic and facultatively anaerobic bacteria. Superoxide dismutase removes the toxic superoxide anion but forms hydrogen peroxide (H₂0₂), which is also toxic. The reaction that describes the action of superoxide dismutase is:

Hydrogen peroxide is frequently used to kill bac­teria, for example, when hydrogen peroxide is ap­plied to a cut to prevent infection. Some bacteria pro­duce enzymes that destroy hydrogen peroxide. Cata­lase converts hydrogen peroxide to water and oxygen. The reaction that describes the action of Cata-

Obligate aerobes and facultative anaerobes usu­ally produce both catalase and superoxide dismutase (Table 10-3). These enzymes permit such microorgan­isms to grow without accumulating toxic forms of oxygen. In contrast, obligate anaerobes, such as Clostridium species, generally lack these enzymes. The inability of these organisms enzymatically to re­move toxic forms of oxygen probably accounts for the fact that they are obligately anaerobic and sensi­tive to oxygen.

Salinity

Halophiles are bacteria that specifically require sodium chloride for growth (FIG. 10-15). Moderate halophiles, which include many marine bacteria, grow best at salt (NaCl) concentrations of about 3% NaCl. Extreme halophiles exhibit maximal growth rates in saturated brine solutions. These organisms grow quite well in salt concentrations of greater than 15% NaCl and can grow in places like salt lakes and pickle barrels (FIG. 10-16). High salt concentrations normally disrupt membrane transport systems and denature proteins. Extreme halophiles must possess physiological mechanisms for tolerating high salt concentrations. For example, Halobacterium, pos­sesses an unusual plasma membrane and many un­usual enzymes that require a high salt concentration for activity.

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