Factors Influencing Bacterial Growth (pp. 296-306)

• Environmental conditions influence bacterial growth
and death rates. Each bacterial species has a specific
tolerance range for specific environmental parame­
ters. Changing environmental conditions cause popu­
lation shifts. Laboratory conditions can be manipu­
lated to achieve optimal growth rates for specific or­
ganisms.

Temperature (pp. 296-301)

• There are maximum and minimum temperatures at
which microorganisms can grow; these extremes of

308 CHAPTER 10 BACTERIAL REPRODUCTION AND GROWTH OF MICROORGANISMS

temperature at which growth occurs establish the temperature growth range.

• Several categories of bacteria are defined based on
optimal growth temperatures: psychrophiles have
optimal growth temperatures of under 20° C; meso-
philes have optimal growth temperatures in the mid­
dle range (20° to 45° C); and thermophiles grow opti­
mally at higher temperatures, above 45° C.

Oxygen (pp. 301-303)

• Aerobic microorganisms grow only when oxygen is available (respiratory metabolism). Anaerobic mi­croorganisms grow in the absence of molecular oxy­gen by fermentation or anaerobic respiration. Obli­gate anaerobes grow only in the absence of molecular oxygen. Facultative anaerobes can grow with or with­out oxygen and are usually capable of both fermenta­tive and respiratory metabolism. Microaerophiles grow only over a very narrow range of oxygen con­centrations; they require oxygen, but high concentra­tions are toxic.

• Microorganisms possess enzyme systems for detoxi­fying various forms of oxygen; catalase is involved in the destruction of hydrogen peroxide; superoxide dismutase destroys the toxic superoxide radical.

Salinity (pp. 303-305)

• Most microorganisms cannot tolerate high salt con­
centrations, but some salt-tolerant bacteria, such as
Staphylococcus, will grow at high salt concentrations.
Halophiles require sodium chloride for growth and
extreme halophiles can grow at very high salt con­
centrations.

Acidity and pH (p. 305)

• The pH of a solution describes its hydrogen ion con­centration. Microorganisms vary in their pH toler- j ance ranges, with fungi generally exhibiting a wider pH range (5 to 9) than bacteria (6 to 9).

• Neutralophiles grow best at near neutral pH. Acido-philes are restricted to growth at low pH values. Some acidophiles grow only at pH 1-2. Alkalophiles grow best at high pH values.

Pressure (p. 305)

• Extreme osmotic pressures can result in microbial death because cells shrink and become desiccated in I hypertonic solutions; in hypotonic solutions, cells I may burst. Osmotolerant microorganisms can grow in solutions with high solute concentrations. Os-mophilic microorganisms require high solute concen- I trations.

• Hydrostatic pressure is the pressure exerted by a col- I umn of water as a result of the weight of the water I column (10 meters water = 1 atmosphere of pres- I sure). Most microorganisms are relatively tolerant to | hydrostatic pressures in most natural systems, except '• deep ocean regions.

Light Radiation (pp. 305-306)

• Exposure to visible light can cause death of some mi- |
croorganisms; some microorganisms produce pig­
ments (often yellow-orange) that protect them against
the lethal action of light radiation. Photosynthetic mi­
croorganisms require visible light to carry out metab­
olism and the rate of photosynthesis is a function of
light intensity.

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