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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

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.



Review Questions

1. Define bacterial growth.

2. What is binary fission?

3. How are microorganisms classified based on optimal growth temperature?

4. Define pH and explain its relation to microbial growth.

5. Define osmotic pressure and explain its relation to mi­crobial growth.

6. How are microorganisms classified based on oxygen requirements?

7. How can oxygen be toxic to cells? How do cells protect themselves from these toxic molecules?

8. What are the phases of microbial growth?

9. What is generation time?


10. Describe some direct and indirect measures of micro­bial growth.

11. What are the similarities and differences between bacteria growing in the environment and those in a continuous culture?

12. What are the advantages and disadvantages of the vi­able plate count method to assess bacterial numbers? I

13. What are the advantages and disadvantages of the di-1 rect microscopic count method to assess bacterial num­bers?

14. What special requirements do bacteria need to survive in very hot environments? In very cold environments?

15. Describe the different parts of the bacterial growth cy-1 cle. What is happening in the cell and in the population of cells in each phase?

16. What does exponential growth mean? What is happen-Ltig during the exponential growth phase?

17. How long would it take a single bacterial cell to form 1,000,000 cells if it had a generation time of 30 minutes!



1. Suppose you wanted to isolate a microorganism that was a mesophilic, degraded cellulose and was mi-croaerophilic. What conditions would you have to provide to isolate such a microorganism in the labo­ratory? Where would you obtain the inoculum for es­tablishing the culture?

2. Some bacteria that live in deep ocean waters are ob­ligate barophiles that tend to lyse or rupture when brought to normal atmospheric pressures. What spe­cial requirements would these bacteria need to survive in their high pressure environment? Why can't they survive at the ocean surface? How can they be cul­tured in the laboratory?


3. Why would you want to distinguish between the num­bers of live bacteria and dead bacteria in a popula­tion? How would you go about doing this? How would you deal with viable nonculturable bacteria?

4. It takes about 60 minutes to replicate the bacterial chromosome. Given that every daughter cell formed by binary fission must have a complete bacterial chro­mosome, how can some bacteria reproduce every 30 minutes?

5. Why does the clinical microbiology laboratory em­ploy so many different methods for isolating and iden­tifying pathogenic microorganisms? Why can't one set of standardized conditions be employed?



Atlas RM and R Bartha: 1993. Microbial Ecology: Fundamentals and

Applications, ed. 3, Menlo Park, California; Benjamin/Cummings. A text describing the ecology of microorganisms that includes chap­ters on the effects of environmental conditions on the growth of mi­croorganisms.

Brock TD (ed.): 1986. Thermophiles: General, Molecular, and Applied Microbiology, New York, John Wiley.

Complete coverage of the thermophilic bacteria by an outstanding re­searcher in the field.

DeLong EF and AA Yayanos: 1985. Adaptation of the membrane

lipids of a deep-sea bacterium to changes in hydrostatic pressure,

Science 228:1101-1103. Discusses the physiological effects of hydrostatic pressure on marine bacteria and the role of their membranes on their ability to adapt to this environment.

Dworkin M: 1985. Developmental Biology of the Bacteria, Menlo Park,

CA; Benjamin/Cummings. Describes the special features of bacterial growth.

Ingraham JL, Î Maaloe, FC Neidhardt: 1983. Growth of the Bacterial

Cell, Sunderland, MA; Sinauer Associates.

Explains biological principles and molecular aspects of bacterial growth.

Jannasch HW and MJ Mottl: 1985. Geomicrobiology of deep-sea

hydrothermal vents, Science 216:1315-1317.

A fascinating report on the microorganisms growing in deep-sea ther­mal vents and how they support the surrounding biological commu­nity.

Postgate JR: 1994. The Outer Reaches of Life, New York; Cambridge

University Press.

Describes the fascinating adaptations of microorganisms that permit survival under extreme environmental conditions.

Slater JH, R Whittenbury, JWT Wimpenny: 1983. Microbes in Their

Natural Environments, Thirty-Fourth Symposium of the Society for

General Microbiology, England, Cambridge University Press. A series of papers on the growth of microorganisms in various natural habitats.

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