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.
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!
CHAPTER REVIEW 309
CRITICAL THINKING QUESTIONS
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
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.