306 Chapter 10 bacterial reproduction and growth of microorganisms

FIG. 10-18 Cells respond to osmotic pressure. In hypotonic solutions, cells may burst.

reach a maximum. Although light intensities above this level do not result in further increases in the rates of photosynthesis, light intensities below the optimal level result in lower rates of photosynthesis.

The wavelength of light also has a marked effect on the rates of photosynthesis. Different photosynthetic microorganisms use light of different wave­lengths. For example, anaerobic photosynthetic bac­teria use light of longer wavelengths than eukaryotic algae are capable of using. Many photosynthetic mi­croorganisms have accessory pigments that enable them to use light of wavelengths other than the ab­sorption wavelength for the primary photosynthetic pigments. The distribution of photosynthetic mi­croorganisms in nature reflects the variations in the ability to use light of different wavelengths and the differential penetration of different colors of light into aquatic habitats.

The rate of photosynthesis is a function of light in­tensity and wavelength.

Exposure to visible light can also cause the death of bacteria (FIG. 10-19). Exposure to visible light can lead to the formation of singlet oxygen, which can re­sult in the death of bacterial cells. Some bacteria pro­duce pigments that protect them against the lethal ef­fects of exposure to light. For example, yellow, or­ange, or red carotenoid pigments interfere with the formation and action of singlet oxygen, preventing its lethal action. Bacteria possessing carotenoid pig­ments can tolerate much higher levels of exposure to

sunlight than nonpigmented microorganisms. Pig-1 mented bacteria often grow on surfaces that are ex­posed to direct sunlight, such as on leaves of trees. Many viable bacteria found in the air produce col­ored pigments.

FIG. 10-19 Pigmentation is important in the ability of bacteria to survive exposure to light.

SUMMARY

Bacterial Reproduction (pp. 287-288) Binary Fission (p. 287)

• Binary fission is the normal form of asexual bacterial reproduction and results in the production of two equal-size daughter cells. Replication of the bacterial chromosome is required to give each daughter cell a complete genome.

• A septum or crosswall is formed by the inward movement of the plasma membrane, separating the two complete bacterial chromosomes in an active protein-requiring process, physically cutting thechro-mosomes apart and distributing them to the two daughter cells. Cell division is synchronized with chromosome replication.

Alternate Means of Bacterial Reproduction (pp. 287-288)

• Other modes of bacterial reproduction are predomi­ nantly asexual, differing in how the cellular material is apportioned between the daughter cells and whether the cells separate or remain together as part of a multicellular aggregation. Budding is character­ ized by an unequal division of cellular material. In hyphae formation the cell elongates, forming rela­ tively long, generally branched filaments; crosswall formation results in individual cells containing com­ plete genomes.

Bacterial Spore Formation (p. 288)

• Sporulation results in the formation of specialized re­ sistant resting cells or reproductive cells called spores. Endospores are heat-resistant nonreproduc- tive spores that are formed within the cells of a few bacterial genera; cysts are dormant nonreproductive cells sometimes enclosed in a sheath. Myxobacteria form fruiting bodies within which they produce prog­ eny myxospores that can survive transport through the air. Arthrospores are spores formed by hyphae fragmentation that permit reproduction (increase in cell number) of some bacteria.

Bacterial Growth (pp. 288-291)

• Binary fission leads to a doubling of a bacterial popu­ lation at regular intervals. The generation or doubling time is a measure of the growth phase.

Generation Time (pp. 288-290)

Bacterial Growth Curve (pp. 291-292)

• The normal growth curve for bacteria has four phases: lag, log or exponential, stationary, and death. During the lag phase, bacteria are preparing for re­production, synthesizing DNA and enzymes for cell

• The time required to achieve a doubling of the popu­lation size is known as the generation time or dou­bling time.

• The generation time is a measure of bacterial growth rate.

• The generation time of a bacterial culture can be ex­pressed as:

division; there is no increase in cell numbers. During the log phase the logarithm of bacterial biomass in­creases linearly with time; this phase determines the generation or doubling time. Bacteria reach a station­ary phase if they are not transferred to new medium and nutrients are not added; during this phase there is no further net increase in bacterial cell numbers and the growth rate is equal to the death rate. The death phase begins when the number of viable bacte­rial cells begins to decline. Batch and Continuous Growth (p. 291)

• In batch cultures, bacteria grow in a closed system to which new materials are not added. In continuous culture, fresh nutrients are added and end products removed so that the exponential growth phase is maintained.

Bacterial Growth on Solid Media (p. 291)

• On solid media, bacteria do not disperse and so nu­ trients become limiting at the center of the colony; bacterial colonies have a well-defined edge. Each colony is a clone of identical cells derived from a sin­ gle parental cell.

Enumeration of Bacteria (pp. 292-296)

Viable Count Procedures (pp. 292-294)

• Numbers of bacteria are determined by viable plate count, direct count, and most probable number deter­ minations. In viable plate counts, serial dilutions of bacterial suspensions are plated on solid growth medium by the pour plate or surface spread tech­ nique, incubated, and counted. Since each colony comes from a single bacterial cell, counting the colony forming units, taking into account the dilution fac­ tors, can determine the original bacterial concentra­ tion.

Direct Count Procedures (pp. 294-295)

• Bacteria enumerated by direct counting procedures do not have to be grown first in culture or stained. Special counting chambers are often used.

Most Probable Number (MPN) Procedures (pp. 295-296)

• In most probable number enumeration procedures, multiple serial dilutions are performed to the point of extinction. Cloudiness or turbidity can be the crite­ rion for existence of bacteria at any dilution level; gas production or other physiological characteristics can be used to determine the presence of specific types of bacteria.

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

307

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.

CHAPTER REVIEW

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!

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?

1. 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?

3. Readings

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, О 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.