Prescott's Microbiology,4/e

		Microbiology, 4/e Prescott, Harley, Klein
		Instructor Resources

6 Microbial Growth

CHAPTER OVERVIEW

This chapter describes the basic nature of microbial growth in the presence of an adequate nutrient supply. Several methods for the measurement of microbial growth are described and different systems used for microbial growth are also described. The chapter finishes with a discussion of the influence of various environmental factors on the growth of microorganisms.

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

o name the various phases of growth that occur in closed culture systems and describe what is occurring in each phase

o determine from experimental data the various parameters (number of generations, specific growth rate constant, mean generation time) that describe microbial growth in mathematical terms

o explain the concept of growth yield and molar growth yield

o describe the various types of continuous culture systems and explain the differences in their function

o describe the influence of various environmental factors (water availability, pH, temperature, oxygen concentration, pressure, radiation) on the growth of microorganisms

o categorize microorganisms according to the environmental factors that are conducive to optimal growth of the organism

CHAPTER OUTLINE

I. Growth-an increase in cellular constituents that may result in an increase in cell size, an increase in cell number, or both

II. The Growth Curve-usually analyzed in a closed system called a batch culture; usually plotted as the logarithm of cell number versus the incubation time

A. Lag phase is the period of apparent inactivity in which the cells are adapting to a new environment and preparing for reproductive growth, usually by synthesizing new cell components; it varies considerably in length depending upon the condition of the microorganisms and the nature of the medium

B. Exponential (log) phase is the period in which the organisms are growing at the maximal rate possible given their genetic potential, the nature of the medium, and the conditions under which they are growing; the population is most uniform in terms of chemical and physical properties during this period

C. Stationary phase is the period in which the number of viable microorganisms remains constant either because metabolically active cells stop reproducing or because the reproductive rate is balanced by the rate of cell death; it may result from:

1. Nutrient limitation

2. Toxic waste accumulation

D. Death phase is the period in which the cells are dying at an exponential rate

E. The mathematics of growth-microbial growth can be described by certain mathematical terms:

1. Mean generation (doubling) time is the time required for the population to double

2. Mean growth rate constant is the number of generations per unit time, often expressed as generations per hour

3. Generation times vary markedly with the species of microorganism and environmental conditions; they can range from 10 minutes for a few bacteria to several days with some eucaryotic microorganisms

III. Measurement of Microbial Growth

A. Measurement of cell numbers

1. Direct count methods do not distinguish between living and dead cells, and may be accomplished by direct microscopic observation on specially etched slides (such as Petroff-Hausser chambers or hemacytometers) or by using electronic counters (such as Coulter Counters, which count microorganisms as they flow through a small hole or orifice)

2. Viable cell counts involve plating diluted samples (using a pour plate or spread plate) onto suitable growth media and monitoring colony formation; this type of method counts only those cells that are reproductively active; because it is not possible to be certain that each colony arose from a single cell, results are usually expressed as colony forming units (CFU)

3. Microbial numbers are frequently determined from counts of colonies growing on membrane filters having pores small enough to trap bacteria

B. Measurement of cell mass may be used to approximate the number of microorganisms if a suitable parameter proportional to the number of microorganisms present is used (suitable parameters may be dry weight, light scattering in liquid solutions, or biochemical determinations of specific cellular constituents such as protein, DNA, or ATP)

IV. Growth Yields and the Effects of a Limiting Nutrient

A. Growth yield is expressed as the mass of cells formed per gram of nutrient consumed

B. Molar growth yield is expressed as the mass of cells formed per mole of nutrient consumed

V. The Continuous Culture of Microorganisms-used to maintain cells in the exponential growth phase at a constant biomass concentration for extended periods of time (these conditions are met by continual provision of nutrients and removal of wastes)

A. A chemostat is a continuous culture device that maintains a constant growth rate by supplying a medium containing a limited amount of an essential nutrient at a fixed rate and by removing medium that contains microorganisms at the same rate

B. A turbidostat is a continuous culture device that regulates the flow rate of media through the vessel in order to maintain a predetermined turbidity or cell density; there is no limiting nutrient

VI. Balanced and Unbalanced Growth

A. Balanced (exponential) growth occurs when all cellular components are synthesized at constant rates relative to one another

B. Unbalanced growth occurs when the rates of synthesis of some components change relative to the rates of synthesis of other components. This usually occurs when the environmental conditions change

VII. The Influence of Environmental Factors on Growth

A. Although most microorganisms only grow in fairly moderate environmental conditions, some, referred to as extremophiles, can grow under harsh conditions that would kill most other organisms

B. Solutes and water activity (a quantitative measurement of the availability of water) is inversely related to osmotic pressure and may have a profound effect on cell growth

1. Osmotolerant organisms can grow in solutions of both high and low water activity

2. Halophiles require environments of low water activity (high osmotic pressure) in order to grow

3. Microorganisms growing in a habitat with low water activity (a_w) usually maintain a high internal solute concentration in order to retain water

C. pH is the negative logarithm of the hydrogen ion concentration

1. Acidophiles grow best between pH 0 and 5.5

2. Neutrophiles grow best between pH 5.5 and 8.0

3. Alkalophiles grow best between pH 8.5 and 11.5

4. Extreme alkalophiles grow best at pH 10.0 or higher

5. Despite wide variations in habitat pH, the internal pH of most microorganisms is maintained near neutrality either by proton/ion exchange or by internal buffering

D. Temperature

1. Temperature has a profound effect on microorganism viability, primarily because enzyme-catalyzed reactions are sensitive to temperature

2. At low temperatures, a temperature rise increases the growth rate by increasing the rate of enzyme reactions

3. At high temperatures, microorganisms are damaged by enzyme denaturation, membrane disruption, and other phenomena

4. Organisms exhibit distinct cardinal temperatures (minimal, maximal, and optimal growth temperatures)

a. Psychrophiles can grow well at 0°C, have optimal growth at 15°C or lower, and usually will not grow above 20°C

b. Psychrotrophs (facultative psychrophiles) can also grow at 0°C, but have growth optima between 20°C and 30°C, and growth maxima at about 35°C

c. Mesophiles have growth minima of 15 to 20°C, optima of 20 to 45°C, and maxima of about 45°C or lower

d. Thermophiles have growth minima around 45°C, and optima of 55 to 65°C

e. Hyperthermophiles have growth minima around 55°C and optima of 80 to 110°C

5. Stenothermal organisms have a narrow range of cardinal growth temperatures; eurythermal organisms have a wide range of cardinal growth temperatures

E. Oxygen concentration

1. Obligate aerobes are completely dependent on atmospheric O₂ for growth

2. Facultative anaerobes do not require O₂ for growth, but do grow better in its presence

3. Aerotolerant anaerobes ignore O₂ and grow equally well whether it is present or not

4. Obligate (strict) anaerobes do not tolerate O₂ and die in its presence

5. Microaerophiles are damaged by the normal atmospheric level of O₂ (20%) but require lower levels (2 to 10%) for growth

F. Pressure

1. Barotolerant organisms are adversely affected by increased pressure, but not as severely as are nontolerant organisms

2. Barophilic organisms require, or grow more rapidly in the presence of, increased pressure

G. Radiation

1. Ultraviolet radiation damages cells by causing the formation of thymine dimers in DNA

a. Photoreactivation repairs thymine dimers by direct splitting when the cells are exposed to blue light

b. Dark reactivation repairs thymine dimers by excision and replacement in the absence of light

2. Ionizing radiation such as X rays or gamma rays are even more harmful to microorganisms than ultraviolet radiation

a. How levels produce mutations and may indirectly result in death

b. High levels are directly lethal by direct damage to cellular macromolecules or through the production of oxygen free radicals

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