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1. Organisms can be grouped as neutrophiles, acidophiles or alkalophiles based on their optimum pH.

Water Availability

1. All microorganisms require water for growth.

2. If the solute concentration is higher in the medium than in the cell, water diffuses out of the cell, causing plasmolysis. (Figure 4.5)

3. Halophiles have adapted to live in high salt environments.

4.4 Nutritional Factors that Influence Microbial Growth

Required Elements (Table 4.3)

1. The major elements make up cell constituents and include carbon, nitrogen, sulfur, and phosphorus. Trace elements are required in very minute amounts.

Growth Factors

1. Bacteria that cannot synthesize cell constituents such as amino acids and vitamins require these as growth factors.

Energy Sources

1. Organisms derive energy either from sunlight or from the oxidation of chemical compounds.

Nutritional Diversity (Table 4.4)

1. Prokaryotes can thrive in every conceivable environmental habitat because they are able to use diverse sources of carbon and energy.

2. Photoautotrophs use the energy of sunlight and the carbon in the atmosphere to make organic compounds.

3. Chemolithoautotrophs use inorganic compounds for energy and derive their carbon from CO2.

4. Photoheterotrophs use the energy of sunlight and derive their carbon from organic compounds.

5. Chemoorganoheterotrophs use organic compounds for energy and as a carbon source.

4.5 Cultivating Prokaryotes in the Laboratory

General Categories of Culture Media (Table 4.5)

1. A complex medium contains a variety of ingredients such as peptones and extracts; examples include nutrient agar, blood agar, and chocolate agar.

2. A chemically defined medium is composed of precise mixtures of pure chemicals; an example is glucose-salts medium.

106 Chapter 4 Dynamics of Prokaryotic Growth Special Types of Culture Media

1. A selective medium inhibits organisms other than the one being sought; examples include Thayer-Martin agar and MacConkey agar.

2. A differential medium contains a substance that certain bacteria change in a recognizable way; examples include blood agar and MacConkey agar. (Figures 4.6 and 4.7)

Providing Appropriate Atmospheric Conditions

1. A candle jar provides increased CO2, which enhances the growth of many medically important bacteria.

2. Microaerophilic bacteria are incubated in a gastight jar along with a packet that generates low O2 conditions.

3. Anaerobes may be incubated in an anaerobe jar or an anaerobic chamber. (Figures 4.8 and 4.9)

Enrichment Cultures (Figure 4.10)

1. An enrichment culture provides conditions in a broth that enhance the growth of one particular organism in a mixed population. Selective agents can be used to make a selective enrichment.

4.6 Methods to Detect and Measure Bacterial Growth (Table 4.7) Direct Cell Counts

1. Direct cell counts generally do not distinguish between living and dead cells.

2. One of the most rapid methods of determining the number of cells in a suspension is the direct microscopic count. (Figure 4.11)

3. Both a Coulter counter and a flow cytometer count cells as they pass through a minute aperture. (Figure 4.12)

Viable Cell Counts

1. Plate counts measure the number of viable cells in a sample by exploiting the fact that an isolated cell will form a single colony. (Figure 4.13)

2. Membrane filtration concentrates bacteria by filtration; the filter is then incubated on an agar plate. (Figure 4.14)

3. The most probable number (MPN) method is a statistical assay based on the theory of probability and is used to estimate cell numbers. (Figure 4.15)

Measuring Biomass

1. Turbidity of a culture can be correlated to the number of cells; a spectrophotometer is used to measure turbidity. (Figure 4.16)

2. Wet weight and dry weight are proportional to the number of cells in a culture.

3. The quantity of a cell constituent such as nitrogen can be used to calculate biomass.

Detecting Cell Products

1. Products including acid, gas, and ATP can be used to indicate growth.

4.7 Bacterial Growth in Laboratory Conditions

The Growth Curve (Figure 4.17)

1. When grown in a closed system, a population of bacteria goes through five phases: lag, log, stationary, death, and prolonged decline.

Colony Growth

1. The position of a single cell within a colony markedly determines its environment; cells on the edge may be in log phase, whereas those in the center may be in death phase.

Continuous Culture

1. Bacteria can be maintained in a state of continuous exponential growth by using a chemostat.

4.8 Bacterial Growth in Nature

Interactions of Mixed Microbial Communities

1. Bacteria often grow in close associations with other kinds of organisms; the metabolic activities of one organism may facilitate the growth of another organism.

Biofilms (Figure 4.20)

1. Bacteria may live suspended in an aqueous environment, but many attach to surfaces and live as a biofilm, a polysaccharide-encased community.

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