4.1 Obtaining a Pure Culture

1. Only an estimated 1% of bacteria can be cultured in the laboratory.

Cultivating Bacteria on a Solid Medium

1. A single bacterial cell deposited on a solid medium, such as one that contains agar, will multiply to form a visible colony. (Figure 4.1)

The Streak-Plate Method (Figure 4.2)

1. The streak-plate method is used to isolate bacteria in order to obtain a pure culture.

Maintaining Stock Cultures

1. Stock cultures can be stored on an agar slant in the refrigerator, frozen in a glycerol solution, or lyophilized.

4.2 Principles of Bacterial Growth

1. Most bacteria multiply by binary fission. (Figure 4.3)

2. Microbial growth is an increase in the number of cells in a population.

3. The time required for a population to double in number is the generation time.

4.3 Environmental Factors that Influence Microbial Growth

Temperature Requirements (Figure 4.4)

1. Organisms can be grouped as psychrophiles, psychrotrophs, mesophiles, thermophiles, or hyperthermophiles based on their optimum growth temperatures.

2. Storage of foods at refrigeration temperatures retards spoilage because it limits the growth of mesophiles.

3. Some microorganisms can inhabit certain parts of the body but not others because of temperature differences.

Oxygen (O2) Requirements (Table 4.2)

1. Organisms can be grouped as obligate aerobes, obligate anaerobes, facultative anaerobes, microaerophiles or aerotolerant anaerobes based on their oxygen (O2) requirements.

2. Although O2 itself is not toxic, it can be converted to superoxide and hydrogen peroxide, both of which are toxic. Superoxide dismutase and catalase can break these down.

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