By taking into account the physiological characteristics of specific bacterial species or types, it is possible to design conditions that favor the growth of those bacteria. This is the basis of enrichment culture technique, a method used to isolate specific groups of bacteria based on designing the culture medium and incubation conditions to preferentially support the growth of a particular bacterial type. Liquid enrichment media tend to select the bacteria that are able to grow best among all the bacteria introduced into the media. For example, to isolate bacteria capable of metabolizing petroleum hydrocarbons, one can design a culture medium containing a hydrocarbon as the sole source of carbon and energy (see Figure). By doing so, one establishes conditions whereby only bacteria that are capable of metabolizing hydrocarbons can grow. Because other bacteria cannot reproduce in this medium, hydrocarbon-utilizing bacteria are thereby selected, resulting in enrichment (increased proportions)
of the selected bacteria. Similarly, a culture medium that favors the growth of autotrophic microorganisms could be designed by providing ammonium ions and carbonate as the sole source of carbon in the medium.
The design of an enrichment procedure takes into consideration the composition of the medium and also environmental factors, such as temperature, aeration, pH, and so forth. For example, temperature can be adjusted to 5° Ñ to favor the growth of microorganisms that live at low refrigerator temperatures or to 37° Ñ to enrich for microorganisms that are capable of growth at the temperature of the human body. Cultures may be aerated by shaking or by bubbling with air to favor the growth of aerobes, or oxygen may be totally excluded to enrich for anaerobes. The enrichment culture technique mimics many natural situations in which the growth of a particular microbial population is favored by the chemical composition of the system and by environmental conditions.
A, To establish an enrichment, a medium is inoculated with a sample, for example, soil or water, that may contain microorganisms with specific characteristics. The medium and the incubation conditions are designed to favor the growth of the microorganisms, for example, microorganisms capable of degrading petroleum hydrocarbons. The desired microorganisms should be able to outcompete others in the sample and increase in number so they then can be isolated and pure cultures established. B, Enrichment cultures are designed to selectively support the growth of specific microorganisms. In a medium with petroleum hydrocarbon as the sole source of carbon and energy, hydrocarbon-degrading microorganisms are selectively enriched (left flask). Control showing oil slick and lack of enrichment for hydrocarbon degraders (right flask). Growth of the hydrocarbon-degrading microorganisms emulsifies the oil so that it disperses through the medium in the flask. A pure culture of the hydrocarbon degrader can be isolated from the enrichment culture.
298CHAPTER 10 BACTERIAL REPRODUCTION AND GROWTH OF MICROORGANISMS
FIG. 10-11A, Microorganisms exhibit specific temperature growth ranges. There is a minimal and a maximal temperature for growth. B, An incubator is used to control temperature for culturing microorganisms.
TABLE 10-2 ■
The optimal growth temperature is defined as the temperature at which the maximal growth rate occurs. This is the temperature that corresponds to the shortest generation time (Table 10-2). The ability of a microbial species to compete for survival in a given system is favored when temperatures are near its optimal growth temperature. It is not surprising that the optimal growth temperature for most human pathogens is 37° C, the temperature of the human body.
Optimal growth temperature is the temperature at which the generation time is shortest and therefore at which the maximal growth rate occurs.
Different microorganisms have different optimal growth temperatures (FIG. 10-12). Some microorgan-
FIG. 10-12Temperature growth ranges for mesophiles, psychrophiles, and thermophiles.