Enriching for Specific Bacteria

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° С to favor the growth of microorganisms that live at low refrigerator temperatures or to 37° С 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.

298 CHAPTER 10 BACTERIAL REPRODUCTION AND GROWTH OF MICROORGANISMS

FIG. 10-11 A, 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.

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