Elective enrichment

In its simplest form, the procedure consists of the following steps  

Some of the experimental details are briefly described in the following paragraphs. The three successive stages used in isolating the desired organisms are outlined first, followed by a more extensive discussion of media. 

An appropriate mineral medium supplemented with the organic compound that is to be studied is inoculated with a sample of water, soil, or sediment. In studies of the environmental fate of a xenobiotic in a specific ecosystem, samples are generally taken from the area putatively contaminated with the given compound so that a degree of environmental relevance is automatically incorporated. Attention has, in addition been directed to pristine environments, and the issues of adaptation or preexposure have already been discussed. 

These procedures may clearly result in the dominance of organisms that carry out only biotransformation of the xenobiotic, although the biodegradation of many of these compounds has also been demonstrated using the same or other organisms. 

After metabolically stable cultures have been obtained, pure cultures of the relevant organisms may then be obtained by any of three basic procedures  

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Attention is directed to organisms that have hitherto evaded isolation or are represented by only a few cultivated examples. Such organisms may well outnumber those that have been isolated as pure cultures, frequently using elective enrichment. A few illustrations are given below ... 

Azotobacters. Burk and Winogradsky in the 1930s showed that these could readily be obtained from soil samples by elective enrichment with benzoate. The degradative pathway for benzoate has been elucidated (Hardisson et al. 1969), and the range of substrates extended to 2,4,6-trichlorophenol (Li et al. 1992 Latus et al. 1995). The enzyme from Azotobacter sp. strain GPl that catalyzes the formation of 2,6-dichlorohydroquinone from... 

While these earlier studies supplied some good information on the nutrition of methane bacteria, they were conducted mainly on species isolated from elective enrichments, and the ecological significance of the species was in doubt except for M. formicicum which was shown to be present in large numbers in sludge (15). 

Experimental aspects of the elective enrichment procedure are discussed in some detail in Chapter 5, but the question of its existence and significance in natural populations already exposed to xenobiotics is conveniently addressed here. It is important to distinguish between induction (or derepression) of catabolic enzymes and selection for a specific phenotype. The former is a relatively rapid response, so that exposure of samples from uncontaminated areas to xenobiotics for a period of weeks or months would be expected to result in the selection of organisms with degradative potential... 

Experiments have used cells with a metabolic capability that may plausibly be predicted as relevant to that of the xenobiotic. For example, elective enrichment failed to yield organisms able to grow at the expense of dibenzo-[l,4]-dioxin, but its metabolism could be studied in a strain of Pseudomonas sp. capable of growth with naphthalene (Klecka and Gibson 1979). Cells were grown with salicylate (1 g/1) in the presence of dibenzo-[l,4]-dioxin (0.5 g/1), and two metabolites of the latter were isolated a s-l,2-dihydro-l,2-diol and 2-hydroxydibenzo-l,4-dioxin. The former is consistent with the established dioxygenation of naphthalene and the role of salicylate as coordinate inducer of the relevant enzymes for conversion of naphthalene into salicylate. 

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