Studies of antimicrobial resistance genes using DNA probes.

In nature there is a theoretical gene pool from which microorganisms can acquire genetic determinants that increase their chances of survival in hostile environments. This gene pool consists of genomic DNA, plasmids, phages, insertion sequences, and transposable elements, all of which exist in a dynamic equilibrium. Among the genes that can be obtained by bacteria are those encoding antimicrobial resistance determinants. The development of resistance determinants and R plasmids was first reviewed by Watanabe in 1971 (45); the review was expanded by Kopecko and colleagues in 1976 to include the role of transposons (16), and there were subsequent reviews by Cohen et al. (5), Davies and Smith (8), Davies and Gray (7),, and others (2, 15). According to Davies and Gray, many antimicrobial resistance genes could have originated in antibiotic-producing strains of soil bacilli. These resistance determinants, which protected the host from the action of an antibiotic, could have been excised from their chromosomal location by insertion elements or transposons and disseminated by these genetic elements to other bacteria (7). The integration of these determinants into thecomplement of genetic material of a bacterium is often a function of appropriate selective pressure. Indeed, the gene pool of resistance determinants available to bacteria seems vast, especially when judged by the number of infections caused by multiply resistant organisms seen in hospitals and medical centers today. Using techniques borrowed from the molecular biology laboratory, clinical and medical microbiologists have begun to seek out and characterize this resistance determinant gene pool and to study the movement of genetic elements, particularly among pathogenic bacteria (2, 19, 31, 41, 46). The most useful technique in this regard is the gene-specific DNA probe. Herein, I review the application of DNA probe technology to the assessment of the size and extent of the resistance determinant gene pool and the use of DNA probes as epidemiologic tools for the tracing of multiresistant bacteria.