Detection of antibiotic resistance genes with DNA probes.

An increasing number of reports describing the use of DNA probes for the clinical diagnosis of specific pathogens are now appearing in the scientific literature. So far as antibiotic resistance is concerned, DNA probes have been used primarily to obtain information regarding the dissemination of particular antibiotic resistance genes and to obtain information regarding their evolutionary relationships (e.g. Towner et al., 1991). However, since primary isolation and growth of an infecting organism is not necessarily required for a successful hybridization test, one of the major potential advantages of DNA probes is that they could be used directly to detect antimicrobial resistance of bacteria present in patient specimens, with obvious concomitant clinical advantages. Such probe-based tests would permit the rapid screening of large numbers of samples, with possibilities for automation; the feasibility of this approach has already been demonstrated with infected urine specimens (Carter et al., 1989). Now that non-radioactive methods of probe-labelling are available commercially, it is particularly clear that DNA probes at least merit further investigation as an alternative test for presence of antimicrobial resistance in bacteria. A major consideration is the type of resistance that can be detected by DNA probes. Resistance resulting from the presence of specific genes, often encoding particular enzymes such as plasmid-mediated /J-lacta-mases, aminoglycoside-inactivating enzymes or dihydrofolate redudases, is readily amenable to detection by DNA probes. In contrast, resistance resulting from random spontaneous mutation of genes on the bacterial chromosome cannot be detected so easily. Although such mutations, which often involve changes in only a single base pair of DNA, can theoretically be detected by means of specific oligo-nucleotide probes, in practice it would be impossible to produce probes capable of detecting all possible spontaneous mutations. Thus DNA probe analysis of antimicrobial resistance for diagnostic purposes is effectively limited to the detection of resistance associated with identifiable structural genes. A second consideration concerns the meaning of the term 'resistance'. Microbiologists and clinicians have become accustomed to the idea that an organism is 'resistant' when it is inhibited in vitro by an antibiotic concentration which is greater than that achievable in vivo. In contrast with this somewhat limited traditional definition, a positive DNA hybridization result provides direct evidence that an organism has the potential for resistance, in that it carries a particular resistance gene. It does not mean that such a resistance gene is necessarily being expressed. Little is known about the frequency with which …