PREVALENCE AND GENETIC LOCATION OF NONTRANSFERABLE TRIMETHOPRIM-RESISTANT DIHYDROFOLATE-REDUCTASE GENES IN SOUTH-AFRICAN COMMENSAL FECAL ISOLATES

In a recent survey of trimethoprim resistance, 357 Gram-negative aerob ic organisms were isolated from healthy volunteers from rural and urba n populations in South Africa. Trimethoprim resistance did not transfe r to an Escherichia coli J62-2 recipient strain by conjugation in a li quid mating: in 161 (45.1%) of the isolates. These isolates which did not transfer their resistance were probed with intragenic oligonucleot ide probes for the types Ia, Ib, IIIa, V, VI, VII, VIII, IX, X and XII dihydrofolate reductase genes. Contrary to all previous data, the mos t prevalent dihydrofolate reductase gene in this group of non-transfer able isolates which hybridized, was the type VII (38%) followed by the type Ia (25%), Ib (12%), V (1.7%) and VIII (1.2%). None of the strain s hybridized to the types IIIa, VI, XI, X and the XII dihydrofolate re ductase probes. Southern blots of plasmid and chromosomal DNA from sel ective isolates revealed that the type VII dihydrofolate reductase gen es were located on the chromosome and were associated with the integra se gene of Tn21. However, the type Ib and V dihydrofolate reductase ge nes were all found on plasmids which could not be mobilized. The type Ia dihydrofolate reductase genes were found on both nontransferable pl asmids and on the chromosome. The nature of the genetic structures ass ociated with a dihydrofolate reductase gene strongly affects the means of spread of the gene in a population.