TRANSFERABLE DRUG RESISTANCE IN MAN AND ANIMALS: GENETIC RELATIONSHIP BETWEEN R-PLASMIDS IN ENTERIC BACTERIA FROM MAN AND DOMESTIC PETS

SUMMARY Enteric bacteria isolated from morbid human and domestic pet populations in Canberra were examined for multiple antibiotic resistance, the transferability of such resistance, and the genetic incompatibility reactions of the presumptive R-plasmid concerned. More than one-third of the isolates obtained from domestic pets were resistant to one or more of the antibiotics tested (ampicillin, chloramphenicol, kanamycin, nalidixic acid, streptomycin, sulfathiazole, tetracycline). Ampicillin resistance was invariably a part of all multiple resistance patterns seen. Half of the resistant bacteria from pets were resistant to 5 or more of the antibiotics, with ApCmSmSuTc the most frequent combination of resistance markers. The occurrence of resistance to Km and Nd was low, probably reflecting the infrequent usage of these antibiotics in veterinary practice. Genetic similarities between identifiable R-plasmids and individual resistance markers in the 2 populations were tested by grouping on the basis of incompatibility reactions against reference plasmids for incompatibility groups Fll, ***lα, N, P and W. The results, though limited in extent by experimental conditions, suggest a close similarity between R-plasmids in domestic pets and man: the majority of assignable plasmids in each population belong to group Fll***, with smaller numbers falling into groups lα and P. Analysis of individual resistance markers reveals that apart from incompatibility group N (which contained a smaller number of markers), the distribution of resistance markers across the other incompatibility groups was similar for isolates from man and animals. These results are consistent with a common origin of R-plasmids in man and domestic pets in Canberra, or one population serving as an infection source for the other, or selection influences operating at the level of host bacteria for the plasmids which result in similarities in the bacterial population and thus of plasmid types.