Published online by Cambridge University Press: 15 May 2009
A high incidence of resistance to trimethoprim has been shown in the normal faecal flora in a population in south India. The dihydrofolate reductase (dhfr) genes mediating transferable resistance to trimethoprim have been identified. Unusually, in this study, the dhfr V was shown to be the predominant resistance gene (dhfr V 50% of tranaconjugants, dhfr Ia 30%), the dhfr Ib was also detected being distinguished from the dhfr V by an oligo-probe. However, when non-transferable resistance was considered, the dhfr Ia was the most prevalent of the dhfrs identified. All those plasmids harbouring the dhfr Ia were shown to possess Tn7. All the plasmids that probed positive for the dhfr V and the dhfr Ib were shown to be associated with the integrase of the Tn21-like transposons, but 8 of the dhfr V genes were not associated with the Tn21 resolvase. The dhfr IV was shown to be present in all seven plasmids that produced low level trimethoprim-resistance. The dhfr V. first characterized in Sri Lanka, would seem to have a local distribution in this region of Asia but is distinguishable from the dhfr Ib only by the use of an oligo-probe.