Common regulatory mechanism of expression and conjugative ability of a tetracycline resistance plasmid in Bacteroides fragilis

Abstract

RESISTANCE to various antibiotics is common in the anaerobic gut bacterium Bacteroides fragilis. A transferable plasmid coding for resistance to antibiotics of the lincosamide/macrol-ide/synergistin groups has been found^1,2 and transfer is most probably a conjugative process. This supports the observation of the recent emergence of resistance to antibiotics in these groups and its rapid spread in certain areas (ref. 3 and our unpublished data). However, in the case of tetracycline resistance the limited genetic and physical data were at variance with the epidemiological data. Although tetracycline resistance is common in B. fragilis⁴ (at present 60% of strains isolated in the USA⁵ and 30% of those isolated in France (our unpublished results) are resistant) intraspecies transfer of tetracycline resistance could not be demonstrated⁶. We report here that transfer of tetracycline resistance between B. fragilis strains is easily achieved if the donor bacteria are grown in the presence of tetracycline before mating. This suggests that tetracycline resistance is borne on a transferable plasmid whose conjugative transfer requires activation of the tetracycline resistance gene. This is supported by our finding that the transfer of tetracycline resistance is either inducible or constitutive, depending on whether tetracycline resistance itself is respectively inducible by tetracycline or constitutive.