Resistance to tetracycline, erythromycin, and clindamycin in the Bacteroides fragilis group: inducible versus constitutive tetracycline resistance.

The transferability of plasmid-mediated tetracycline, erythromycin, and clindamycin resistance was studied in 63 clinical isolates of the Bacteroides fragilis group. Of 48 strains which were tetracycline resistant (Tcr), the regulation of both the expression of Tcr and its transferability was shown to be under inducible control by tetracycline. In 29 of the strains, Tcr was transferable; in the majority of these (26 strains), transferability was inducible (Trai) and it was constitutive (Trac) in only 3 strains. All four possible phenotypes were found (Tci Trai, Tci Trac, Tcc Trai, and Tcc Trac), which indicates independent control of both Tcr expression and its transferability. Resistance to erythromycin and clindamycin was cotransferred with Tcr in 14 of the 48 Tcr strains and transferred independently of Tcr in only 1 strain.