Two independent conjugal transfer systems operating in Bacteroides fragilis V479-1

Bacteroides fragilis V479-1 (also designated strain 92) has previously been shown to contain a conjugative plasmid, pBF4, that specifies resistance to clindamycin (Cc). A report of inducible tetracycline (Tc) resistance in this strain suggested that this phenotype was also plasmid associated (G. Privitera et al., Nature [London] 278:657-659, 1979) and prompted further investigation. Mating experiments with V469-1 and a Cc-sensitive derivative of V479-1, V598, showed that Tc resistance transfer occurred by a conjugation-like event which was insensitive to DNase, was not mediated by donor culture cell-free filtrates, and required cell-to-cell contact. Results from transfer experiments with V479-1 indicated that Tc and Cc resistance determinants were not linked and segregated independently in matings. Progeny recovered from matings with the V479-1 or V598 donor strain were able to transfer the Tc resistance marker in secondary crosses. Tc resistance transfer from V479-1 or V598 was greatly stimulated by pregrowth in the presence of Tc but not Cc. pBF4-mediated Cc resistance transfer was not affected by pregrowth in the presence of Cc or Tc. Filter blot DNA hybridization studies revealed that pBF4 sequences were not involved in either the Tc resistant phenotype or its associated conjugal transfer properties. The Tc resistance transfer element was not associated with pBF4 or any other extrachromosomal DNA element.