Transduction of antibiotic resistance markers among Actinobacillus actinomycetemcomitans strains by temperate bacteriophages Aaφ23

Actinobacillus actinomycetemcomitans (Aa) strain ST1 carries the tetracycline (Tc) resistance transposon Tn916 and the Aaφ ST1 prophage, which is closely related to temperate bacteriophage Aaφ23. High titre phage preparations were obtained from this strain by mitomycin C induction and were used to transduce the TcR determinant to the TcS recipient strains ZIB1001 and ZIB1015 (MIC 2 μg Tc/ml). TcR transductants (MIC ≥ 32 μg Tc/ml) were detected at frequencies of 3 × 10−6 to 5 × 10−8 per pfu. All TcR transductants examined contained the entire Tn916 inserted at several different locations within the Aa genome. They appear to have resulted from generalized transduction. In addition both bacteriophages, Aaφ23 and AaφST1, were capable of transducing the chloramphenicol (Cm) resistance marker of plasmid pKT210 (transduction frequencies of 2 × 10−5 to 3 × 10−7 per pfu) to the recipient strain ZIB1001 (MIC 8 μg Cm/ml). Eleven CmR ZIB1001 transductants (MIC ≥ 100 μg Cm/ml) studied carried a plasmid indistinguishable from pKT210 by restriction analyses. In view of the high prevalence of this phage family, and the increasing use of tetracycline in periodontitis therapy, these findings may have clinical importance.