Resistance transfer in mixed cultures of Staphylococcus aureus

Transfer of resistance markers was regularly observed in mixed cultures of pairs of constructed strains of Staphylococcus aureus. This transfer was shown to be due to general transduction and required a lysogenic donor strain, carrying a transducing phage of serotype B and a transducible plasmid, and an acceptor strain without either strong restriction or an incompatible plasmid. Transfer was sometimes bidirectional when both strains carried a transducing phage. Unidirectional transfer occurred when the acceptor strain was immune. When the acceptor was sensitive to the donor phage, reversed transfer of a marker from the acceptor to the donor could take place. The kinetics of transfer are governed by the rate of phage production, the rate of phage adsorption and the frequency of transduction. A mathematical model has been developed to describe the interplay of these factors. The numbers of transductants observed in mixed cultures agreed with those expected from the model. Transfer was also observed in mixed cultures of constructed strains and wild-type strains. Transfer between wild-type strains is also possible, but was not observed in many combinations because of lack of transducing phage, strong restriction or other factors. Transfer was also observed in local lesions in mice infected with two strains and in mixtures inoculated on to human skin.