Evidence for two mechanisms of plasmid transfer in mixed cultures of Staphylococcus aureus

Non-lysogenic Staphylococcus aureus strain 1030 was lysogenized with 12 different bacteriophages. Lysogeny was associated with acquisition of phase inducibility to mitomycin C treatment or ultraviolet irradiation, with the presence of plaque-forming phage in culture supernatants and with considerable narrowing in susceptibility to the typing bacteriophages and also with increased sensitivity to trimethoprim and sulphadiazine. The presence of prophages in the donor and/or the recipient could either promote or inhibit transfer of plasmids between mixed cultured. Transfer frequencies in mixed culture after 18 h incubation could be as high as 7 . 0 x 10(-1) resistant recipients/final donor, and evidence was adduced for a mechanism distinct from transfer by spontaneous transduction. It is suggested that this method of gene transfer be described as 'phage-mediated conjugation'. Chromosomal genes were not transferred by this method. Two similar plasmids cp-2 and cp-3 were able to promote their own transfer through clones of 1030; plasmids coding for resistance to either neomycin or tetracycline could be transferred to a recipient by the presence of cp-2 or cp-3 simultaneously in the donor. The presence of plasmids in 1030 was associated with a small increase in sensitivity to trimethoprim or sulphadiazine.