Associated Diploids Involving Penicillinase Plasmids in Staphylococcus aureus

SUMMARY Staphylococcus aureus penicillinase plasmid diploids of the type (α. i+p+ A/β.i–pC+), when constructed in strain 147, normally segregated one of the two parental genotypes at a frequency of about 1/5000 divisions, and recombinants were rare. In about 2% of the diploid clones, however, segregation occurred at a much higher frequency and in these clones both plasmids might be lost together. Examination of these ‘unstable’ diploids suggests that their two plasmids are associated to form a single structure, so that they are subsequently lost or transduced together. The ease of formation of associated diploids suggests that the penicillinase plasmids may be circular.