Studies on recently isolated cultures of methicillin-resistant Staphylococcus aureus

Of 19 recently isolated cultures of methicillin-resistant Staphylococcus aureus, 18 showed inducible low-level resistance to minocycline, 15 showed high-level resistance to streptomycin, and 4 showed resistance to low levels of streptomycin. Two cultures produced yellow pigment and may have been derived in vivo by loss of a gene(s) determining orange pigment. Treatment of three cultures with serial exposures to N-methyl-N'-nitro-N-nitrosoguanidine resulted in a widening of phage typing pattern that included all reactions in group I, the great majority in group III, but none in group II. The widening in phage lysis was possibly due to the elimination of defective prophages. Transfer of tetracycline resistance occurred from 12 out of the 19 cultures to a recipient in mixed culture; this transfer required either Ca2+ or Mg2+, was abolished by citrate, and enhanced by high cell density. It was probably mediated by defective bacteriophages. No evidence was obtained for the occurrence of recombination within the methicillin-resistant clone in nature. Eleven methicillin-resistant cultures stored for at least 5 years on agar slopes at 20 degrees C had all lost this resistance at high frequency.