with those incubated in their absence. There was no difference in the colony counts, suggesting that these agents did not produce proliferation of S.

Drug resistance of staphylococci. II. Joint elimination and joint transduction of the determinants of penicillinase production and resistance to macrolide antibiotics. J. Bacteriol. 89:988-992. 1965.-Strains of Staphylococcus aureus, which show high resistance to macrolide antibiotics (erythromycin, oleandomycin, leucomycin, and spiramycin) and the capacity to produce penicillinase, have been isolated from clinical sources. The determinants of penicillinas] production (PCase+) and resistance to macrolide antibiotics (MACr) of these strains were irreversibly eliminated by treatment with acridine or with ultraviolet light. Among the 18 strains tested, PCase+ and MACr were eliminated from all strains except one, which lost only PCase+ but not MACr. The characters PCase+ and MACr were jointly transduced with the aid of phage lysates, obtained from the resistant donors by ultraviolet irradiation, into staphylococcal strains sensitive to PC and MAC. Segregation of PCase+ and MACr was rarely observed after transduction. From these results, it is suggested that the determinants of both PCase+ and MACr of staphylococci are located close together on a single genetic element, i.e., a plasmid (or episome), which exists extrachromosomally.

The determinant for streptomycin resistance in some strains of Staphylococcus aureus isolated from clinical specimens was found to be lost during storage as stock cultures or by high temperature treatment. This fact has added an additional proof that many genetic determinants for drug-resistance in staphylococci are easily and irreversibly lost.

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