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SUMMARY: Attempts were made to obtain Gram-negative rods from the Oxford strain of Staphylococcus aureus in the presence of benzylpenicillin. Two such isolations were made in 476 test cultures and one in 192 controls. Thedifferenceisnot significant. No reversion to coagulase-positive staphylococci could be demonstrated of a rod- shaped Gram-negative organism previously reported as showing this effect. The question of the presence of Gram-negative rods in staphylococcal cultures exposed to penicillin is discussed and it is considered that the phenomenon is due to contamination. The isolation of Gram-negative rods from a strain of Staphylococcus aureus serially subcultured many times in the presence of increasing concentrations of penicillin was first reported by Klimek, Cavallito & Bailey (1948). Similar observations were described by Gale & Rodwell (1949) and by McVeigh & Hobdy (1952). More recently, it was reported that Gram-negative rods could be found at various times during the development of individual cultures of S. aureus in the presence of benzylpenicillin, cephalosporin C or bacitracin (Crawford & Abraham, 1957), and this phenomenon was studied further, with special precautions against contamination, by Briggs, Crawford, Abraham &- Gladstone (1957). They used only benzylpenicillin and were able to isolate Gram-negative rods with some regularity from overnight cultures, even when the cultures had been derived from single-cell isolations of the staphylococcal strain. Single-cell isolations were also made of the Gram-negative rods, and during subsequent maintenance of these cultures under different conditions Gram-positive cocci resembling the original strain were once more isolated from them. There can be no doubt that it is possible to isolate Gram-negative rods from penicillin-treated cultures of Staphylococcus aurew : the finding is, however, open to two interpretations. One is that contamination becomes manifest through the selective action of the drug against the original inoculum; the second, that the rods are in some way directly derived from the staphylococci by mutational or adaptive mechanisms. In view of the special precautions taken by Briggs et al. and their report of the reappearance of staphylococci in bacillary cultures, their work provides the strongest evidence hitherto that the second interpretation may be the correct one. In view of the fundamental importance of the problem an attempt to confirm their findings was made.

An Investigation into the Development of Gram-negative rods in Penicillin-treated Cultures of Staphylococcus aureus