EFFECT OF PLASMIDS CONFERRING PRESERVATIVE RESISTANCE ON PERFORMANCE OF BACTERIAL STRAINS IN COMPENDIAL PRESERVATIVE EFFICACY TESTS

R plasmids, which increased resistance of Staphylococcus aureus and Es cherichia coli to a variety of pharmaceutical preservatives, were show n, using the protocol of the Preservative Efficacy Test of the British Pharmacopoeia (1988), to influence the antibacterial efficacy of 0.00 1% benzalkonium chloride, 0.001% cetrimide and 0.0001% phenyl mercuric nitrate solutions, but not 0.0005% chlorhexidine gluconate. Some stra ins, which failed the BP (1988) criteria for ophthalmic and parenteral products, satisfied the less stringent A or B criteria of the BP (199 3). However, the death rates of all strains tested complied with the c riteria of the BP (1988) for 0.003% benzalkonium chloride, 0.003% cetr imide and 0.001% phenylmercurie nitrate solutions. The data illustrate the undoubted effects R plasmids can have on the survival of bacteria contaminating pharmaceutical products but suggest that the levels of resistance conferred are not sufficient to affect, adversely; the acti vities of in-use concentrations that are required by compendial preser vative efficacy tests.