[Plasmid resistance to antiseptics. Demonstration of methods]

Susceptibility to chlorhexidine, benzalkonium chloride and mercury chloride was studied for 70 Gram negative strains (56 Enterobacteriaceae and 14 Pseudomonas aeruginosa) recovered from humans or animals. Minimal inhibitory concentration (MIC) was determined on solid agar (replica plating) and in a liquid medium (laser nephelometry). For resistant strains, plasmidic mediation was looked for using conjugation. Isolation of lysates by the miniprep analytic method and physicochemical characterization of plasmids by agar electrophoresis were carried out using conjugative strains. MICs obtained using the two methods differed by one dilution at the most. Both methods yielded reproducible results. Regardless of the origin of strains, a resistant population was identified only for mercury chloride. This resistance was plasmid-mediated and transferred singly or associated with resistance to other antibiotics.