Epidemiology and susceptibilities to mercury preservatives of staphylococci isolated from used eye-drops preserved with thiomersal.

Minimum inhibitory concentrations (MICs) of seven independent isolates of Staphylococcus hominis isolated in the same week from used eye-drops, preserved with thiomersal and collected from wards and clinics in the same hospital, ranged between 1 and 0.03 mg L-1 for thiomersal, 1 and 0.01 mg L-1 for phenyl mercuric nitrate and 10 and 3 mg L-1 for mercuric chloride. Although MIC values determined on solid nutrient medium indicated a 100-fold variation in susceptibility to the bacteriostatic effect of phenyl mercuric nitrate, after 5 h in an aqueous solution containing the bactericidal concentration of 10 mg L-1 phenyl mercuric nitrate, the survival levels of the six S. hominis isolates were similar, with a mean of 13.4% (s.d. 11.0), compared with 100 and 0.8%, respectively, for the most resistant and most sensitive control staphylococcal strains tested. Antibiotic susceptibilities and plasmid profiles of the S. hominis isolates indicated they were the same strain. It is concluded that laboratory indicators of preservative efficacy, such as MIC determination or susceptibility to bactericidal concentrations of preservatives, do not necessarily correlate with the epidemiology of contaminating bacterial strains or their survival in preserved pharmaceuticals.