microbiological evaluation of some ophthalmic preparations containing different concentrations of benzalkonium chloride

zinc sulphate and boric acid ophthalmic preparations, containing benzalkonium chloride as preservative, have been investigated against gram-negative and positive bacteria. viability of bacteria was found after 7 days, although it was completely suppressed after 24 hours of inoculation. zinc sulphate generally increases the antimicrobial activity of benzalkonium chloride, whereas boric acid or its combination with zinc sulphate reduce it. pseudomonas aeruginosa was shown to be viable against zinc sulphate, boric acid, and their combination at a concentration of 0.0 i % to 0.005% of the preservative. escherichia coli and staphylococcus aureus were also found to be resistant but to a lesser extent than pseudomonas aeruginosa. this viability may be dangerous in case of multidose ophthalmic preparations.