Seven-day viable analysis of some ophthalmic preparations with different concentrations of benzalkonium chloride.

The efficacy of zinc sulphate and boric acid ophthalmic preparations, containing benzalkonium chloride as preservative have been investigated against Gram-negative Escherichia coli and Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus bacteria. The viability of bacteria was checked after 7 days though they were completely suppressed after 24 hours of inoculation. Zinc sulphate increased the antimicrobial activity of benzalkonium chloride, whereas the boric acid or its combination with zinc sulphate reduced the same. The Pseudomonas aeruginosa was found to be viable against zinc sulphate, boric acid or its combination at a concentration of 0.01% to 0.005% of the preservative. Escherichia coil and Staphylococcus aureus also exhibited resistance but to a lesser extent than Pseudomonas aeruginosa. This viability may be dangerous in case of multidose ophthalmic preparations.