The antibacterial activity of topical anesthetics

Topical anesthetics are commonly used prior to obtaining bacterial cultures in ulcerative keratitis. We performed an in vitro study designed to test both the bacteriostatic and bactericidal effects of commercially available preserved topical anesthetic agents.

Proparacaine, tetracaine, cocaine, and sterile water solutions were applied to filter paper disks, which were then placed on Mueller-Hinton agar plates that had previously been inoculated with known quantities of Pseudomonas aeruginosa and Staphylococcus aureus. After 24 h of incubation, zones of inhibition were measured and recorded.

Proparacaine strongly inhibited the growth of S. aureus at all concentrations (0.5%, 0.25%, 0.125%) and inhibited growth of P. aeruginosa at 0.5% and 0.25% but not at 0.125% concentration. Tetracaine also inhibited S. aureus at 0.5% and inhibited P. aeruginosa at 0.5% and 0.25% concentrations. Cocaine exhibited no inhibition of S. aureus and exhibited mild inhibition of P. aeruginosa growth only at the 4% concentration.

The in vitro antibacterial effect of topical anesthetics suggests one possible reason why bacterial culture yields in clinical ulcerative keratitis are suboptimal. We propose that clinicians consider the use of a 1% or 2% cocaine solution instead of standard commercial topical anesthetics in the management of individual cases of ulcerative keratitis and in future clinical bacterial keratitis studies.