Povidone-iodine concentration and in vitro killing time of bacterial corneal ulcer isolates

The concentration and dosing of povidone-iodine (PI) solution used in surgical site prophylaxis are variable. Prior in vitro work has demonstrated that dilute PI solutions (<1%) had greater bactericidal activity than stock solutions (10%). Studies using pathologic clinical isolates from the eye have yielded mixed results. The purpose of the current study was to evaluate the efficacy of different concentrations of PI on pathologic ocular surface isolates.

We conducted an in vitro microbiology study using clinical isolates from corneal ulcers. Bacteria were recovered from trypticase soy agar with 5% sheep erythrocytes, chocolate agar, and thioglycollate broth media. A standardized concentration of each bacterial sample (1 × 10⁸ cfu/ml) was exposed to various dilutions of PI. Quantitative cultures were performed to determine the number of organisms surviving PI exposure.

None of the isolates survived exposure to the PI 0.25% solution for 30 seconds. Micrococcus luteus and Staphylococcus aureus survived both 30-second and 1-minute exposure to PI 5% and 10%. The exposure time required to produce no growth was variable with concentrations of <0.25%. In some isolates, the 10% solution was faster than the more dilute solutions (0.1%, 0.05%).

Our results are consistent with prior in vitro studies of PI, from nonocular sources, and suggest that PI has similar bactericidal action on pathologic bacteria from the ocular surface. In vitro exposure to dilute PI (0.25%) resulted in no growth after 30 seconds, whereas 10% and 5% solutions took longer to kill several of the isolates. Future investigations of PI use in ophthalmology as an antimicrobial agent should include the study of low-concentration PI (0.25%).