ANTIBIOTIC SUPPLEMENTATION OF INTRAOCULAR IRRIGATING SOLUTIONS - AN IN-VITRO MODEL OF ANTIBACTERIAL ACTION

Purpose: The addition of antibiotics to infusion solutions for catarac t surgery is becoming increasingly popular. The authors developed an i n vitro model to evaluate antibacterial effects of this use of antibio tics. Methods: Clinical isolates and/or reference strains of the follo wing organisms were examined: coagulase-negative Staphylococcus spp, S taphylococcus aureus, Streptococcus pneumoniae, Streptococcus viridans , Streptococcus spp, Enterococcus spp, Proprionibacterium acnes, Morax ella nonliquifaciens, and Pseudomonas aeruginosa. Standardized suspens ions of each organism were incubated with a control solution (Balanced Salt Solution) or Balanced Salt Solution containing the following ant ibiotics: vancomycin (20 mu g/ml) or gentamicin (8 mu g/ml) or gentami cin and vancomycin combined (8 and 20 mu g/ml, respectively). Suspensi ons were incubated for 30, 60, acid 120 minutes at room temperature. S amples were centrifuged, and the organisms were washed with Balanced S alt Solution before quantitative culturing. Each organism also was inc ubated for 48 hours in Mueller-Hinton broth with the same antibiotic c oncentrations. Results: Most of the organisms were not affected by exp osure to the antibiotics for up to 140 minutes. P. aeruginosa and M. n onliquifaciens were exceptions, decreasing in colony numbers even with 30 minutes of exposure. Several Staphylococcus spp yielded variable r esults. All organisms demonstrated nearly complete inhibition of growt h when exposed for an extended time to the appropriate antibiotic in b roth. Conclusions: Exposure to antibiotics for a short period of time, such as during intraocular surgery, generally has no effect on organi sms commonly responsible for endophthalmitis. The use of antibiotics i n this manner should be critically reassessed until further study.