Polymyxin and Bacitracin in the Irrigation Solution Provide No Benefit for Bacterial Killing in Vitro

Background: Many surgeons add topical antibiotics to irrigation solutions assuming that this has a local effect and eliminates bacteria. However, prior studies have suggested that the addition of antibiotics to irrigation solution confers little benefit, adds cost, may potentiate anaphylactic reactions, and may contribute to antimicrobial resistance. We sought to compare the antimicrobial efficacy and cytotoxicity of an irrigation solution containing polymyxin-bacitracin with other commonly used irrigation solutions. Methods: Staphylococcus aureus and Escherichia coli were exposed to irrigation solutions containing topical antibiotics (500,000-U/L polymyxin and 50,000-U/L bacitracin; 1-g/L vancomycin; or 80-mg/L gentamicin), as well as commonly used irrigation solutions (saline solution 0.9%; povidone-iodine 0.3%; chlorhexidine 0.05%; Castile soap 0.45%; and sodium hypochlorite 0.125%). Following 1 and 3 minutes of exposure, surviving bacteria were manually counted. Failure to eradicate all bacteria in any of the 3 replicates was considered not effective for that respective solution. Cytotoxicity analysis in human fibroblasts, osteoblasts, and chondrocytes exposed to the irrigation solutions was performed by visualization of cell structure and was quantified by lactate dehydrogenase (LDH) activity. Efficacy and cytotoxicity were assessed in triplicate experiments, with generalized linear mixed models. Results: Polymyxin-bacitracin, saline solution, and Castile soap at both exposure times were not effective at eradicating S. aureus or E. coli. In contrast, povidone-iodine, chlorhexidine, and sodium hypochlorite irrigation were effective against both S. aureus and E. coli (p < 0.001). Vancomycin irrigation was effective against S. aureus but not against E. coli, whereas gentamicin irrigation showed partial efficacy against E. coli but none against S. aureus. Within fibroblasts, the greatest cytotoxicity was seen with chlorhexidine (mean [and standard error], 49.38% +/- 0.80%; p < 0.0001), followed by Castile soap (33.57% +/- 2.17%; p < 0.0001) and polymyxin-bacitracin (8.90% +/- 1.40%). Povidone-iodine showed the least cytotoxicity of the efficacious solutions (5.00% +/- 0.86%). Similar trends were seen at both exposure times and across fibroblasts, osteoblasts, and chondrocytes. Conclusions: Irrigation with polymyxin-bacitracin was ineffective at bacterial eradication, and statistically inferior to povidone-iodine. Chlorhexidine lavage conferred the greatest in vitro cytotoxicity.