In Vitro Evaluation of the Synergistic Activity of Neomycin-Polymyxin B Association against Pathogens Responsible for Otitis Externa

The most recent guidelines recommend, for otitis externa antibiotic therapy, the use of topical formulations in that they are very safe, have a quicker effect and do not induce bacterial resistance compared to systemic therapy. The choice of the class of antibiotics in empiric therapy of otitis externa must take into consideration the polymicrobic nature of the infection that includes both bacteria (Grampositive and Gram-negative) and mycetes. For this reason, in this study we evaluated the synergic activity of neomycin in association with polymyxin B against the pathogens commonly responsible for otitis externa, compared to that of a single antibiotic (ciprofloxacin). The polymyxinB/neomycin association shows clear synergic effects with values of both Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) reduced by 3-4 times with respect to the single antibiotic; and in P. aeruginosa the synergistic effect of the neomycin/polymyxin B association with respect to neomycin was more evident (5-6 times), with an intrinsic in vitro activity constantly higher than that of ciprofloxacin alone or in association with hydrocortisone. From the analysis of the data obtained in vitro, we can conclude that the possibility of using a topical formulation containing a synergistic association of antibiotics, such as neomycin-polymyxin B, in such a way as to obtain the maximum effect in the minimum time with an increase in the spectrum of action of non-bacterial pathogens, is an optimal choice for the clinician for the empiric therapy of otitis externa.