In vitro susceptibility of ciprofloxacin-resistant strains of Pseudomonas aeruginosa to high concentration ciprofloxacin-containing ototopical solution

Context: Pseudomonas aeruginosa is the most frequently isolated pathogen in acute otitis externa (AOE), with ototopical antibiotics being the mainstay of treatment. The incidence of ciprofloxacin resistance in P. aeruginosa has been steadily increasing. This study aimed to evaluate the in vitro susceptibility of P. aeruginosa to a ciprofloxacin-containing ototopical solution. Methods: In this prospective study we examined the in vitro growth of ciprofloxacin-resistant P. aeruginosa isolates when exposed to varying concentrations of a ciprofloxacin-containing ototopical solution. Results: Concentration of the ototopical solution was strongly associated with the diameter of inhibition on sensitive strains. Sensitive strains of P. aeruginosa showed inhibition of growth with an extrapolated MIC value of 0.0686 µg/µL. Resistant strains demonstrated inhibition to all dilutions of the ototopical solution, including undiluted concentration. Ciprofloxacin-sensitive strains demonstrated resistance at higher ototopical concentrations compared with the reported serum concentration after standard systemic administration. Conclusions: The current use of commercially available ciprofloxacin topical ear drops for treating AOE does not appear to hold promise for the fight against resistant strains of P. aeruginosa and might be at risk of becoming ineffective in the treatment of sensitive species. Therefore, further investigation is needed to gain a deeper understanding of ototopical solutions, as well as to establish a database of MICs for topical antibiotics.