Declining susceptibility to neomycin and polymyxin B of pathogens recovered in otitis externa clinical trials

Background: Otitis externa is usually treated empirically with topical neomycin/polymyxin B/hydrocortisone. The predominant pathogens associated with this infection are Pseudonionas aeruginosa and Staphylococcus aureus. Methods: Two multicenter clinical trials (one in adults and adolescents, and one in children), conducted between 1995 and 1996, compared neomycin/polymyxin B/hydrocortisone with ofloxacin for the treatment of otitis externa; two similar trials were conducted between 1999 and 2000. Assessments included the minimum inhibitory concentrations (MICs) of each antimicrobial drug for the major pathogens, bacterial eradication, and clinical efficacy. Results: The MICs of all bacterial isolates (including P aeruginosa) for neomycin and polymyxin B increased markedly in the 1999 to 2000 studies compared with the 1995 to 1996 studies. In the later studies, mean MICs for all major pathogens tested had increased above the breakpoint for polymyxin B ( greater than or equal to 4 mug/ml). In contrast, MICs of all isolates for ofloxacin remained similar between the two study periods and were within the susceptible range for this drug. Conclusions: Although the bacterial eradication rates for both treatments in each study were equivalent, the clinical cure rate for neomyciti/polymyxin B/hydrocortisone was lower (87%) than for ofloxacin (93%). Therefore, the organisms most often causing otitis externa appear to be developing resistance to neomycin and polymyxin B but not to ofloxacin.