Understanding the risk of emerging bacterial resistance to over the counter antibiotics in topical sore throat medicines.

AIMS: The aims of this study were to explore development of bacterial resistance and cross resistance in four common human pathogens following realistic exposure to antibiotics found in over-the-counter (OTC) sore throat medicines: gramicidin, neomycin, bacitracin and tyrothricin. METHODS AND RESULTS: Bacterial exposure to in-use (concentration in the product before use) and diluted concentration (i.e. during use) of antibiotic where conducted in broth for 24 h or until growth was visible. Changes in bacterial susceptibility profile before and after exposure was determined using standardised ISO microdilution broth. Antibiotic testing was performed according to EUCAST guidelines. We demonstrated that test bacteria were able to survive exposure to the in-use concentrations of some antibiotics used in OTC medicines. Exposure to during use concentrations of bacitracin resulted in stable increase in MIC (>8-fold) in S. aureus and A. baumannii. Exposure to tyrothricin resulted in a stable increase in MIC (2.4-fold) in K. pneumonia, and exposure to neomycin resulted in a stable increase MIC (5000-fold higher than the baseline) in S. pyogenes. Clinical cross-resistance to other antibiotics (ciprofloxacin, fusidic acid, gentamicin, cefpodoxime, amoxicillin/clavulanic acid, cefotaxime) was also demonstrated following exposure to bacitracin or tyrothricin. Bacitracin exposure lead to a stable bacterial resistance after ten passages. CONCLUSIONS: Our results indicate OTC antibiotic medicines have the potential to drive resistance and cross-resistance in-vitro. SIGNIFICANCE AND IMPACT OF THE STUDY: Tackling antibiotic resistance is a high worldwide priority. It is widely accepted that the overuse and misuse of antibiotics increase the risk of the development and spread of antibiotic resistance within communities. A number of over-the-counter (OTC) sore throat products, widely available across the world for topical use in respiratory indications, contain locally delivered antibiotics. Our findings showed that these antibiotics in OTC medicines present a risk for emerging cross-resistance in a number of bacterial respiratory pathogens.