Over-the-counter (OTC) throat products may be contributing to antibiotic resistance, according results of a study presented at the 29th European Congress of Clinical Microbiology & Infectious Diseases, held April 13-16, 2019, in Amsterdam, The Netherlands

The overuse and misuse of antibiotics increases the risk of developing and spreading antibiotic resistance, which is a high priority issue for the World Health Organization. This phenomenon is especially relevant for viral and self-limiting respiratory infections such as sore throat. Approximately 80% of cases of sore throat have a viral etiology and cases in which the cause is a bacterial infection commonly resolve without antibiotics. However, many OTC products available for topical in sore throat contain locally delivered antibiotics. Therefore, this study aimed to understand the development of bacterial resistance in 4 common pathogens to 4 locally delivered antibiotics found in sore threat medicines and to measure cross-resistance.

The 4 common human pathogens studied were Staphylococcus aureus (ATCC6538), Acinetobacter baumannii (ATCC19568), Streptococcus pyogenes (ATCC19615), and Haemophilus influenza (ATCC10211). The 4 locally delivered antibiotics studied were gramicidin, neomycin, bacitracin, and tyrothricin. Bacteria were first exposed to decreasing concentrations of the antibiotics for 24 hours, then surviving bacteria were subcultured and tested for antibiotic susceptibility. If there was a change in antibiotic susceptibility after the 24-hour exposure, the bacteria were again subcultured and tested for a change in antibiotic susceptibility profile (susceptible/resistant) after the 1st, 5th, and 10th exposures using the EUCAST protocol.

For neomycin, bacitracin, and tyrothricin, the minimum inhibitory concentrations (MIC) for S aureus and A baumannii were below in-use antibiotic concentrations; no MIC value could be determined for gramicidin. For S pyogenes and H influenza, the MIC results were also below in-use antibiotic concentrations. Although S aureus did not grow in tyrothricin or neomycin, growth was observed at 100% (3 mg/mL), 90%, 75%, and 50% gramicidin after 24 hours. Similarly, S aureus growth was observed in bacitracin (250 U) after 144 hours and showed decreased clinical susceptibility to gentamicin, fusidic acid, and ciprofloxacin afterward. Further, although S pyogenes did not grow in gramicidin, bacitracin, or tyrothricin, growth was observed in neomycin at both 0.125 mg/mL and 0.025 mg/mL (5% and 1% of in-use concentration, respectively) after a 96-hour exposure, but no cross‑resistance was observed with other antibiotics. No cross-resistance with other antibiotics was seen after exposure of A baumannii to gramicidin in-use concentration (3 mg/mL) either. No observed H influenza growth was observed in any of the 4 antibiotics tested.

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Overall, the study authors concluded that, “Growth of common human pathogens occurred in the presence of some of the OTC antibiotics investigated and exposure to in-use and diluted concentrations of bacitracin was associated with clinical cross-resistance to other antibiotics. This raises doubt about the continued OTC availability of these antibiotics for sore throat.”

Reference

Westgate R, Evangelista C, Atkinson R, Shephard A, Adegoke O, Maillard J. Understanding the risk of emerging bacterial resistance to topical antibiotics. Presented at: 29th European Congress of Clinical Microbiology & Infectious Diseases; April 13-16, 2019; Amsterdam, The Netherlands. Poster 2749.

