Factores de confusión potenciales en los ensayos de susceptibilidad antibiótica utilizados actualmente para...

Introduction. Melissococcus plutonius is the causative agent of European foulbrood (EFB), a disease of honey bees that is endemic in many areas of the USA. Only one antibiotic, oxytetracycline (OTC), is approved for EFB management, and there have been reports of recalcitrance.Gap Statement. Resistant strains of M. plutonius have been identified in Canada and Japan, but methodology differs between studies, making reliable comparisons difficult. Additionally, no M. plutonius isolates from the USA have yet been tested for susceptibility to OTC, despite decades of use.Aim. Here, we determine the impact of media, time and persistence on the results of commonly used growth and antibiotic resistance assays using regionally representative M. plutonius isolates.Methodology. Twelve genetically diverse isolates of M. plutonius were tested for susceptibility to OTC using previously published assays, but with variations in media and time to determine factors that may be impacting results.Results. Media composition and incubation time dramatically impact antibiotic susceptibility assays for M. plutonius, differing widely between strains, likely due to differences in OTC stability. Assays that ended when growth appeared on antibiotic-free agar showed that all strains remained susceptible to OTC with an MIC of 2-4 µg ml^-1. However, M. plutonius remains viable after OTC efficacy wanes, with some strains able to persist at room temperature for at least 3.5 years.Conclusion. To standardize antibiotic susceptibility testing for M. plutonius, we recommend the use of M110 media due to stability and speed of growth. However, all strains of M. plutonius persist on M110 beyond the window of OTC efficacy, complicating assay results and interpretation, and additional research is needed to determine the clinical implications of these findings.