Oxytetracycline-resistance in the honey bee pathogen Paenibacillus larvae is encoded on novel plasmid pMA67

The gram-positive bacterium, Paenibacillus larvae, causes a serious honey bee disease, American Foulbrood. For several decades, commercial and hobbyist beekeepers have controlled this disease with the antibiotic oxytetracycline. However, in recent years there have been reports of oxytetracycline-resistant P larvae. In this study, we report that the reason for the oxytetracycline resistance in P larvae is the presence of a novel plasmid carrying a tetracycline resistance gene-tetL. We tested 36 strains of P. larvae from the USA and Canada and found this plasmid in all 21 oxytetracycline-resistant strains and in none of the 15 oxytetracycline-sensitive strains. We cloned and expressed the P larvae tetL gene in Escherichia coli and showed it was functional. Sequencing of the entire plasmid, which we named pMA67, revealed that it is likely a mobilizable rolling-circle replication plasmid. This work provides the first sequence information for any P. larvae plasmid, a new tetL ortholog with significant sequence divergence from tetL genes found in other species, and the first tetracycline-resistance gene found in the entire Paenibacillus genus.