Isolation and characterization of a novel mcr-5 carrying Escherichia coli plasmid from chicken feces in Germany

Questions: Colistin is considered as an important antibiotic of the last-resort, which will be only used for the treatment of severe human infections with multidrug-resistant Gram-negative bacteria. Since 2015, several mobile colistin resistance genes were described coding for enzymes of the phosphoethanolamine-transferase family. To date, eight different mcr-genes have been characterized, mediating resistance to colistin in different bacterial genera (especially in Enterobacteriaceae).

Material & Methods: By molecular screening on mcr-1 to -5 using the multiplex PCR of Rebelo et al. (2018), an E. coli isolate recovered in 2013 from chicken feces was identified to carry a mcr-5 resistance gene. Antimicrobial resistance testing according to the CLSI-guideline was performed. MIC-data were interpreted using the ECOFFS of EUCAST. The genome of the mcr-5-plasmid was deduced by whole-genome sequencing using different platforms (MiSeq, Illumina and MinIon, Nanopore). Bioinformatic analyses were performed to determine the genome structure and composition of the plasmid and isolate.

Results: Within this study, a novel mcr-5 plasmid-prototype was identified in the E. coli isolate from the German national monitoring of zoonoses in food and livestock in 2013/2014. The genome of the plasmid pEC1897-13 was 38 kb in size. Bioinformatics revealed that the plasmid belongs to the IncFII group, but represents a novel pMLST-allele that is closely related to the allele FII-82. Interestingly, pEC1897-13 obviously comprises all necessary components of a functional IncF conjugative-transfer system. However, up to now no self-transmission of the plasmid was observed by filter mating studies.

Conclusion: The impact of the plasmid pEC1897-13 for the transmission of colistin resistance is unknown. In contrast to most of the described mcr-5 carrying plasmids, pEC1897-13 carries a complex IncF-like transfer system that might be functional under specific circumstances although currently it is not transferred under tested experimental conditions.