Replicon sequence typing of IncF plasmids carrying virulence and resistance determinants.

OBJECTIVES IncF plasmids are frequently encountered in clinical enterobacterial strains associated with the dissemination of relevant antimicrobial resistance and virulence genes. These plasmids are usually heterogeneous in size and carry multiple replicons, and technical difficulties can impair the comparison and detection of related plasmids by restriction fragment length polymorphism analysis. We devised a rapid sequence-based typing scheme to categorize the members of this plasmid family into homogeneous groups. METHODS We compared the available IncF replicon sequences, identifying the combination of the different IncF replicon alleles as the discriminating characteristic of these plasmid scaffolds. An IncF typing method based on PCR amplification and sequence typing of the IncF replicons was devised. A collection of IncF plasmids carrying resistance and/or virulence genes, identified in strains from different sources and geographical origins, was tested with this typing system. RESULTS We devised a replicon sequence typing (RST) scheme discriminating IncF plasmid variants. This system was tested on the collection of IncF plasmids, demonstrating that it was useful for the discrimination of plasmids carrying the same resistance gene (i.e. the bla(CTX-M-15) gene), but also recognized strictly related virulence plasmids (i.e. IncFIme plasmids). The PCR-based replicon typing (PBRT) system was also updated by including new primer pairs to allow the identification of the Salmonella, Klebsiella and Yersinia IncF plasmids. CONCLUSIONS The ability to recognize and sub-categorize IncF plasmids by RST in homogeneous groups on the basis of their phylogenetic relatedness can be helpful in analysing their distribution in nature and discovering their evolutionary origin.