IncFIB-4.1 and IncFIB-4.2 Single-Replicon Plasmids: Small Backbones with Large Accessory Regions

Purpose: To establish a typing scheme for IncFIB replicon and to dissect genomic features of IncFIB-4.1/4.2 single-replicon plasmids. Methods: A total of 146 representative fully sequenced IncFIB-replicon-containing plasmids were selected to construct a phylogenetic tree of repB(IncFIB) sequences. A collection of nine IncFIB-4.1/4.2 single-replicon plasmids from China were fully sequenced here and compared with the first sequenced IncFIB-4.1/4.2 single-replicon plasmids from GenBank to dissect their genomic diversity. Results: In this study, a repB sequence-based scheme was proposed for grouping IncFIB replicon into seven primary types and further into 70 subtypes. A collection of nine IncFIB-4.1/4.2 single-replicon plasmids were fully sequenced here and compared with the first sequenced IncFIB-4.1/4.2 single-replicon plasmids from GenBank. These 11 plasmids had small backbones and shared only three key backbone markers repB together with its iterons, parABC, and stbD. Each plasmid contained one large accessory region (LAR) inserted into the backbone, and these 11 LARs had significantly distinct profiles of mobile genetic elements (MGEs) and resistance/metabolism gene loci. Antibiotic resistance regions (ARRs; the antibiotic resistance gene-containing genetic elements) were found in seven of these 11 LARs. Besides resistance genes, ARRs carried unit or composite transposons, integrons, and putative resistance units. IncFIB-4.1/4.2 single-replicon plasmids were important vectors of drug resistance genes. This was the first report of three novel MGEs: In1776, Tn6755, and Tn6857. Conclusion: Data presented here provided a deeper insight into diversity and evolution of IncFIB replicon and IncFIB-4.1/4.2 single-replicon plasmids.