Evolution of colicin BM plasmids: the loss of the colicin B activity gene

Abstract

Colicins, a class of antimicrobial compounds produced by bacteria, are thought to be important mediators of intra-and interspecific interactions, and are a significant factor in maintaining microbial diversity. Colicins B and M are among the most common colicins produced by Escherichia coli, and are usually encoded adjacently on the same plasmid. In this study, the characterization of a collection of E. coli isolated from Australian vertebrates revealed that a significant fraction of colicin BM strains lack an intact colicin B activity gene. The colicin B and M gene region was sequenced in 60 strains and it was found (with one exception) that all plasmids lacking an intact colicin B activity gene have an identical colicin gene structure, possessing a complete colicin B immunity gene and a 130 bp remnant of the B activity gene. A phylogenetic analysis of the colicin M and B operons and characterization of the plasmids suggested that ColBM plasmids with a truncated B activity gene have evolved on at least three separate occasions. Colicin B immunity was found to be non-functional in strains that have lost colicin B activity, and colicin M was still produced despite the absence of the SOS box believed to regulate its production in colicin BM strains. The presence of a remnant of the microcin V operon next to the truncated colicin B activity gene indicated that these plasmids evolved as a consequence of gene transfer between colicin BM and microcin V plasmids. We suggest that these transfer events most likely involved the transfer of some microcin V genes and associated virulence factors onto ColBM plasmids.

Pathogenomics of the Virulence Plasmids ofEscherichia coli

“…This supports the idea that these plasmids have arisen from a RepFIB/FIIA ancestral plasmid on multiple occasions. This is also supported by the findings by Christenson and Gordon showing that ColBM plasmids have arisen on at least three separate occasions (31). The coevolution of similar plasmid types on multiple occasions also provides further evidence that the possession of a particular set of core virulence factors provides the host bacterium with the tools that it needs to cause a specific disease type.…”

CbrA Is a Flavin Adenine Dinucleotide Protein That Modifies the Escherichia coli Outer Membrane and Confers Specific Resistance to Colicin M

“…Nearly half of the E. coli natural isolates produce colicins, and pColBM plasmids are among the most frequently occurring plasmids (11). The large pColBM plasmids encode virulence factors, such as iron transport systems, hemolysins, hemagglutinins, and a complement resistance factor, which are lost when pColBM is lost.…”

“…olicin M (Cma) is frequently encoded on plasmids of naturally occurring Escherichia coli isolates (11). The encoding gene, cma, usually forms an operon with the gene encoding colicin B, cba.…”

Molecular Characterization of Commensal Escherichia coli Adapted to Different Compartments of the Porcine Gastrointestinal Tract

“…Further, most of the plasmids encoding the bacteriocins Ia/Ib, B, and M are F-type plasmids (6,18). Further, it is well known that colicins B and M almost always co-occur (6). Colicins E2 and E7 are typically encoded on small nonconjugative plasmids.…”