Enteropathogenic E. coli (EPEC) is one of the leading causes of lethal diarrheal disease among infants in developing countries. As one of the attaching and effacing E. coli (AEEC, along with enterohemorrhagic E. coli or EHEC), EPEC is characterized, in part, by the locus of enterocyte effacement (LEE) pathogenicity island. This group of chromosomally-encoded genes are required for EPEC to cause disease and includes the intimin protein, used in adherence, and a type III secretion system that delivers effector proteins in to the host cell. Plasmid-encoded genes supplement the EPEC arsenal and virulence plasmids, such as the EPEC adherence factor (EAF) virulence plasmid, have been the focus of several studies. However, despite growing concerns about antibiotic resistance in human-associated bacteria, little is known about the diversity and gene content of resistance plasmids in EPEC and related pathogens.

A recent paper by Tracy Hazen and others seeks to correct this gap in knowledge. In it, she describes the sequence of the 90,229 bp resistance plasmid (called pB171_90) isolated from EPEC strain B171. This plasmid contains genes that confer resistance to antibiotics such as spectinomycin-streptomycin, tetracycline, and sulfonamides, as well as narrow spectrum mercury resistance. By comparing the gene content of the pB171_90 plasmid to genes in other E. coli, Tracy and colleagues found that the pB171_90 plasmid genes differed in their distributions. This mosaic plasmid combines genes common to AEEC strains with antibiotic resistance genes that are more frequently associated with non-AEEC strains. Genes found in this plasmid were more widespread than expected. Several genes thought to be unique to EPEC B171 were found throughout E. coli as well as other enteric bacteria. The high sequence similarity between this plasmid and those in other strains suggests that resistance plasmids have been exchanged between EPEC and other gastrointestinal pathogens EHEC O157:H7 and S. dysenteriae. This study highlights the need for sequence-based characterization of resistance plasmids to characterize the complex compositions and distributions of resistance plasmids in pathogenic bacteria.

Hazen, T. H. et al. (2017) Characterization of a large antibiotic resistance plasmid found in enteropathogenic Escherichia coli strain B171 and its relatedness to plasmids of diverse E. coli and Shigella. Antimicrob Agents Chemother doi:10.1128/AAC.00995-17