Abstract
Bacterial surface polysaccharides play significant roles in fitness and virulence. In Gram-negative bacteria such as Escherichia coli, major surface polysaccharides are lipopolysaccharide (LPS) and capsule, representing O- and K-antigens, respectively. There are multiple combinations of O:K types, many of which are well-characterized and can be related to ecotype or pathotype. In this investigation, we have identified a novel O:K permutation resulting through a process of major genome reorganization in a clade of E. coli. A multidrug-resistant, extended-spectrum β-lactamase (ESBL)-producing strain – E. coli 26561 – represented a prototype of strains combining a locus variant of O89 and group 1 capsular polysaccharide. Specifically, the variant O89 locus in this strain was truncated at gnd, flanked by insertion sequences and located between nfsB and ybdK and we apply the term O89m for this variant. The prototype lacked colanic acid and O-antigen loci between yegH and hisI with this tandem polysaccharide locus being replaced with a group 1 capsule (G1C) which, rather than being a recognized E. coli capsule type, this locus matched to Klebsiella K10 capsule type. A genomic survey identified more than 200 E. coli strains which possessed the O89m locus variant with one of a variety of G1C types. Isolates from our collection with the combination of O89m and G1C all displayed a mucoid phenotype and E. coli 26561 was unusual in exhibiting a mucoviscous phenotype more recognized as a characteristic among Klebsiella strains. Despite the locus truncation and novel location, all O89m:G1C strains examined showed a ladder pattern typifying smooth LPS and also showed high molecular weight, alcian blue-staining polysaccharide in cellular and/or extra-cellular fractions. Expression of both O-antigen and capsule biosynthesis loci were confirmed in prototype strain 26561 through quantitative proteome analysis. Further in silico exploration of more than 200 E. coli strains possessing the O89m:G1C combination identified a very high prevalence of multidrug resistance (MDR) – 85% possessed resistance to three or more antibiotic classes and a high proportion (58%) of these carried ESBL and/or carbapenemase. The increasing isolation of O89m:G1C isolates from extra-intestinal infection sites suggests that these represents an emergent clade of invasive, MDR E. coli.
Highlights
Escherichia coli is a much-studied bacterial species yet remains incompletely understood
Culture of 26561 on a range of agar media revealed a colony characteristic that resembled the “hypermucoviscous” (HMV) phenotype displayed by certain Klebsiella pneumoniae strains (Fang et al, 2004; Lee et al, 2006) which presents as a mucoid “string” of ≥5 mm adherent to a microbiological loop
For E. coli 26561, viscosity assays showed three main groupings of medium-dependent relative viscosity: LOW values were obtained during growth in LB, TSB, MHB, and NB; MEDIUM values were obtained by culture in PCB and defined medium sourced from either Sigma-Aldrich (DMEM-S); HIGH values through culture in M9 minimal medium, MacConkey broth or DMEM-T
Summary
Escherichia coli is a much-studied bacterial species yet remains incompletely understood This is exemplified by E. coli’s “open” genome such that – despite greater than 10,000 genome sequences (complete and incomplete) being readily accessible – sequencing of any further isolates can be predicted to add new content to the pan-genome (Rasko et al, 2008). This extent of genome sequence resource is leading to identification of novel genomic features as well as providing for systematic characterization of variable determinants such as surface polysaccharides. LPS and CPS are described variously as fitness or virulence factors
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