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Abstract
Colibacillosis is an economically important syndromic disease of poultry caused by extra-intestinal avian pathogenic Escherichia coli (APEC) but the pathotype remains poorly defined. Combinations of virulence-associated genes (VAGs) have aided APEC identification. The intestinal microbiota is a potential APEC reservoir. Broiler chickens are selectively bred for fast, uniform growth. Here we simultaneously investigate intestinal E. coli VAG carriage in apparently healthy birds and characterise systemic E. coli from diseased broiler chickens from the same flocks. Four flocks were sampled longitudinally from chick placement until slaughter. Phylogrouping, macro-restriction pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST) were performed on an isolate subset from one flock to investigate the population structure of faecal and systemic E. coli. Early in production, VAG carriage among chick intestinal E. coli populations was diverse (average Simpson's D value = 0.73); 24.05% of intestinal E. coli (n = 160) from 1 day old chicks were carrying ≥5 VAGs. Generalised Linear models demonstrated VAG prevalence in potential APEC populations declined with age; 1% of E. coli carrying ≥5 VAGs at slaughter and demonstrated high strain diversity. A variety of VAG profiles and high strain diversity were observed among systemic E. coli. Thirty three new MLST sequence types were identified among 50 isolates and a new sequence type representing 22.2% (ST-2999) of the systemic population was found, differing from the pre-defined pathogenic ST-117 at a single locus. For the first time, this study takes a longitudinal approach to unravelling the APEC paradigm. Our findings, supported by other studies, highlight the difficulty in defining the APEC pathotype. Here we report a high genetic diversity among systemic E. coli between and within diseased broilers, harbouring diverse VAG profiles rather than single and/or highly related pathogenic clones suggesting host susceptibility in broilers plays an important role in APEC pathogenesis.
Highlights
Colibacillosis is a syndromic disease of birds characterised by fibrinous lesions around visceral organs caused by a group of extraintestinal pathogenic Escherichia coli (ExPEC) known as avian pathogenic E. coli (APEC)
The current study aims to address several new questions: to determine whether there are significant changes with time in the intestinal potential’ APEC (pAPEC) reservoir among avian faecal E. coli populations; if certain E. coli strains, virulence-associated genes (VAGs) and/or profiles are selected for in the gastrointestinal tract through time and how this relates to the strains and VAG profiles seen among systemic E. coli isolated from birds, which die as a consequence of APEC infection
Two out of the 3 VAGs are involved in iron acquisition (Figure 5). This is the first study to address the longitudinal diversity of intestinal E.coli populations with a focus on APEC VAG carriage, while simultaneously characterising systemic E. coli isolated from visceral organs of diseased birds in UK broiler flocks
Summary
Colibacillosis is a syndromic disease of birds characterised by fibrinous lesions around visceral organs caused by a group of extraintestinal pathogenic Escherichia coli (ExPEC) known as avian pathogenic E. coli (APEC). Despite a number of studies aimed at elucidating the APEC pathotype, it remains poorly defined. Genes involved in bacterial adhesion, invasion, toxin production, serum survival and iron acquisition have all been shown to contribute to APEC pathogenesis [2,3,4,5,6,7]. It is likely that combinations of virulence associated genes (termed VAG profiles or virulotypes) are needed to give rise to pathogenic E. coli, as no single gene has been exclusively associated with APEC. A recent study demonstrates APEC strains arise from multiple E. coli lineages following the acquisition of distinct VAGs, highlighting the potential high genetic diversity among these bacteria [8]. Serotyping has been used as a method for identifying APEC but several authors suggest it fails to discriminate APEC and avian faecal E. coli and a significant proportion of E. coli is untypable [9]
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