Comparative studies of Campylobacter jejuni genomic diversity reveal the importance of core and dispensable genes in the biology of this enigmatic food-borne pathogen
Introduction
Campylobacter jejuni, a Gram-negative, spiral shaped bacterium [1], is one of the leading bacterial causes of food-borne human gastroenteritis. C. jejuni is currently estimated to cause 5–14% of diarrhea worldwide, which translates into 400–500 million cases per year [2]. Most cases of C. jejuni mediated gastroenteritis (campylobacteriosis) are characterized by nausea, abdominal cramps, diarrhea, and fatigue. Although campylobacteriosis is most often self-limiting, certain strains of C. jejuni have been implicated as an antecedent to the development of Guillain-Barré Syndrome (GBS), an acute autoimmune mediated polyneuropathy characterized by ascending paralysis [3, 4].
While outbreaks of campylobacteriosis occur, predominantly through consumption of contaminated milk and untreated water [5], most Campylobacter infections are sporadic in nature and linked to the improper handling and consumption of poultry. The linkage between human infection and the handling of raw poultry is not unexpected, as C. jejuni is a common commensal organism of chickens. In fact, C. jejuni colonize the intestinal tract of a variety of animals, including common livestock (cattle, sheep, pigs), domestic animals (dogs, cats), poultry, and wildlife (rabbits, pheasant, quail) [6, 7, 8, 9]. A number of methods (e.g. Penner serotyping, Lior serotyping, fla-short variable region [SVR] sequencing, pulsed-field gel electrophoresis [PFGE], multilocus sequence typing [MLST]) are useful for the discrimination of C. jejuni isolates in epidemiological investigations. These methods have enabled investigators to identify the strain responsible for an outbreak [10, 11]. The use of MLST, in particular, has provided researchers with the benefit of a defined molecular fingerprint to compare strains. The recent explosion of genome sequences and comparative genomic data has increased our understanding of the epidemiology and metabolic capacity of this organism.
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The number of Campylobacter species and strain specific genome sequences is increasing
The importance of Campylobacter in human gastrointestinal illness has resulted in at least 18 isolates from 8 different Campylobacter species having been or being sequenced (Table 1). The genomes of Campylobacter organisms are characterized by a low mol% (G + C) content (between 29.5% and 44.5%), small size (ranging from 1.5 Mb for Campylobacter lari RM2100 to 2.1 Mb for Campylobacter concisus 13826), and relatively few open reading frames (between 1425 and 1931 ORFs). The availability of sequenced
MLST provides greater insight into genetic diversity and population structure
MLST allows researchers to differentiate strains based on alleles at seven ‘unlinked’ housekeeping loci [12]. Each unique allele is assigned a number based on its order of discovery and the combination of allelic numbers is the basis for its sequence type (ST). The ST is indicative of the isolate's genotype. This method is advantageous because it yields data that are accurate, reproducible, unaffected by changes in gene order, and readily comparable among laboratories. In addition to its
Identification of hypervariable plasticity regions
The availability of genome sequences has made it possible to construct whole genome DNA microarrays for comparative genomic hybridization (CGH) analysis. Analysis of 11 C. jejuni clinical isolates by CGH revealed extensive genetic diversity and enabled the researchers to identify the genetic core of this organism [17]. Approximately 84% of the 1654 genes analyzed were common to all strains tested and encoded proteins involved in housekeeping functions, including metabolic, biosynthetic,
C. jejuni genomes are syntenic and some contain integrated elements
Presumably, all C. jejuni strains sequenced to date are pathogenic. As such, it is not possible to compare the genomic sequence of pathogenic and non-pathogenic strains. However, sequencing and comparative genomic analysis of five Campylobacter genomes (C. jejuni NCTC 11168, C. jejuni RM1221, C. coli RM2228, C. lari RM2100, and Campylobacter upsaliensis RM3195) revealed major structural differences between the strains [22•]. While the genome of C. jejuni RM1221 was syntenic with the previously
Nucleotide variations alter pathogenic behavior
The availability of C. jejuni genome sequences has provided the genetic basis for investigating the metabolism, gene regulation, and physiology of these organisms. These data indicate that many of the previously identified putative virulence determinants, including cytolethal distending toxin (CDT), various adhesins (e.g. CadF, JlpA, and PEB1), and the flagellar structural proteins, are conserved among strains [17]. Despite conservation of these genes, it is likely that the variations in the
Conclusions and future perspectives
The identification of genetic markers predictive of ecological source and virulence potential are important to detecting and preventing the dissemination of C. jejuni via food sources. As we have reviewed, MLST, DNA microarrays, and genome sequencing of C. jejuni strains have demonstrated the genetic diversity of this important food-borne pathogen. Comparative genomic studies have demonstrated C. jejuni population structure relates to ecological source (livestock versus non-livestock sources)
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We thank Charles L Larson (School of Molecular Biosciences, Washington State University) and Philip F Mixter (School of Molecular Biosciences, Washington State University) for critical review of this manuscript. Comparative genomic analyses of C. jejuni is supported from funds awarded to MEK from the National Institute of Health, Department of Health and Human Services under contract number NO1-AI-30055.
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