Microbe Profile: Campylobacter jejuni – survival instincts

Campylobacter jejuni is considered to be the most common bacterial cause of human gastroenteritis worldwide. C. jejuni can cause bloody diarrhoea, fever and abdominal pain in humans along with postinfectious sequelae such as GuillainBarré syndrome (a paralytic autoimmune complication). C. jejuni infections can be fatal, particularly among young children. C. jejuni are distributed in most warmblooded animals, and therefore the main route of transmission is generally foodborne, via the consumption and handling of meat products (particularly poultry). C. jejuni is microaerophilic and oxygensensitive, although it appears to be omnipresent in the environment, one of the many contradictions of Campylobacter. OPEN ACCESS


PROPERTIES
Campylobacter jejuni is a Gram-negative bacterium classified as microaerophilic and capnophilic, and requires atmospheric conditions of 3-10 % oxygen and 5-10 % carbon dioxide for optimal growth, with a growth temperature range between 30 and 47 °C. Cells of C. jejuni can be curved, rod-shaped or spiral, with a size range of 0.2-0.8 µm width and 0.5-5.0 µm length, with amphitrichous flagella. The bacterium is oxidase-positive and energy is typically obtained from amino acids or tricarboxylic acid cycle intermediates rather than the utilization of carbohydrates. GENOME C. jejuni strain NCTC11168 was among the first bacterial genomes sequenced with an estimated size of 1.64 Mb (30.6 % G+C) and 1654 predicted ORFs. This genome was later re-annotated to 1643 ORFs. The NCTC11168 genome is unusual with a lack of insertion, phageassociated and repeat sequences. However, it has a high prevalence of homopolymeric tracts, resulting in the slipstrand mispairing of genes encoding surface structures such as the capsule and the lipooligosaccharide (LOS). No type III or IV secretion systems have been identified, but a general N-linked glycosylation system that posttranslationally modifies over 70 proteins was identified. C. jejuni displays extensive ongoing genetic variation, arising from intragenomic mechanisms as well as genetic exchange between strains and other species [2]. The genomic diversity and plasticity of C. jejuni is demonstrated with the large number of sequence types, host generalist and specialist strains. Further C. jejuni strains have since been sequenced from human, animal and environmental sources and have varied genotypic and phenotypic characteristics (e.g. C. jejuni 81-176 harbouring plasmids pVir and pTet). Recent studies have also identified an increasing number of strains with type VI secretion systems (T6SSs) [3]. However, in contrast to other enteric pathogens, relatively few genomes have been sequenced and the true global diversity and representation of the C. jejuni pan-genome remains to be determined.

PHYLOGENY
The Epsilonproteobacteria class of Proteobacteria consist of the renowned species Helicobacter and Campylobacter. There are currently 17 species and six subspecies assigned to the genus Campylobacter. The most frequently reported are C. jejuni (subspecies jejuni) and C. coli. The vast majority of epsilonproteobacterial species exist in deep-sea hydrothermal vents (e.g. Sulfurimonas species), perhaps indicating an ancient origin of the genus Campylobacter. These bacteria characteristically exhibit chemolithotrophy: organisms that can derive their cellular carbon from carbon dioxide, and are thus able to grow without organic compounds or light. Some deep-sea vent Epsilonproteobacteria also share potential virulence genes from pathogenic counterparts, e.g. CiaB and haemolysin. Interestingly, many of the deep-sea vent species have the unusual N-linked general protein glycosylation system first identified in C. jejuni NCTC11168. Thus, evolutionary links exist between important human/animal pathogens such as Campylobacter and Helicobacter, and their non-pathogenic chemolithoautotrophic deep-sea vent distant relatives.

KEY FEATURES AND DISCOVERIES
Despite specific microaerobic growth requirements [4], C. jejuni is ubiquitous in the ambient environment and must have genetic regulatory mechanisms to withstand aerobic stresses. C. jejuni possesses a range of enzymes that are involved in the breakdown of reactive oxygen species, e.g. KatA and SodB. C. jejuni also comprises regulators involved in the oxidative stress response such as PerR, Fur, RrpA, RrpB and CosR. Many of these regulators have dual functions and are often linked to metabolic functions such as iron homeostasis. One key attribute of C. jejuni aiding its survival is the ability to form and survive in biofilms and under stress to form viable but non-culturable cells.
Key structures such as LOS, capsule and flagella have been investigated with the aim of understanding C. jejuni survival and pathogenesis. Flagella, which are modified by an O-linked glycosylation system, have been hypothesized not only to play a role in motility, but may also act as a secretion system discharging effectors such as CiaB. More recently, T6SSs have been identified in an increasing number of C. jejuni strains and the presence of T6SSs has been associated with a response to oxidative stress [5]. C. jejuni produces a cytolethal distending toxin which has been reported to arrest at the G 1 /S or G 2 /M transition of the cell cycle, depending on the cell type. Adherence factors such as FlpA and CadF, which bind to fibronectin on epithelial cells, have also been studied. It has been reported that C. jejuni primarily uses a microtubule-dependent process for penetration, and once inside the host cell, exists within a Campylobacter-containing vacuole (CCV). C. jejuni is able to modify the CCV to meet its metabolic needs and also interfere with the canonical endocytic pathway, preventing C. jejuni killing within lysosomes [6]. Outer membrane vehicles have also been explored which can act as cargo delivery vehicles, possibly containing and delivering virulence-related proteins [7].
In summary, the lack of a convenient animal model reproducing human disease has been a major hurdle in understanding the pathogenesis of Campylobacter infection, with only recently new models being available [8]. Perhaps the biggest impact of C. jejuni research is the discovery and exploitation of the N-linked glycosylation system and its subsequent transfer to Escherichia coli cells for glycobiotechnological applications [9]. These include the development of low-cost recombinant glycoconjugate vaccines against many bacterial species, including C. jejuni.

OPEN QUESTIONS
Many questions remain and still perplex campylobacteriologists including: • How can a microaerophilic organism survive and be omnipresent in the natural environment? • How does C. jejuni causes disease, including diarrhoea and post-immune complications? • Does C. jejuni survive intracellularly during human infection? If so what are the molecular mechanisms? • Why are birds, particularly poultry, so susceptible to colonization by C. jejuni? • How can birds tolerate trillions of C. jejuni cells without having overt disease, yet only 500 cells cause severe disease in human hosts?
• Why does campylobacteriosis present differently in lowincome countries (watery diarrhoea) compared to highincome countries (bloody diarrhoea)? • Has farming intensification in high-income countries led to the expansion of selective clonal lineages that are different to strains from low-income countries that often have T6SSs?