The genus Campylobacter was first proposed in 1963 by Sebald and Veron, and contained at that time just two species (C. fetus and "C. bubulus", which is now known as C. sputorum biovar sputorum), both formerly referred to as Vibrio species. The taxonomy of the genus has changed almost beyond recognition since this time, partly as a result of an increased interest in the group since their clinical and economic importance was recognised; and partly as a consequence of the use of increasingly sophisticated methods being applied to investigate taxonomic relationships. At present, the genus contains 16 species and six subspecies. It should also be noted that many species formerly referred to as Campylobacter spp. or Campylobacter-like organisms have been assigned to other genera, most of which are phylogenetically related. These include Arcobacter, Helicobacter, and Sulfurospirillum.

The following is intended as an overview of the genus Campylobacter. It is certainly not an exhaustive treatment of the subject but, I hope, a useful introduction to these fascinating, diverse, and important bacteria.

General features. Cells of most species are Gram-negative, slender, spirally curved rods, but those of some species are predominantly curved or straight rods. Old or damaged cells in old cultures may appear as spherical bodies. Most species are motile with a characteristic corkscrew-like motion by means of an unsheathed flagellum at one or both ends of the cell. C. gracilis is non-motile, whilst C. showae (and occasionally C. hyointestinalis) has multiple flagella.

"Candidatus C. hominis"

"C. hyoilei"

C. coli

This species usually accounts for some 5-10% of diarrheal cases caused by campylobacters, but in some regions this figure can be up to 40%. May also cause septicemia and abortion in humans. In animals it has been associated with hepatitis. Some strains have been found in lesions of a proliferative enteritis-like condition in pigs: these strains were first presumed to be a novel species, "C. hyoilei", until they were confirmed as C. coli strains. C. coli is closely related to C. jejuni, with which it shares most biochemical features. These two species are most commonly distinguished by their reaction in the hippurate hydrolysis test in which C. coli is negative. However, a few strains of C. jejuni may also give a negative result. Although a few other phenotypic traits provide additional discrimination (e.g. growth on a minimal medium, hydrogen suphide production in triple sugar iron agar), these are not commonly used.

A phenotypically and genetically diverse species that is best thought of as a "species-complex". First found in cases of periodontal disease, C. concisus has also been isolated from cases of diarrhoea in humans. No animal reservoir has yet been identified. The species is probably an important opportunistic pathogen, since it is often isolated from cases of human gastroenteritis in infants and the elderly.

Another species closely associated with periodontal disease. Rarely described from cases of human diarrhoea and septicemia. No animal reservoir identified as yet.

This species is divided into two subspecies. C. fetus subsp. fetus and C. fetus subsp. venerealis. In veterinary microbiology, accurate differentiation between th two subspecies is important since the former causes abortion in herd animals (eg. cattle, sheep), but the latter causes infertility - a disease which can cause considerable economic losses. Discrimination of the subspecies is far from easy, even where genetic methods are applied.

C. fetus subsp. venerealis is only rarely reported from human disease. By contrast, C. fetus subsp. fetus has been found in cases of human diarrhoea, septicemia, and abortion.

This species is atypical in that cells are straight, not curved or spiral as with other campylobacters, and also since oxidase activity is not detected by standard methods. C. gracilis is most commonly found in deep tissue infections in humans.

Genetically and phenotypically highly related to C. upsaliensis. Both species may be found in healthy or diarrhoetic cats and dogs. Unlike C. upsaliensis, C. helveticus has not yet been confirmed in cases of human diarrhoea. It is however possible that such episodes have been misdiagnosed as C. upsaliensis, since the species are so alike.

"Candidatus C. hominis"

An unusual species that was described on the basis of a distinct phylogenetic position only, and was not cultured from the specimens in which it occurred (human faeces). Thus, the species was assigned to the taxonomic rank Candidatus, which is reserved for the description of taxa that appear to be novel species but which cannot be examined by standard taxonomic methods. To date, "Candidatus C. hominis" has been found only in humans without signs of enteric disease and it has been suggested as a commensal. Time will tell.

 "C. hyoilei"

This name was proposed for a group of isolates associated with a condition in pigs resembling proliferative enteritis. Although initially believed to be a novel species, a subsequent taxonomic investigation demonstrated that " C. hyoilei" strains were in fact C. coli.

This species is divided into two subspecies. C. hyointestinalis subsp. hyointestinalis has been isolated from cases of human diarrhoea and from healthy and diarrhoetic cattle and pigs. C. hyointestinalis subsp. lawsonii is mainly found in pig stomachs, possibly in association with ulcerative complaints; no human infections have yet been reported with this subspecies. Interestingly, the species as a whole shows a remarkable sequence diversity in 16S rRNA genes and phylogenetic analyses based on these sequences generally place the two subspecies into distinct clades.

This species is divided into two subspecies. C. jejuni subsp. jejuni is generally regarded as one of the most frequent bacterial causes of human diarrhoea worldwide and is widely found in food animals - poultry, cattle, pigs, goats and even ostriches have been found to harbour this organism. Environmental persistence may also play a role in re-infecting food animals and contaminated water supplies are a well-documented source of outbreaks! In addition to diarrhoea, C. jejuni subsp. jejuni may cause septicemia, meningitis, and abortion in humans and serious neurological disorders such as Guillain-Barre syndrome are believed in some cases to arise as a post-infectious complication. In animals, C. jejuni subsp. jejuni may cause gastroenteritis and hepatitis in some birds.

By contrast, C. jejuni subsp. doylei has to date been found only in humans, where it has been isolated from gastric ulcers, diarrhoea and blood. Curiously, the two subspecies are relatively easy to distinguish with simple biochemical tests, a situation in stark contrast to that experienced with differentiating C. jejuni subsp. jejuni from C. coli.

This species can cause gastroenteritis and septicemia in humans, and has been found in episodes of gastroenteritis in some birds. C. lari is phenotypically and ecologically diverse. Although first noted for its resistance to nalidixic acid (the first quinolone antibiotic), some strains sensitive to this compound have been isolated (so-called Nalidixic Acid Sensitive Thermophilic Campylobacters - NASTC). Moreover, unusual urease-producing strains (UPTC) of C. lari have also been described. Although the best-known reservoir of C. lari include various birds (seagulls, chickens), shellfish, river- and sea water have also been found to harbour this organism.

This species was recently (March 2000) described from the faeces of asymptomatic workers in an abbatoir. Further investigations by the isolating laboratory of mainly pig and cattle carcasses processed at the abbatoir (and additionally, of a range of other animals, including poultry, sheep, and domestic pets) have not as yet identified an animal source. C. lanienae is most closely related to C. hyointestinalis and C. fetus and is phenotypically difficult to differentiate from these species.

Reports of human infections have, to date, proved to be cases of misdiagnosed C. concisus infections (these two species are closely related and phenotypically similar). So far, C. mucosalis is one of the few Campylobacter species that has not yet been isolated from human disease. In pigs, C. mucosalis was first thought to be a possible cause of proliferative enteritis, but this condition is now widely thought to be caused by an entirely different bacterium, Lawsonia intracellularis.

This species is strongly associated with periodontal disease (gingivitis, periodontitis) in humans. It has an unusual and distinctive paracrystalline cell surface, which has been postulated as an important virulence factor. Cytotoxic activity has also been demonstrated. It has not been found in any animal species.

This species occurs in the oral cavity of humans and has been isolated from dental plaque, and infected root canals. No reports of an animal source as yet.

This species is unusual in that strains may be assigned to one of three named biovars (bv.=biochemical varieties) depending on their reaction in catalase and urease tests. Bv. sputorum (catalase and urease negative) strains occur in the human oral cavity, and have also been isolated from cases of diarrhoea, abscesses and other skin lesions. Strains have also been isolated from the genital tract of bulls, abortions in sheep and sheep and pig faeces: most of these animal strains were once referred to as "bv. bubulus", a designation that is now considered invalid.

Bv. fecalis strains are catalase positive and urease negative. They are found in sheep and cattle faeces and no human infections have yet been described.

Bv. paraureolyticus strains are catalase negative and urease positive. They have been isolated from healthy cattle faeces and from human diarrhoea.

C. upsalienesis

 This species has been isolated from cases of human diarrhoea, septicemia, spontaneous abortion and haemolytic-uremic syndrome. C. upsalienesis has not yet been identified in food animals but is a common inhabitant of dogs and cats, in which it may also cause gastroenteritis. The frequency at which C. upsalienesis is found in domestic pets leads many researchers to believe that these animals are an important source of human infection. C. upsalienesis is closely related to C. helveticus and distinguishing the two species is problematic.