Characteristics of the organism and its antigens

The Campylobacter genus comprises several species of gram-negative, highly motile, bacteria with an S-shaped morphology and microaerobic growth requirements. Clinically the most important Campylobacter species are the thermophilic organisms, C. jejuni and C. coli, which colonize and invade the small intestine and colon and are the most common cause of human acute bacterial enteritis. Asymptomatic infection is common in underdeveloped countries but in susceptible patients diarrhea occurs, usually 3 days after infection, and lasts 2-3 days, although excretion may continue for several weeks. The mechanisms by which these Campylobacters cause enteritis are, as yet, unknown but may involve adherence, invasion and/or toxin production. Several other Campylobacter species, including C. lari, C. hyointestinalis, C. concisus and C. upsaliensis have also been implicated as causative agents of enteritis. All these enteric Campylobacters may also cause blood and systemic infections. C. fetus subsp. fetus usually causes opportunistic septicemic infections in humans. Campylobacteriosis is generally self-limiting but arthritis and glomerulonephritis are not uncommon reactive complications and about 50% of cases of Guillain-Barre syndrome are thought to be associated with a prior C. jejuni infection.

In the following text the term 'Campylobacter' will refer to the C. jejuni/coli group of organisms unless otherwise stated. The strains in this group are diverse and two serotyping schemes have been developed for use in epidemiologic tracing. The Lior serotyping scheme is based on heat-labile antigens and utilizes slide agglutination of live bacteria with serotype-specific rabbit antisera preabsorbed with homologous heated and heterologous unheated cross-reactive antigens. Flagellin is considered to be one of the antigens involved in this serotyping scheme. The Penner serotyping scheme is based on heat-stable antigens and involves the passive hemagglutination of erythrocytes, sensitized with boiled saline bacterial extracts, by serotype-specific rabbit sera. Lipopoly-saccharides (LPSs) are though to be the dominant heat-stable antigen involved in this scheme.

Several surface protein antigens of the Campylobacters have been described. The most well charac terized antigens are the flagellins. The Campylobacters have single, bipolar flagella. In animal models using isogenic mutants fully active flagella are essential virulence factors. There are two flagellin genes (flaA and flaB) in tandem in all strains, although only one of these genes may be expressed. The flagellins have a molecular weight of approximately 62 kDa and are the major protein surface antigen. The antigenicity of Campylobacter flagellin is complex and epitopic analysis indicates that common as well as serotype-specific linear epitopes are expressed. The common epitopes are antigenically cross-reactive with flagellins from most Campylobacter species, including C. fetus fetus, C. lari and Helicobacter pylori (formerly C. pylori). Antigenic variations occur between these flagellins, and a microheterogen-eity in charge properties due to post-translational modification, probably glycosylation, also demonstrates some antigenic variation. Additionally there is a reversible transition between flagellated and aflagellated variants.

There are several other conserved groups of protein antigens. The variable molecular weight (4247 kDa) major outer membrane protein (MOMP) is surface exposed. The antigenicity of this protein mainly relies on its native conformation and it is antigenically cross-reactive with similar proteins in C. coli and C. lari but not other Campylobacter species. A group of surface extractable antigens (2632 kDa), called the PEB proteins (PEB 1-4), have been isolated and partly characterized. PEB 1 has been cloned and sequenced and may have adherence properties. Some of the Campylobacters produce detectable toxins, one of which has some antigenic cross-reactivity with cholera toxin, but is structurally different, comprising a 66 kDa polypeptide, probably a holotoxin, and a series of antigenically related subunits. Recently the expression of pili on C jejuni has been described but the role of these structures in pathogenicity has yet to be determined.

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