Characteristics of the organism and its antigens

Salmonellae (family Enterobacteriaceae, genus Salmonella) are facultatively anaerobic, motile gramnegative rods. They are now classified into three main species {S. typhi, S. cholerae-suis and S. enteritidis) with over 2000 different serotypes considered different varieties of S. enteritidis. Pathogens of humans and animals, they can be found in the digestive tract, blood and internal organs, and can survive in the environment to cause water- or food-borne infections. Host-adapted salmonellae can cause severe invasive disease, e.g. S. typhi or the paratyphoid bacilli causing typhoid fever in humans, S. gallinarum or S. pullorum causing fowl typhoid. Others have a broader host range, causing invasive disease in one host species and more limited infections in others, e.g. S. typhimurium or S. enteritidis, which cause invasive disease in rodents and are common agents of food poisoning. These salmonellae of animal origin can also cause invasive disease in humans in the extremes of age or in situations of lowered host defences. Salmonella septicaemias are invasive infections with abscess formation, typically by S. choleraesuis. Asymptomatic carriers of S. typhi are the reservoir of human typhoid fever. Carriage of other serotypes in animals, including ovarian carriage in birds with infected eggs, causes contamination of foodstuffs and gastroenteritis.

Serological classification of salmonellae is based on the main determinants present on the somatic (O) and flagellar antigens. O antigens are present on the long side-chains of lipopolysaccharide (LPS or endoxin). The lipid A of LPS is the main determinant of toxicity. 'Rough' mutants lacking the O side-chains are serum sensitive and avirulent. Some salmonellae (e.g. S. typhi, S. dublin, S. paratyphi C) express the Vi capsular antigen, a homopolymer of a-l,4-deoxy-2-N-acetylgalacturonic acid, which can mask the O antigen and is important in virulence and immunity. Enterobacterial common antigen (ECA)

may also be present. With few exceptions, salmonellae are motile with peritrichous flagella. Many serotypes show flagellar phase variation, reversibly expressing flagella with one or two different antigenic specificities. Important in classification, the relevance to pathogenicity of this highly conserved variability is unknown. Many salmonellae express fimbriae; other somatic components are also antigenic, and renewed attention is being given to the role of cell wall proteins in pathogenesis, immunity and diagnosis.

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