Salmonella

Some serotypes tend to be more species specific, whereas others can affect a wide range of species. For example:
Salmonella enteritidis
Salmonella typhimurium
- Widespread in most species.
Salmonella dublin
Cattle
Salmonella cholerae suis
- Pigs
- Usually speticaemic.
- A cause of ulcerative enteritis in the pig.
- Not very common now.
Salmonella montevideo
- Produces outbreaks from contaminated imported meat and bone meal.

Important member of the enterobacteria
Cause disease in humans and animals
Reservior of infection in poulty, pigs, rodents, cattle, dogs
Cause enteritis and systemic infection (septicaemia and abortion)
Salmonella may be carried sub-clinically
Some human strains cause enteric fever (S. Typhi causes typhoid), also gastroenteritis, septicaemia or bacteraemia

Gram negative bacilli
Facultative intracellular pathogens
Non-lactose fermentor, oxidase negative
Do not produce urease or indole from tryptophan
Utilise citrate as a carbon source
Reduce nitrates to nitrites
Grow on MacConkey
Ferment glucose to produce acid and gas
Usually produce hydrogen sulphide
Most motile with flagellae (H antigen)
H antigen can be in phase 1 or phase 2, depending on a genetic switch allowing for one of the H antigen genes to be transcribed at any one time

Single species, Salmonella enterica
Over 2400 pathogenic serotypes or serovars identified
Grouped into 9 groups according to O antigen (lipopolysaccharide) by the Kauffmann-White scheme - determined by slide agglutination of the bacteria with specific antisera
Categorised into serovars depending on and H (flagellar) antigen, e.g. Salmonella enterica subspecies enterica serovar Tymphimurium; must also determine phase of H antigen

Faecal-oral transmission
Comparitively large dose required for infection due to gastric acid, normal intestinal flora and local immunity
Enterocolitis:
- Acute enteritis
- Bacteria multiply in the intestine and damage epithelial cells
- Cytotoxin may cause epithelial cell damage
- Enterotoxin may induce fluid secretion into intestinal lumen
- Degeneration of microvilli
Systemic disease:
- Bacteria internalised by intestinal epithelial cells
- Stimulate immune response on reaching the lamina propria
- Inflammatory response with phagocytosis of bacteria by neutrophils and macrophages
- Bacteria either destroyed by the phagocytic cells or survive and multiply in the cells to cause systemic disease
- Intracellular carriage if bacteria no completely removed
- Invasive potential of certain strains e.e Salmonella Dublin associated with carriage of a large plasmid, encoding genes to allow intracellular survival in macrophages and also to allow iron acquisition
- Salmonellae are facultative intracellular organisms, allowing them to move from the gut in macrophages and cause a bacteraemia and lesions throughout the body
- Possession of Pathogenicity islands associated with virulence

Some serotypes are host-specific, some are not
S. Tymphimurium infects many species; causes severe diarrhoea; non-invasive; causes of food poisoning in humans, e.g. from infected poultry
S. enteritidis: non species-specific; losses in young birds; causes food poisoning in humans
S. Dublin: invasive serovar; infects cattle
S. Cholerae-Suis: primarily infects pigs; also causes severe human disease
S. Pullorum: infects poultry; egg-transmitted; causes bacillary white diarrhoea, known as pullorum disease
S. Gallinarum: infectes older birds; known as howl typhoid
S. Pullorum and S. Gallinarum now rare in UK due to eradication programs including the Pullorum test (whole blood slide agglutination to detect antibody to both S. Pullorum and S Gallinarum
S. Abortis-ovis: infects sheep
S. Abortus-equi: infects horses outside of the UK
S. Typhi, S. Paratyphi: infect humans

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