Salmonella

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• 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.

• Salmonellosis
• * Can cause haemorrhagic disease by secondary thrombocytopenic disease.

• Salmonella in Osteomyelitis
• In arthritis of horses

Overview

• 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

Characteristics

• 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

Classification

• 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

Pathogenesis

• Faemcal-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 invade the body
  • 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