Salmonella

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Overview

• Important member of the enterobacteria
• Cause disease in humans and animals worldwide
• Reservior of infection in poulty, pigs, rodents, cattle, dogs
• Bacteria may be present in water, soil, animal feed, raw meat
• 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 Somatic, 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 (isolates must be in phase 1 to be typed)
• Most animal and human isolates in Groups B to E

Pathogenicity

• Faecal-oral transmission
• Infection frequently transmitted from faeces of rodents and birds
• Young, immunocompromised animals particularly susceptible
• Comparitively large dose required for infection due to gastric acid, normal intestinal flora and local immunity
• Enterocolitis:
  • Acute enteritis
  • Bacteria adhere to intestinal epithelial cells in the ileum and colon, probably via fimbrae, O antigen and flagellar H antigen
  • Multiply in and destroy epithelial cells
  • Cytotoxin may cause epithelial cell damage by inhibiting protein synthesis and causing calcium escape from cells
  • Enterotoxin may induce fluid secretion into intestinal lumen
  • Degeneration of microvilli
• Systemic disease:
  • Bacteria invade and replicate in host cells and resist phagocytosis and destruction by complement
  • Bacteria internalised by intestinal epithelial cells by inducing ruffling of cell membranes and uptake into vesicles
  • The organisms replicate within the vesicles and are released from the 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
  • Resistance to phagocytosis and destruction by complement allows spread within the body
  • LPS O antigens prevent damage to bacterial cell wall by complement
  • LPS also causes endotoxaemia, and may contribute to local inflammatory response damaging intestinal cells to cause diarrhoea
  • Endotoxic shock during septicaemic salmonellosis due to LPS
  • 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
• Carriage:
  • Salmonellae can persist in the gut or gall bladder
  • Excreted in faeces after clinical signs disappeared - active carriage
  • Bacteria can survive intracellularly, avioding the immune system and antimicrobials
  • May have latent carriage and intermittent excretion in faeces
  • Stresses e.g. transportaion, illness, parturition, overcrowding promote excretion in carrier animals
  • Tortoises, terrapins, snakes and other reptiles ofter carry Salmonellae
  • Asymptomatic carriage allows faecal spread of infection

Clinical infections

• Zoonotic
• Outbreaks from contaminated imported meat and bone meal
• 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
• Most human infections contracted from animals, especially poulty and cattle
• Bovine Salmonellosis:
  • Syndrome of fever and diarrhoea (with dysentery), often fatal, in calves and adult cattle
  • May cause abortion of pregnant cattle in absence of other signs
  • Caused by infection with various Salmonella serotypes, e.g. S. Dublin and S. Typhimurium
  • An important zoonosis and reportable
  • Carrier animals important for spread
• A cause of ulcerative enteritis in the pig.
• Can cause haemorrhagic disease by secondary thrombocytopenic disease.
• Salmonella in Osteomyelitis
• In arthritis of horses