Salmonella and shigella

Gram-negative rods related to enteric tract infections:
Dr.Jawad Kadhim Tarrad
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Salmonella:
Important properties:
• Gram-negative rods .
• It is enapsulated organism(not all species).
• It is motile.
• Aerobic or facultative anaerobic.
Antigenic structure and virulence factors:
• O-antigens are used to divided salmonella into 6 serogroups( A,B,C1,C2,D, and E) and it has 2500 serotypes.
• H-antigens is flagellar protein..
• Vi-(virulence) is capsular polysaccharide antigen.
• Endotoxin acts as enterotoxin that responsible for enteric fever, food poisoning and sepsis.
Natural habitat and transmission:
• Natural habitat is enteric tract of animals and human carriers(not in health person without infection previously). The carriers are important sources of infection.
• Salmonella infection is transmitted via fecal-oral route during ingestion of contaminated food (eg. meat, poultry, eggs, dairy products ) while by water-borne is less frequent.
Pathogenesis:
Food poisoning:
• The disease is caused by S.enterica serotype Enteritidis (serogroup D) and S.enterica serotype Typhimurium(serogroup B). The bacteria produce enterotoxin and cause food poisoning.
• The ingestion of large number of organism is needed to cause disease. Infectious dose is at least 105 cells. Infectious dose is high because organisms are inactivated by stomach acid. The organism invades epithelial and subepithelial tissues of the mucosa (binds to M-cells)of small and large intestines then penetrate it and migrate to lamina propria of ileocecal region.
• Predisposing factors include lowered stomach acidity either from taking antacids or from gastrectomy.