Gastrointestinal infections - salmonellosis, shigellosis
Salmonellosis
Salmonella species, members of enterobacteriaceae family (gram -ve) have two main pathogenic species:
- Salmonella typhi (causes typhoid)
- Salmonella enteritidis (causes salmonella food poisoning)
Very few bacteria are required to cause infection, especially in cases of patients with decreased gastric acid (e.g. atrophic gastritis). Salmonellae have a type III secretion system that can transfer bacterial proteins into M cells and enterocytes, that active host cell Rho GTPases, triggering actin rearrangement and bacterial uptake into phagosomes where the bacteria grow.
Salmonella typhi (Typhoid fever)
- Infection most common in endemic areas
- Humans are the sole reservoir, transmission thanks to contaminated food or water
- Gall bladder colonization is associated with gallstones, acute infection shows anorexia, abdominal pain, bloating, nausea, vomiting and bloody diarrhea. Rose spots (erythematous maculopapular lesions) are found on chest and abdomen, we may also see complications such as encephalopathy, meningitis, seizures, endocarditis and myocarditis.
Typhoid fever, terminal ileum (19th-century watercolour): swollen, ulcerated Peyer's patches; the ulcer on the left has perforated.
Image: “Typhoid ulceration of the lower portion of the ileum Wellcome L0061611” by Godart, Thomas, CC BY 4.0, via Wikimedia Commons.
Remember: typhoid targets the lymphoid tissue: Peyer's patches of the terminal ileum swell into plateau-like elevations, then ulcerate with ulcers oriented along the long axis of the bowel (tuberculous ulcers run around it), and may bleed or perforate. The infiltrate is mononuclear, not neutrophilic; liver and spleen show typhoid nodules (macrophage aggregates).
Shigellosis
Gram -ve bacilli (fac anaerobes, nonmotile, no capsule) and the most common cause of bloody diarrhea. Their pathogenesis is listed below:
- Highly transmissible via fecal oral route
- Shigella are resistant to the acid of the stomach, they are taken up by M cells in the intestine where they then escape to lamina propria
- Here they infect epithelial cells via basolateral membranes and release Shiga Toxin which inhibits eukaryotic protein synthesis and causes death
- Histology shows hemorrhagic and ulcerated mucosa (similar to Crohn's)
Clinical picture is a self limiting fever with watery, bloody stool that can last for as long as a month. Complications are uncommon but include reactive arthritis (arthritis + urethritis + conjunctivitis)