Salmonella-Epidemiology, Pathogenesis, Diseases, and Treatment

Salmonella-Epidemiology, Pathogenesis, diseases, and Treatment


  • Salmonella belongs to the Enterobacteriaceae family, are gram-negative rods, facultative anaerobes, oxidase negative, have peritrichous flagella.
  • Its antigenic structure contains lipopolysaccharide that has outer somatic O polysaccharides, a common antigen is core polysaccharides and lipid A i.e., an endotoxin.
  • The isolates that are responsible for human gastrointestinal diseases belong to the species Salmonella enterica.
    • More than 2500 unique O serotypes are present.
    • E.g., of Serotypes of S. enterica, are S. Typhi, S. Typhimurium, S. enteritidis, S. paratyphi.


  • Salmonella usually colonizes in animals such as livestock, rodents, poultry (especially eggs), reptiles, birds, domestic animals, and humans.
  • Transmission is carried by an animal to animal spread, and from animal reservoirs to humans.
  • The infective dose is 1000 or more to cause disease by Salmonella species.
  • People who consume antacids regularly can be infected in the small inoculum.
  • Salmonella is a major cause of foodborne intestinal infections, poultry products i.e., eggs act as vehicles of infection for gastroenteritis.
  • In the U.S, per year 40,000 to 50,000 cases are reported, the peak of infections varies seasonally especially in summer.
  • Children less than 5 yrs, adults above 20- 30, and older people above 70 are susceptible to the highest rate of infections.
  • On account, 5% of fully recovered patients from Salmonella gastroenteritis can shed organisms even after 20 weeks.


  • Salmonella is ingested through contaminated food, and passes through the stomach, gets attached to the small intestine mucosal membrane, and invades into microfold cells (M cells), in Peyer patches, and also enterocytes.
Arrow indicates Salmonella entering the membrane ruffles
Arrow indicates Salmonella entering the membrane ruffles
  • Bacteria multiply in endocytic vacuoles
  • The bacteria can also transport across the cytoplasm and get released into the blood.
  • Virulence Factors: Attachment, engulfment, and replication are regulated by two clusters of genes on pathogenicity islands I & II
    • Genes encode by pathogenicity I  for salmonella secreted invasion proteins (Ssps) & type III secretion system (helps to injects proteins inside the host cells).
    • Pathogenicity island II encodes for evasion of bacteria through host immune response & also second type III secretion system.
    • Inflammatory response mediates the release of the prostaglandins and triggers the cAMP which causes hyper fluid secretion
Pathogenesis of Salmonella
Pathogenesis of Salmonella

Clinical Diseases

  • The four forms of Salmonella infection are gastroenteritis, septicemia, enteric fever, and asymptomatic colonization.
Types of Salmonella Infections
Types of Salmonella Infections


  • It’s a common form of Salmonellosis.
  •  Appears in 6- 48 hrs, after consumption of salmonella-contaminated food or water.
  • Symptoms: Nausea, vomiting, non-bloody diarrhea, headache, abdominal cramps, fever, and myalgia.
    • Symptoms can persist for 2-7 days.
  • The acute form of the disease has colonic involvement.


  • All species of Salmonella can cause bacteremia.
  • Common ones are S.Typhi, S. Paratyphi, and S. choleraesuis.
  • Higher chances of infections are in pediatric (younger children), geriatric(old people), and immunocompromised patients ( Sickle cells disease, HIV infections).
  • In 10% of patients, localized suppurative(formation of an abscess or pus )infections (e.g., endocarditis, osteomyelitis, and arthritis) can occur.

Enteric Fever

  • It is also known as Typhoid Fever caused by S. Typhi and Paratyphoid fever(a milder form of fever) is induced by S. paratyphi, S. Schottmuelleri ( previously known as S. paratyphi B ).
  • Bacteria passed through the intestine’s cell lining & get internalized by macrophages.
  • They multiply after reaching the spleen, liver & bone marrow.
  • After ingesting bacteria, in 10–14 days patients experience slowly increasing fever, along with symptoms like anorexia, malaise, headache, and myalgia.
  • Persists for 1 week or longer, followed by gastrointestinal symptoms.
  • The infection cycle starts with the bacteremia phase followed by colonizing the gall bladder and then intestines reinfections.

Asymptomatic Colonization

  • Strains of Salmonella species can stay after recovering of the patients from enteric fever ( caused by S. Typhi and S. Paratyphi).
  • In 5- 10% of patients, chronic colonization of Salmonella spp can occur for more than 1 year


  • The primary method is culturing of Salmonella spp from the blood or feces of the patient on MacConkey Agar (colorless colonies are obtained).
  • A Blood Test is more accurate in giving a positive result and characterized biochemicals tests are also useful.
Gram Staining of the Salmonella Typhi from positive blood sample
Gram Staining of the Salmonella Typhi from the positive blood sample
  • Widal test is widely used for the diagnosis of Salmonella infections for enteric fever.


  • For gastroenteritis, fluid and electrolyte replacement is used.
  • Antimicrobial treatment is useful as a prophylactic measure to prevent the systemic spread of infection.
  • In 1948, Chloramphenicol was used to treat Typhoid.
  • Ampilicin & trimethoprim-sulfonamide, are used for the treatment of infections, after the development of the chloramphenicol resistant strains.
  • New Cephalosporins (ceftriaxone) & quinolones i.e., norfloxacin & ciprofloxacin) are also effective.
  • Recovery: After proper antimicrobial therapy, temperature becomes normal in 3-5 days,  it takes 10-14 days to be generally well.
  • Prevention: Typhoid vaccines are available.

References and Sources


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