Salmonella sps (S. typhimurium and S. paratyphi): Notes on Typhoid fever, the diagnosis, treatment, and prevention of Typhoid

           Notes on Salmonella - Enteric or Typhoid fever, the diagnosis, treatment, and prevention               of  Typhoid 

  •  Salmonella bacteria typically live in the intestines of animals and humans and are usually transmitted to humans through contaminated food or water. Main types of Salmonella associated with human infections are Salmonella enterica serotype Typhi (S. Typhi) and Salmonella serotypes Paratyphi A, B, or C typhi that cause typhoid fever and paratyphoid fever respectively.
  • Salmonella are found as a gut flora/normal flora in many vertebrate species. 
  • The bacteria are often associated with contaminated food, particularly undercooked or raw eggs, poultry, meat, and unpasteurized dairy products. 
  • Common symptoms of a Salmonella infection include diarrhea, abdominal cramps, fever, nausea, and vomiting. These symptoms can appear 6 hours to 6 days after infection and may last between 4 to 7 days. In some cases, the infection can be more severe and lead to complications, especially in vulnerable populations such as infants, elderly individuals, and those with weakened immune systems.
  • Salmonella enterica serotype Typhi (S. Typhi) serotype is responsible for causing typhoid fever, a more severe and systemic form of salmonellosis. Salmonellosis is a common gastrointestinal infection caused by the bacteria, symptoms include diarrhea, abdominal cramps, fever, and sometimes vomiting.
  • Typhoid fever or enteric fever is a more severe and potentially life-threatening illness caused by the bacterium Salmonella typhi. The common symptoms typhoid infection include high-grade and prolonged fever, diarrhea (gastroenteritis /food poisoning), in severe condition septicemia may occur leading to the death when treatment is not available.
  • The infection is more prevalent in the subtropical and tropical parts of the world, common in countries with inadequate water treatment and sanitation facilities.

- Gram-negative bacilli (rods), motile, facultative anaerobic bacteria
- Non-spore-forming
- S. typhi is capsulated and only some strains of S. paratyphi are capsulated

Cultural characteristics of the bacteria:
- Gram-negative bacilli, aerobic and facultative anaerobic, most of the species produce hydrogen sulfite gas (H2S)
- Form large translucent, colorless colonies on Mac Conkey agar and deoxycholate citrate agar
- Black colonies appear on selective media Wilson and Blair Bismuth Sulphite medium
- Brilliant green agar is used for the isolation of Salmonella spp
- Selenite F broth is an enrichment medium for the growth of Salmonella spp
- The optimal temperature for the growth is 37 degrees Celcius

Biochemical properties:
- S. typhi is anaerogenic  (that do not produce gas/air)
- Catalase positive
- Oxidase negative
- Non-lactose fermenter (NLF), ferment glucose, mannitol, and maltose with acid
- Produces hydrogen sulphide (H2S), S. paratyphi A does not produce H2S gas
- MR positive VP negative, Indole negative, citrate positive, urease negative

Virulence Factors:
- Salmonella typhi and S. paratyphi possess flagellar antigen (H), somatic antigen (O) and capsular antigen (Vi)
- S. typhi consists of AB enzymatic toxin known as typhoid toxin/endotoxin 

 The bacteria reach the gut through ingestion of contaminated food or water, the incubation period is two to three weeks
Attachment to the microvilli and penetration to the ileal mucosa of the intestine
- Multiplication of the bacteria via the thoracic duct and invades the bloodstream causing bacteremia during the first week of the infection
Infections occur in the liver, gall bladder, kidney during the second week
- Enter the gall bladder and involves the Peyer's patches of the ileum
- Ulceration of the ileum also called “typhoid ulcers” may occur in serious and chronic conditions
- During the infection period a person develops high-grade fever, nausea, vomiting, abdominal pain, some may experience diarrhea. Some patients may develop rose spots around the abdomen

- Endemic infections occur in countries with poor sanitation and hygiene, 
- Fewer cases are observed in the developed countries
- Typhoid fever is more prevalent than paratyphoid fever
S. paratyphoid B and C are rare
- S. paratyphii A is more prevalent in Asian countries, South America, and Eastern Europe 
- The bacteria infects all age groups but is mostly seen in the 5- 20 years age group 
- The patient may shed the bacteria in the feces from three weeks to three months after the cure, these people are called the convalescent carriers and are the source of the infection
- Typhoid fever can occur as an endemic or epidemic type

Symptoms (Typhoid fever and paratyphoid):
  • High-grade fever (102 to 104 degrees Celsius) and body aches are very common. Chills, stomach pain and abdominal discomfort, diarrhea.
  • Loss of appetite are other symptoms that usually appear within a week after exposure to the bacteria.
  • Muscle weakness, headache, sweating, fatigue can last up to weeks after the onset of the disease
  • Skin rashes, a rose-colored rash can sometimes appear.

Laboratory Diagnosis:

i) Blood culture
  • Blood specimen is the standard and most commonly used specimen for the diagnosis of the infection.
  • 5- 10 ml of blood is collected during the first to second weeks of infection, inoculated in bile broth, and incubated overnight. 
  • Mac Conkey agar is the most commonly used media for the subculture method, in the identification and differentiation of enteric gram-negative bacteria. 
  • The optimal incubation period is 37-degree Celcius for 24 hours. 
  • Salmonella typhi and S. paratyphi are non lactose fermenters, round colorless colonies appear on the Mac Conkey agar usually after 24 hours of the incubation period. Bacteria usually show non-hemolytic properties when cultured on blood agar.
ii) Stool culture
  • Specimens are collected during the first to second weeks of the infection, directly plated on Mac Conkey, XLD (xylose lysine deoxycholate) selective for Salmonella. Salmonella-Shigella agar (SS) selective for Salmonella and Shigella
  • Incubation at 37 degree Celsius for 24 hours 
  • Colonies appear usually after 24 hours of incubation
iii) Urine culture
  • Urine sample is centrifuged and the deposit is plated in the Wilson-Blair media, inoculated in selenite and tetrathionate broth.
  • Biochemical tests are performed for the proper identification of the enteric bacilli, catalase, and oxidase tests are done.

Detected by gram staining method, S. typhi and S. paratyphi appear as gram-negative bacilli /rods 

Widal test (Serological test)
- Usually positive after the first week of infection 
- Detection of the antibodies against O and H antigens 
- Confirmed by the presence of O and H agglutinins for typhoid and H for paratyphoid (A and B) bacilli in the patient’s serum 

Molecular diagnosis:
- Nucleic acid-based test including commercially available PCR test


Drugs/ Medications
- Chloramphenicol was effective for typhoid fever until it became resistant during the 1970s
- Ampicillin, Amoxycillin, Cotrimoxazole. Fluoroquinolones and Cephalosporins are effective and in use.

Typhoid fever vaccine:
 Typhoid vaccines are recommended while traveling to the different parts of the world where the infection is highly prevalent.
Two types of vaccines are available for the prevention of typhoid infection. 
i) Typhoid injections (inactivated typhoid vaccine) for 2 years and older people. A vaccine dose is recommended every 2 years
ii) Oral vaccine capsules ( live typhoid vaccine) for 6 years and older people. A vaccine dose is recommended every 5 years


Preventing Salmonella infections involves practicing good food hygiene, thoroughly cooking foods (especially eggs and poultry), avoiding the consumption of raw or undercooked eggs and meats, and maintaining proper personal hygiene, such as regular handwashing.
Minimize the risk of consuming contaminated food and water, especially in areas with inadequate sanitation. Opt for bottled or boiled water, and ensure that food is thoroughly cooked before consumption.
Wash your hands frequently, especially before eating or preparing food, and after using the restroom.
Vaccines are recommended for travelers to regions where the disease is endemic and for individuals at high risk of exposure. Tab vaccine which was first introduced during the first World war
Divalent typhoid- paratyphoid A vaccine and the monovalent typhoid vaccine (preferred in Asia)