Salmonella sps (S. typhi and S. paratyphi serotypes): Medical Microbiology Lecture Notes

Typhoid and paratyphoid fever (Salmonella typhi and Salmonella paratyphi A, B and C) Salmonellae is found as a gut flora in many vertebrate species
- Human infection includes enteric fever, septicemia, gastroenteritis/ food poisoning

Morphology:
- Motile, facultative, gram-negative rods/bacillus
- Non-spore-forming
- S. typhi is capsulated and only some strains of S. paratyphi are capsulated

Cultural characteristics:
- Aerobic and facultative anaerobic, most of the species produce hydrogen sulphite 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 degree celcius

Biochemical properties:
- S. typhi is anaerogenic  (that do not produce gas/air)
- Catalase positive
- Oxidase negative
- Non-lactose fermenters, ferment glucose, mannitol, and maltose with acid
- Produces H2S, S. paratyphi A does not produce H2S gas
- Non- lactose fermenters, 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 

Pathogenic mechanism:
- 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

Epidemiology:
- Endemic infections occur in countries with poor sanitation and hygiene, 
- Fewer cases observed in the developed countries
- Typhoid fever is more prevalent than paratyphoid fever
S. paratyphoid B and C are rare
- S. paratyphi A is more prevalent in Asian countries, South America, and Eastern Europe 
- The bacteria infects all age group 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 convalescent carrier and are the source of the infection
- Typhoid fever can occur as endemic or epidemic type


Laboratory Diagnosis:
Isolation and Identification

Blood culture
- 5- 10 ml of blood is collected during the first to second weeks of infection, inoculated in bile broth, and incubated overnight. 
- Sub cultured on Mac Conkey agar at 37-degree Celcius for 24 hours of incubation 

Stool culture
- Specimens are collected during the first to second weeks of the infection, directly plated on Mac Conkey, DCA, and Wilson-Blair media at 37-degree celcius. 
- Colonies appear usually after 24 hours of incubation.

Urine culture
The urine sample is centrifuged and the deposit is plated in the Wilson-Blair media and inoculated in selenite and tetrathionate broth.
Biochemical tests are performed for the proper identification of the enteric bacilli, catalase, and oxidase tests are done.

Microscopy 
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

Treatment:
- Chloramphenicol until it became resistant during the 1970s
- Ampicillin, Amoxycillin, Cotrimoxazole. Fluoroquinolones and Cephalosporins

Prevention:

- Improved sanitation, safe and clean water supply
- Tab vaccine which was first introduced during the first World war
- Divalent typhoid- paratyphoid A vaccine or the monovalent typhoid vaccine (preferred in Asia)