Question

If a patient is suspected to have non-typhoidal gastroenteritis infection from Salmonella enterica serovar Typhimurium (Samonellosis), describe how the lab will identify the pathogen, including growth conditions, relevant media, bacterial characteristics, and pertinent biochemical tests. (using a swab of blood and/or stool sample)

Answer 1

The laboratory diagnosis of gastroenteritis is made by isolation and identification of causative agent from blood, faece, urine, bone marrow, bile etc. And by demonstration of circulating antigens and antibody in the patient serum.

The growth condition -

Salmonella enterica have simple nutritional requirements . They can survive during temperature range 5.2-46degree Celsius. With optimal range 35-43degree Celsius.

They can also survive wide range of pH 3.8-9.5 but the optimal range is 7-7.5.

Their growth is inhibited by benzoic acid, sorbic or propionic acid.

They secrete a cholera like enterotoxin that induces increased level of cAMP and that some strains produce a heat stable enterotoxin. A cytotoxin that inhibits protein synthesis in intestinal cells.

General characteristics -

They are gram negative , non spore forming, rod shaped bacteria.

Facultative anaerobic.

They have incubation period of 12-24 hours.

Biochemical tests are-

The serological type of bacteria is confirmed by serological testing. Feces, blood are played on several non selective and selective agar media( blood, McConkey ,eosin -methylene blue, bismuth sulphite , Salmonella -Shigella and brilliant green share) as well as into enrichment broth as selenite or tetrathionate. Any growth is enrichment broth is subsequently subcultured onto various agar. The biochemical reactions of suspicious colonies are then determined on triple sugar iron agar or lysin -iron agar and then a presumptuous identification is made.