Salmonella is a rod-shaped, gram-negative motile rod that moves with the use of its peritrichous flagella. The genus Salmonella can be divided into two species (S. enterica and S. bongori), according to its phenotypic profile. The genus Salmonella is a member of the Enterobacteriaceae family. Salmonella is one of the most common causes of foodborne illness in the world.
Most human diseases are caused by bacteria belonging to the subspecies Salmonella enterica. Salmonella lives in the intestines of many animals such as cows, dogs, pigs, and birds, but Salmonella typhi only lives in humans. Salmonella infection acquired in humans by;
- consuming contaminated meat or animal products (for example, eggs)
- direct contact with infected animals or environments
- contaminated food and water, utensils, hands of someone who handles food.
Thphoidal vs. Non-Typhoidal Salmonella
Salmonella bacteria are classified as either “typhoidal” or “nontyphoidal,” based on their serotype.
Main diseases caused by Salmonella
- Enteric fever (Typhoid fever and Paratyphoid fever)
Important properties of Salmonella species
- Gram-negative rods
- Do not ferment lactose
- Antigens of Salmonella species
a. Cell wall O antigen
b. Flagellar H antigen
c. Capsular Vi (Virulence) antigen)
Laboratory diagnosis of typhoid fever
Blood culture is the mainstay for the diagnosis of typhoid fever. The presence of specific antibodies against Salmonella and / or the presence of characteristic signs and symptoms may suggest typhoid fever but not definitively.
The definitive diagnosis of typhoid fever depends on the isolation of S. typhi from the culture of a blood or bone marrow aspirate.
- Bone marrow aspirate
- Duodenal aspirate
- Stools (especially useful for the diagnosis of typhoid carriers).
- Blood for culture should be taken before the patient is given antimicrobial therapy.
- Patients with a history of fever for 7 to 10 days are more likely than others to have a positive blood culture.
- 10-15 ml from school children & adults
- 2-4 ml from toddlers and preschool children (Remember- children have higher level of bacteremia than adults).
Reading and Reporting Blood Culture Results for Typhoid Fever
Check the inoculated culture bottles for turbidity, gas formation, and other evidence of growth after 1, 2, 3, and 7 days.
- During days 1, 2 and 3, only the vials showing signs of positive growth are grown on agar plates (commonly used media for subculture are Blood Agar, Chocolate Agar, and MacConkey Agar)
- On day 7 all flasks should be subcultured before being discarded as negative.
- Blood agar: S.typhi and S. paratyphi usually produce non-hemolytic smooth white colonies on blood agar.
- MacConkey agar: Salmonellae produce lactose non-fermenting smooth colonies on MacConeky agar.
2. Widal Test
This test has moderate sensitivity and specificity. It can be negative in up to 30% of culture-proven typhoid cases. The Widal test measures the levels of agglutinating antibodies against the O and H antigens. Since S. typhi shares O and H antigens with other Salmonella serotypes and has cross-reactive epitopes with other Enterobacteriaceae, false positive results from the widales tests can occur in other clinical conditions like malaria, typhus fever, bacteremia caused by other organisms as well.
Biochemical Test of Salmonella Typhi
|Gelatin Hydrolysis||Negative (-ve)|
|Gram Staining||Negative (-ve)|
|Growth in KCN||Negative (-ve)|
|MR (Methyl Red)||Positive (+ve)|
|MUG Test||Negative (-ve)|
|Nitrate Reduction||Positive (+ve)|
|TSIA (Triple Sugar Iron Agar)||Alkali/Acid|
|VP (Voges Proskauer)||Negative (-ve)|
|Esculin Hydrolysis||Negative (-ve)|
|Acetate Utilization||Negative (-ve)|
|Arginine Dehydrolase||Negative (-ve)|
|Esculin Hydrolysis||Negative (-ve)|
|ONPG (β-galactosidase)||Negative (-ve)|
|Ornithine Decarboxylase||Negative (-ve)|
|Tyrosine Hydrolysis||Negative (-ve)|
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