Last updated on April 5th, 2020
Salmonella is a Gram-negative, rod-shaped, motile bacilli which move with the use of its peritrichous flagella. The genus Salmonella can be divided into two species (S. enterica and S. bongori), based on their phenotypic profile. The genus Salmonella is a member of the family Enterobacteriaceae. Salmonella is one of the most common causes of foodborne disease in the world.
Most human diseases are caused by the bacteria belonging to the sub species Salmonella enterica. Salmonella lives in the intestine of many animals such as cow, dog, pig and birds but Salmonella typhi only lives in humans. Human acquired Salmonella infection by;
- consuming contaminated meat or animal products (eg. 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. Presence of specific antibodies against Salmonella and or presence of characteristics signs & symptoms can be suggestive of typhoid fever but not definitive.
Definitive diagnosis of typhoid fever depends on the isolation of S.typhi from blood or bone marrow aspirate culture.
- Bone marrow aspirate
- Duodenal aspirate
- Stools (especially useful for the diagnosis of typhoid carriers).
In this post, I am discussing the isolation and identification of Salmonella from blood culture only.
For blood culture
Salmonella is present in the blood of more than 80% of patients with typhoid fever. The overall volume of blood cultured is critical to increase yield (isolation rate) of the causative pathogen. Reducing the blood volume reduces the sensitivity of the blood culture.
- 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).
The optimum ratio of the volume of blood to traditional culture broth is 1:10 (e.g. 5 ml blood in 45 ml broth). For commercial blood culture system please read and use recommended amounts of blood (do not exceed).
Blood should be inoculated immediately into a blood culture bottle at the time of drawing blood using same syringe that has been used for collection. Inoculated culture bottles should be incubated at 37°C
Do not refrigerate the sample or keep in cool places during transport.
Reading and reporting blood culture results for Typhoid fever
Check the inoculated culture bootles for turbidity, gas formation and other evidence of growth after 1, 2, 3 and 7 days.
- For days 1, 2 and 3, only bottles showing signs of positive growth are cultured on agar plates (commonly used media for subculturing are Blood Agar, Chcolate Agar, and MacConkey Agar)
- On day 7 all bottles should be subcultured before being discarded as negative.
Subculture plates should be incubated at 37°C for 18-24 hours in an aerobic incubator. If growth is observed in the culture plates, colony morphology should be noted and biochemical tests performed to identify the isolate
Remember, S.typhi is not the only bacterial pathogen found in the blood. Find information about organisms that are commonly isolated from blood
- 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.
Suspected colonies obtained on the above culture media are screened by means of the following media/tests:
|Organism||Triple Sugar Iron Agar||Motility Indole Urease Test||Citrate|
|Other Salmonella spp.||Alk||Acid||V||V||+||–||–||V|
Note: V= variable; Alk= Alkaline
Isolated Salmonellae can be characterized by detecting the presence of their somatic (O) and flagellar (H) antigens using specific antiserum. Specific O antigen for S.typhi is O9, O2 for S.paratyphi A, O4 for S.paratyphi B and O6/7 for S.paratyphi C.
B. Widal Test
This test has only moderate sensitivity and specificity. It can be negative in up to 30% of culture-proven cases of typhoid fever. Widal test measures agglutinating antibody levels against O and H antigens. As S.typhi shares O and H antigens with other Salmonella serotypes and has cross-reacting epitopes with other Enterobacteriaceae, false positives widal test results may occur in other clinical conditions such as malaria, typhus fever, bacteremia caused by other organisms as well.