Salmonella: Properties, Disease and Laboratory diagnosis

Salmonella is a Gram-negative, rod-shaped, motile bacilli which moves 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 cause of food borne 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.

Classification of Salmonella

Thphoidal vs. Non-Typhoidal Salmonella

Salmonella bacteria are classified as either “typhoidal” or “nontyphoidal,” based on their serotype.

Main diseases caused by Salmonella

  1. Enteric fever (Typhoid fever and Paratyphoid fever)
  2. Enterocolitis
  3. Septicemia
  4. Osteomyelitis

Important properties of Salmonella species

  1. Gram negative rods
  2. Do not ferment lactose
  3. 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.


  • Blood
  • Bone marrow aspirate
  • Duodenal aspirate
  • Stools (especially useful for the diagnosis of typhoid carriers).

In this post, i am discussing 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. 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 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 schoolchildren & adults
  • 2-4 ml from toodlers and preschoolchildren (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 bootle 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 peformed 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

Colony characteristics

  1. Blood agar: S.typhi and S. paratyphi  usually produce non-haemolytic smooth white colonies on blood agar.
  2. 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
Slant Butt H2S Gas Motility Indole Urease
S.typhi Alk Acid + (weak) +
S.paratyphi A Alk Acid + +
Other Salmonella spp. Alk Acid V V + V
E.coli Acid Acid + + +
Klebsiella spp. Acid Acid ++ V + +
Citrobacter spp. V Acid ++ V + +
Proteus spp Alk Acid + + + V ++ V

Note: V= variable; Alk= Alkaline

The salmonellae that cause typhoid fever and paratyphoid fever have the following antigenic compositions.
Antigen composition of Salmonellae causing Typhoid and Paratyphoid fever
Antigen composition of Salmonellae causing Typhoid and Paratyphoid fever

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, bacteraemia caused by other organisms as well.

Find more about Widal Tests here:

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