Bacteriology-Salmonella| Quick Revision Notes for NEET-PG

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A genus of rod-shaped (bacillus) gram-negative bacteria of the family Enterobacteriaceae

Motile by peritrichous flagella, except(non-motile strains)

      • S. gallinarum
      • S. pullorum


Biochemical RXN

      1. Indole -ve
      2. Methyl red +ve
      3. VP test -ve
      4. Citrate test +ve
        • Exception- Citrate test is negative in Salmonella typhi
      5. TSI media-H2S is produced
        • Exception- H2S is not produced in Salmonella paratyphi A and S. cholerasuis
      6. Do not ferment lactose

Disease caused

      1. They are never commensals, always pathogenic.
      2. Disease-causing species are classifies as
        1. Typhoidal salmonella-Cause enteric fever(Typhoid fever and paratyphoid fever are types of enteric fever, the latter being less severe)
          • Typhoid fever-caused by
            • salmonella typhi (the Most common cause of enteric fever)
          • Paratyphoid fever-caused by
            • S. paratyphi A (2nd MC cause of enteric fever)
            • S. paratyphi B (3rd MC cause of enteric fever)
            • S. paratyphi C (rarest cause of enteric fever)
          • Source of infection-Human carriers(No animal reservoir)
        2. Non-typhoidal salmonella-causes
          • Gastroenteritis-caused by
            • Salmonella typhimurium
            • Salmonella enteritidis, etc
            • IP>16-18hrs
            • Food-meat, poultry and their products
          • Septicemia-caused by
            • Salmonella choleraesuis(Present in pigs)
          • These organisms have both human & animal reservoir
      3. Enteric fever
        1. Mode of infection-faeco-oral route
        2. Incubation period-1-2 weeks
        3. Infective dose- 105-108 organisms
        4. Pathogenesis- Following intake of infective dose, organisms reach the Peyer patches where they are engulfed by macrophages but resist being killed after which they enter blood causing primary bacteremia, after which they spread to and multiply in the reticuloendothelial system including liver, spleen, lymph nodes. After multiplying in RES, they again enter the blood causing secondary bacteremia. Now, the patient will have signs and symptoms.
        5. Some of the circulating bacteria seed into the gall bladder and shed up in feces.
        6. Clinical features-
          1. stepladder pyrexia
          2. Relative bradycardia
          3. Leucopenia
          4. Rose spots-maculopapular brown colored rash
        7. Complications-Intestinal perforations and hemorrhage (usually in 3rd week).These are rare nowadays due to early diagnosis and treatment.
        8. Diagnosis of enteric fever-
          • Gold standard-Blood culture(100% specific).Sensitivity depends upon the duration of the fever(in weeks)
            1. First Week 90%
            2. Second Week 75%
            3. Third Week 60%
            4. Forth Week onwards 30-40%
          • How blood culture is done?
            • 5-10 ml of blood is diluted to form 50-100 ml of broth(1:10 ratio). Dilution is done in order to prevent the inhibitory effect of antibodies, complement, and antibiotics which may interfere with the growth of the pathogen. After this, serial subculture on solid media is done for up to 7 days
          • Urine culture-positive generally in the 2nd-3rd week in 25% individuals
          • Stool culture-Stool sample is added to enrichment media(Selenite F. Broth/ tetrathionate broth) and incubated for 48-72 hrs. Then subculture on selective media(eg. Mc Conkey’s agar), observe(colorless colonies on MC. Agar) and infer(Non-lactose fermenting colonies)
            • Mc Conkey’s agar-non-lactose fermenting colonies
            • DCA(deoxycholate citrate agar)-colorless colonies with black center
            • Wilson and Blair medium-black colonies with a metallic sheen
          • Antigen detection(possible in 1st week)-In the following sample with the help of coagglutination test and ELISA
            • Serum
            • Urine
          • Antibodies to O and H antigen detection-by WIDAL test(tube agglutination test)-

O antigen

H antigen

Polysaccharide in nature

Protein in nature

Less antigenic

More antigenic

Low antibody titer

High antibody titer

Only IgM is formed

Both IgM and IgG is formed

Antibodies to O antigen appears early

And go early(disappears within 6 weeks)

Antibodies to H antigen appears late,

Stays for longer duration and disappears late


Agglutination is slow

Agglutination is fast

The test is done in Felix tube(round bottom)

Test performed in Dreyer’s tube(conical bottom)

Antibody Titre of 1:100 of O agglutinins is significant

The antibody titer of 1:200 of H agglutinins is significant

    • Vi agglutinins(1:10) or more indicate carrier state
    • Positive widal test-
      • Take 2 serum samples at the interval of 10-14 days. These are acute and convalescent/paired sera. Antibodies appear by 7th to 10th day of illness and declines after 3rd and 4th weeks
      • 4 fold rise in the antibody titer is interpreted as a positive Widal test where,
      • Titre = 1/maximum dilution at which agglutination occurs
    • False positive-
      • Due to vaccination
    • False negative-
      • It is due to early antibiotic treatment or anamnestic reaction(accelerated exposure of specific antibody after reexposure to an agent the subject has been previously been exposed to)
    • Carriers
      • Carrier is a person hosting a pathogen without clinical features
      • Carrier state develops in 5% of patients of enteric fever
      • Types of carriers
        1. Convalescent carriers-patient who has suffered from the disease and is currently in the recovery phase
        2. Temporary-carriers who shed organisms for 3 months to 1yr (Typhoid Mary)
        3. Chronic- ppl who sheds org. For >1yr
      • Types of carriers
        1. Fecal carriers shed the organisms in feces (more common)
        2. Urinary carriers-shed the or. In urine(less common and more dangerous)
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