Ans. B: Widal test S. typhi is Gram negative, non acid fast, non capsulated and non-sporing bacilli, which measure approximately 2-4 mm x 0.6 mm. The organism is motile. Typhoid bacilli grow rapidly on ordinary media and optimum temperature of growth is 37degC. Blood culture The organisms can be best isolated during the first 7-10 days of the illness, but in at least half the cases the organisms can still be isolated in the second and the third weeks. Periodic subcultures are made after day 2, 5 and 7 on MacConkey agar. No growth after seven days may be regarded as negative. A positive blood culture is diagnostic, while the same significance cannot be attached to isolation from feces or urine. Cultural characters The growth on MacConkey agar/ DCA is pale yellow (non lactose fermenting) colonies, 2-3 mm in diameter, moist, circular and smooth convex surface. These are catalase positive and oxidase negative. The biochemical reactions which differentiate S. typhi from other genera are: Glucose fermentation + Mannitol fermentation + Lactose fermentation ? Sucrose fermentation - H2S production - Phenylalanine deaminase ? Citrate utilization + Widal test In this test the patient's serum is tested by tube agglutination test for its titres of antibodies against H,0 and Vi suspensions of enteric fever bacteria that are commonly encountered in an area such as S.typhi and S. paratyphiA. False positives Immunization with typhoid vaccine Repeated subclinical infection Past clinical infection Healthy carriers of S. typhi Anamnestic response Patients of cirrhosis and hepatitis False negatives Too early collection of blood sample Patients on antibiotics 5-10% patients do not respond by antibody formation Immunosuppressed patients