Typhoid fever is caused by Salmonella typhi, a Gram-negative bacterium. S. Typhi has several unique features and the genetic basis of many of which is known as a result of early genetic studies and the recent sequencing of the whole genome.  The bacteria S. typhi can be identified in the laboratory by several biochemical and serological tests prevalent for its detection.

During an acute infection,  the bacterium S. Typhi multiplies in mononuclear phagocytic cells before being released into the human bloodstream.  The typhoid organisms pass through the pylorus and reach the small intestine, these bacteria rapidly penetrate the mucosal epithelium either through microfold cells or enterocytes and arrive in the lamina propria.

The Clinical illness is accompanied mainly by a fairly sustained but low level of secondary bacteraemia in the blood.

There are Many factors that also influence the severity and overall clinical outcome of the typhoid infection. They mainly  include the duration of illness before the initiation of appropriate medical therapy, the  proper choice of antimicrobial treatment, age,  also the previous exposure or vaccination history, the changed  virulence of the bacterial strain, the total quantity of inoculum ingested and host factors

Types of diagnostic tests:

• Specimens: For the blood culture it is essential to inoculate the media at the time of drawing blood from the patient. Once the specimens have been inoculated, blood culture bottles should not be kept cold. They should be incubated at 37°C  and in tropical countries be left at room temperature, before being processed in the laboratory for results.
• Blood: The blood cultured is the primary factor in the isolation of S. Typhi from typhoid patients as the children have higher levels of bacteraemia than adults. The Blood has to be collected by using the highly sterile technique of venous puncture and inoculated immediately into a blood culture collection sample bottle with the same syringe that has been used for collection and further contamination should be prevented.
• Serum: For tests regarding serological purposes, 1 to 3 ml of blood should be inoculated into a tube without anticoagulant. A second sample of blood serum should be collected at the convalescent stage, mostly 5 days later. After blood clotting has occurred the serum should be separated again and stored in aliquots of 200 ml at +4°C.  The sample Testing has to be taken place immediately if then the storage can continue for a week without affecting the antibody present. The serum should be frozen at -20°C if longer-term storage is required for preventing any contamination.
• Stool samples: in some cases, even Stools can be collected from acute patients and they are especially very useful for the diagnosis of typhoid carriers. The isolation of S. Typhi from stools is suggestive of typhoid fever.  These Stool specimens should be collected in a sterile wide-mouthed plastic container. The chances of obtaining positive results increase with the stools collected. Specimens should preferably be processed within two hours after sample collection. A stool culture may increase the total yield of culture-positive results by up to 5% in acute typhoid fever.