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Background  Enteric fever is a major public health problem, causing an estimated 21million new cases and 216,000 or more deaths every year. Current diagnosis of the disease is inadequate. Blood culture only identifies 45 to 70% of the cases and is time-consuming. Serological tests have very low sensitivity and specificity. Clinical samples obtained for diagnosis of enteric fever in the field generally have <1 organism/ml of blood, so that even PCR-based methods, widely used for detection of other infectious diseases, are not a straightforward option in typhoid diagnosis. We developed a novel method to enrich target bacterial DNA by selective removal of human DNA from blood samples, enhancing the sensitivity of PCR tests. This method offers the possibility of improving PCR assays directly using clinical specimens for diagnosis of this globally important infectious disease.    Methods  Blood samples were mixed with ox bile for selective lysis of human blood cells and the released human DNA was then digested with addition of bile resistant micrococcal nuclease. The intact  Salmonella  Typhi bacteria were collected from the specimen by centrifugation and the DNA extracted with QIAamp DNA mini kit. The presence of  Salmonella  Typhi bacteria in blood samples was detected by PCR with the  fliC-d  gene of  Salmonella  Typhi as the target.    Results  Micrococcal nuclease retained activity against human blood DNA in the presence of up to 9% ox bile. Background human DNA was dramatically removed from blood samples through the use of ox bile lysis and micrococcal nuclease for removal of mammalian DNA. Consequently target  Salmonella  Typhi DNA was enriched in DNA preparations and the PCR sensitivity for detection of  Salmonella  Typhi in spiked blood samples was enhanced by 1,000 fold.    Conclusions  Use of a combination of selective ox-bile blood cell lysis and removal of human DNA with micrococcal nuclease significantly improves PCR sensitivity and offers a better option for improved typhoid PCR assays directly using clinical specimens in diagnosis of this globally important infection disease which we believe could be of importance in improving clinical care and providing effective evaluation of novel vaccines.

A novel method of selective removal of human DNA improves PCR sensitivity for detection of Salmonella Typhi in blood samples