Highly-sensitive detection of Salmonella typhi in human blood using a portable optical system for detection of nucleic acids amplification

Diseases such as enteric fever continues to be major cause of deaths globally, particularly in poor resource settings. Traditional cell viability test using bacterial culture method followed by confirmatory biochemical tests such as motility, triple sugar iron agar (TSI), citrate, urease test, agglutination tests, forms the gold standard method for diagnosis of enteric fever. However, these existing practices are time consuming. Highly sensitive detection of Salmonella typhi (S. typhi) in blood of 50 CFU/mL was achieved using our protocol involving a magnetic nanoparticle based preconcentration, a loop mediated isothermal amplification assay (LAMP) for signal augmentation and signal detection using an in-house designed optical detection system. Primers specific for STY2879 gene were used to amplify the nucleic acid isolated from S. typhi cells. The protocol involves detection of nucleic acid amplification for both pre and four hour post-incubation that confirms (a) cell viability and (b) specificity of S. typhi and (c) quantification of S. typhi. No cross reactivity of the primers were observed against 106 CFU/mL of common pathogenic bacterial species found in blood such as such as E. coli, P. aeruginosa, S. aureus, A baumanni, E. faecalis, S. paratyphi A and K. pneumonia. This detection system shows a promising future in the field of food and medical diagnostics. The instrument can be extended to other pathogens in the future with modification of the primers and minimal modification of the pre-concentration and lysis protocol.