Detection and differential identification of typhoidal Salmonella using bacteriophages and resazurin

This study was aimed at developing an easy to use and inexpensive biosensor for the detection of typhoidal Salmonella. The technique was designed to be used without expensive instrumentation if necessary. Bacteriophages specifically infecting three typhoidal Salmonella serovars were isolated and purified. Log-phase cultures were mixed with a high titre of a single phage (10⁹ PFUs) in separate wells of a microtitre plate and incubated at room temperature (30 °C) for 1 h. After incubation, resazurin was added and the plates were incubated further for 1 h. Absorbance at 570 nm of each test well was measured using a commercial microplate reader and compared with that of the control well. A significant difference (p < 0.05) between the absorbance of test and control wells indicated the presence of target bacteria. With visual inspection, a delay in colour change from blue to pink was considered a positive result. The system could detect 5 × 10⁴ CFUs in 120 min without pre-enrichment and 10 CFUs with a pre-enrichment of 6 h.