Detection of salmonellas by DNA hybridization with a fluorescent alkaline phosphatase substrate

R. J. CANO, M.J. TORRES, R.E. KLEM, J.C. PALOMARES AND J. CASADESUS. 1992. This study evaluates a DNA hybridization assay for salmonella with A tto P hos ™ (JBL Scientific, San Luis Obispo, CA), a fluorescent substrate for alkaline phosphatase. The probe used (50 ng/ml) was a biotinylated 600 bp fragment consisting of a tandem repeat of an insertion sequence (IS200) found in most Salmonella spp. evaluated. The hybridization was carried out at 65°C for 2 h without prior prehybridization and hybrids were detected by the addition of a streptavidin-alkaline phosphatase conjugate. Circles (5 mm) were cut from the membrane and placed in a cuvette containing 1 ml of 1 mmol/1 A tto P hos ™. The reaction was evaluated after 30 min at 37°C with a fluorometer with an excitation wavelength of 440 nm and an emission wavelength of 550 nm. The sensitivity of the probe was estimated to be 10 000 copies of target DNA or 5 times 10^-20 mol of DNA. All 74 salmonella strains tested reacted with the probe but none of the 98 heterologous species tested gave positive results. The results of this study indicate that our assay method, which employs a biotinylated tandem repeat of IS200 and A tto P hos ™, is a specific and highly sensitive quantitative method for the detection of salmonellas.