Salmonella Lab Report

To develop a high-throughput spectrophotometric screen for Salmonella contamination, modified amino acid decarboxylase broths with pH indicators supplemented with selective agents were studied in microwell plates. The goal was to develop a rapid, reliable, and efficient procedure for resource-poor settings or food industry with large sample numbers. Detection depends on the broth changing from neutral to acidic pH due to sugar fermentation reflecting cell growth and then to basic due to decarboxylase generating amines, indicating presence of decarboxylase-positive bacteria. The visible absorption spectra of three pH indicators that change color at neutral pH were compared from pH 4.5 to 9.

In the modified decarboxylase broth, phenol red showed the largest 550

Most of tested Salmonella serovars rapidly and specifically metabolized ornithine and lysine and showed positive decarboxylase activity (red broth turning pink) within 6e8 h. All Salmonella serovars possessed arginine decarboxylase, but arginine metabolism took at least 24 h to attain pink broth. Salmonella

Typhi and Salmonella Paratyphi A are void of ornithine and lysine decarboxylases, respectively.

Therefore, both ornithine and lysine were used as decarboxylase substrates in the selective enrichment media formulated to detect all Salmonella serovars. Three selective inhibitors: magnesium chloride, novobiocin, and magnesium chloride with malachite green (from Rappaport-Vasilliadis soy broth) were chosen to be included in seven new amino acid decarboxylase broths. They were tested for their ability to screen for Salmonella contamination in the selective enrichment step. Collectively, the broth formulations with different amino acid substrates and selective inhibitors not only identified decarboxylase-positive bacteria, but further distinguished between decarboxylase-positive salmonellae and