Miniaturized Most Probable Number and Enrichment Serology Technique for the Enumeration of Salmonella spp. on Poultry Carcasses

Abstract

The ability of two 8-tube most probable number (MPN) techniques to quantitatively recover Salmonella spp. from 26 fresh, naturally contaminated chicken skin samples was compared. Individual macerated skin samples were tested in parallel using a traditional (tMPN) and a miniaturized (mMPN) analytical procedure. In the tMPN assay, replicate aqueous portions from each macerated sample were preenriched individually in buffered peptone water, selectively enriched in Müller-Kauffmann tetrathionate brilliant green broth (MKTBG), and plated on Rambach agar. Each MKTBG was also postenriched in M Broth, and the resulting postenrichment culture screened for the presence of Salmonella cells by enrichment serology (ES). Although a similar analytical approach was used in the mMPN assay, it differed from the tMPN in the use of smaller test volumes dispensed in microplates, and on a sedimented portion of skin macerate as test material. Of the 26 Salmonella-contaminated samples examined in this study, the tMPN coupled to Rambach agar or ES identified 23 and 24 positive samples, respectively. Under homologous conditions, the mMPN detected all 26 positive Salmonella contaminated samples. The most probable numbers in 100-g skin samples analyzed by the tMPN ranged from 18/100 g to 9,530,000/100 g with a median value of 570/100 g. Levels of contamination by the mMPN procedure ranged from 90/100 g to 556,000/100 g with a median value of 1,200/100 g. Statistical analysis of experimental data underlined the equivalence of the tMPN and the mMPN procedures and nonequivalence of the Rambach plating and ES conditions. It is suggested that the microplate mMPN coupled to ES offers a reliable and more cost-effective analytical approach for the quantitative recovery of Salmonella on broiler carcasses.