Evaluation of a two-step protocol for rapid detection of Salmonella in ice-cream and Cheddar cheese

A two-step protocol for repair, multiplication and rapid defection of Salmonella species in ice-cream and Cheddar cheese was evaluated. The first step involves selective preenrichment using lactose broth supplemented with sodium pyruvate and yeast extract for the repair of injured cells and brilliant green for repression of the competing flora (LBPYEBG) This enrichment is incubated for 7 h at 40 degrees C. The second step involves direct plating of 7 h selective preenrichment onto XLD agar and simultaneous inoculations into Salmonella 1-2 Test(R) for both isolation and presumptive identification of Salmonella the next day. The two-step protocol requires only 26 +/- 2 h for isolation and presumptive identification of Salmonella. This protocol was successfully used in detecting as few as 2 colony forming units (cfu) of non-stressed S, enteritidis per mi in 250 mi of enrichment. initial inocula of 3 cfu ml(-1) of freeze-thaw-injured S. enteritidis inoculated into various ice-creams grew to significantly (p less than or equal to 0.05) higher numbers after 6 and 7 h in the LBPYEBG enrichment than in the conventional lactose broth. This was also found to be true with a naturally contaminated ice-cream (MPN of 0.09 g(-1) of S, enteritidis) S. typhimurium HF artificially incorporated into Cheddar cheese grew more rapidly in the LBPYEBG enrichment than in the conventional lactose broth. This indicates the usefulness of this protocol for rapid detection of Salmonella spp from ice-cream and Cheddar cheese. (C) 2000 Academic Press.