Development of a 24 h screen to detect viable salmonellas in faeces

摘要

A 24 h screen to detect viable salmonellas in faeces was developed by studying growth dynamics of salmonellas and competing flora in combinations of enrichment media and artificially‐inoculated pig faeces. Muller‐Kauffmann tetrathionate (MK) broth, incubated overnight at 42°C, maintained the lowest ratio of salmonella: competing flora and identified all inoculated samples. A 4 h postenrichment in M broth plus novobiocin reduced the number of false‐positive results in subsequent ELISAs. Adjusting the negative cut‐off values and incubation time of the chromogenic substrate from that recommended in the ELISA instructions reduced the rate of false‐positive results further and allowed the detection of 103salmonellas per ml in the presence of up to 107ml−1aerobic‐competing cells. Suspension of faeces diluted 1 in 2 and 1 in 5, rather than 1 in 10 in MK broth did not necessitate further adjustments to the ELISA baseline values. The proposed screen protocol is an overnight incubation of faeces suspended 1 in 10 in MK broth, a 1 in 100 subculture into M broth plus 10 μg ml−1novobiocin (MbN) for 4 h, steam inactivation of MbN cultures and testing by ELISA, and can detect three salmonella c