A new rapid automated method for the detection of Listeria from environmental swabs and sponges

Many food and meat processors test environmental swabs and sponges to confirm the absence of Listeria spp. Spectral pattern changes in a liquid growth medium, resulting from esculin hydrolysis by Listeria in contaminated swabs and sponges, were automatically monitored by the BioSys instrument in a semifluid layer (SFL). The blackening of SFL in modified MOX broth resulted in sharply declining curves, which were easily detected by the instrument. The instrument detected all nine strains of Listeria monocytogenes tested. None of the gram negative organisms (Proteus, Escherichia coli, Pseudomonas, Citrobacter and Yersinia) were detected by the system, nor were most gram positive organisms, including Bacillus, Streptococcus, and Lactobacillus strains, Staphylococcus aureus, Enterococcus faecium and E. faecalis, which hydrolyze esculin, produced black colonies on PALCAM and Oxford media and were also detected in the system. A total of 122 sponges and swabs collected at food processing plants were evaluated by this method. Of these, 99 were negative, and 11 were positive. L. innocua was the dominant Listeria species in these environmental samples. Good correlation was obtained between numbers of Listeria and detection times of esculin hydrolysis: 1000 CFU/swab were detected in 10-13 h, whereas 1-10 CFU/swab were detected in less than 22 h. The total assay time was 26 h.