Performance Evaluation of Easy Plate™ LS for Selective Detection of Listeria Species

Introduction:

Listeria monocytogenes is one of the major foodborne pathogens responsible for listeriosis through contaminated food. Environmental monitoring for Listeria species plays a critical role in preventing contamination and controlling the presence of L. monocytogenes. The newly developed ready-to-use (RTU) medium, Easy Plate™ LS, is convenient, environmentally friendly, and enables rapid detection and enumeration of Listeria species within just 24 hours.

Purpose:

The selectivity of Easy Plate™ LS was evaluated and compared with three conventional agar media (ALOA, PALCAM, and a chromogenic agar) and two commercially available RTU media (RTU-A and RTU-B). The inclusivity study included 28 Listeria monocytogenes strains and 20 strains of other Listeria species, while the exclusivity study evaluated 28 non-target strains. Each strain was enriched overnight, diluted in BPW, inoculated at approximately 100 CFU per plate, and incubated at 35 or 37°C for 24, 28, and 48 hours.

Result:

All L. monocytogenes strains were successfully detected on every medium within 24 hours. For other Listeria species, Easy Plate™ LS achieved growth rates of 75% (15/20) at 24 hours and 85% (17/20) at 28 hours, outperforming the two comparator RTU media, which showed approximately 50% recovery at 24 hours. ALOA demonstrated the highest recovery at 90% (18/20), while PALCAM and the chromogenic agar achieved 65–70% recovery at 24 hours. Some species, including L. seeligeri, L. ivanovii, and L. grayi, required extended incubation for optimal recovery. In the exclusivity study, Easy Plate™ LS, RTU-A, and ALOA showed no growth of non-target strains up to 48 hours, whereas prolonged incubation on some other media resulted in the growth of Enterococcus and Bacillus species.

Significance:

Easy Plate™ LS demonstrated high selectivity and reliable detection of Listeria species while minimizing false-positive growth from non-target microorganisms. By eliminating the need for complex media preparation and providing rapid results within 24 hours, Easy Plate™ LS is expected to improve the efficiency and reliability of environmental monitoring programs for Listeria in food production facilities.