Concentration of Listeria monocytogenes in skim milk and soft cheese through microplate immunocapture

Microplate immunocapture is an inexpensive method for the concentration of foodborne pathogens using an antibody-coated microplate. The objective of this study was to determine the efficacy of microplate immunocapture as an alternative to traditional enrichment for concentrating Listeria monocytogenes to levels detectable with selective plating or real-time PCR. L. monocytogenes isolates serologically characterized as Type 1 (1/2a) and Type 4 (untypeable) were grown overnight and diluted to 100 to 106 colony-forming units (CFU)/mL. The isolates were used to optimize microplate immunocapture in tryptic soy broth with 0.6% yeast extract (TSBYE), skim milk, and queso fresco samples. Following microplate immunocapture, the bacteria were streaked onto polymyxin-acriflavine-LiCl-ceftazidime-aesculin-mannitol (PALCAM) agar, followed by incubation at 37 °C for 24 ± 2 h. The bacteria also underwent real-time polymerase chain reaction (PCR). The optimized microplate immunocapture method was tested in triplicate for its ability to capture L. monocytogenes in broth and food samples. Overall recovery rates for L. monocytogenes in food samples at cell populations of 100, 102, and 104 CFU/25 g using microplate immunocapture with real-time PCR were 88.9%, 94.4%, and 100%, respectively. Recovery in these matrices using microplate immunocapture with selective plating was comparatively lower, at 0%, 44.4%, and 100%, respectively. Conventional culture method showed 100% detection at each inoculation level. Microplate immunocapture combined with real-time PCR shows high potential to reduce the time required for detection, with concentration of L. monocytogenes to detectable levels within 1-4 h. The incorporation of a short enrichment step may improve recovery rates at low cell levels.