Flow cytometry for automated analysis of milk containing Listeria monocytogenes

This paper describes the application of flow cytometry to the analysis of milk for the presence of Listeria monocytogenes. A total of 939 raw milk samples from 54 farms were analyzed for the presence of L. monocytogenes by cold enrichment vs flow cytometry, which incorporated a selective enrichment step. Analyzed samples consisted of milk from individual farm bulk tanks; string samples which represented milk from 25 to 40 cows; and combination samples representing milk from 200 cows. Raw milk samples were directly enriched for 24 h at 37 degrees C and analyzed by flow cytometry following fluorescence labeling of bacterial populations, or were cold enriched for 1 month at 4 degrees C. Following cold enrichment, samples were streaked to McBride's Listeria agar, and suspect colonies were biochemically identified as L. monocytogenes. L. monocytogenes was isolated from only 15 of the analyzed samples, all of which were common to one farm. Results of flow cytometry analysis yielded a 5.86% false positive rate and a 0.53% false negative rate when compared with culture procedures.