|Knowles, Donald - Don|
Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2003
Publication Date: 8/20/2003
Citation: MURAOKA, W.T., GAY, C., KNOWLES JR, D.P., BORUCKI, M.K. PREVALENCE OF LISTERIA MONOCYTOGENES SUBTYPES IN BULK MILK OF THE PACIFIC NORTHWEST. JOURNAL OF FOOD PROTECTION. 2003. v. 66(8). p. 1413-1419. Interpretive Summary: Raw bulk milk from Pacific Northwest dairies was tested for the presence of Listeria monocytogenes at three timepoints. L. monocytogenes isolates were serotyped and subtyped by PFGE to determine the prevalence, epidemiology, and molecular characteristics of regional subtypes. A majority of isolates were of serotype 1/2a and a surprisingly large number of isolates from different herds had the same molecular subtype. This may indicate a common source of L. monocytogenes contamination in area dairy farms and/or bulk milk samples. Alternatively, this may be due to the ability of this particular subtype to persist on dairy farms for an extended period of time. The PFGE subytpe of 23 samples of L. monocytogenes isolates obtained from the Washington Department of Health was determined and compared to the subtypes found in bulk milk to determine if the same subtypes associated with raw milk were also associated with human listeriosis. Although genetic similarities existed between some of the L. monocytogenes strains, no identical genetic match was found.
Technical Abstract: To determine the prevalence of Listeria monocytogenes in the Pacific Northwestern United States, bulk milk from 474 herds in three states was sampled at two time-points. Sample collections occurred in November 2000 and June 2001, and the prevalence was 4.9% and 7.0%, respectively. All isolates were subtyped by serotyping and pulsed-field gel electrophoresis (PFGE). Fifty-one of the 57 isolates belong to serogroup 1/2a, while six belonged to serogroup 4. Subtyping by PFGE revealed that isolates from 31 herds shared ten patterns; there was a weak, but significant association between PFGE subtype and geographical distance. Six herds were positive for L. monocytogenes at both time points. Of these six herds, four had indistinguishable PFGE patterns at both time points. Twenty-five of the 33 herds that were positive in June 2001 were sampled again in June 2002. L. monocytogenes was recovered from 17 of the 25 herds (68%), eight of which had identical ApaI restriction enzyme digestion profiles (REDP) as isolates recovered from the previous year. ApaI REDP of the bulk milk isolates were compared to isolates recovered from environmental and human samples that were collected by the Washington Department of Health (n = 23). Analysis of digestion profiles using ApaI revealed that only two isolates from the Department of Health had digestion profiles similar to isolates from bulk milk; however further analysis using a second enzyme (AscI) was capable of discriminating between isolates from the two sources. Thus, we found no direct REDP matches between bulk milk and clinical isolates.