Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2007
Publication Date: 11/1/2007
Citation: Moreno-Enriquez, R.I., Garcia-Galaz, A., Accedo-Felix, E., Gonzalez-Rios, H., Call, J.E., Luchansky, J.B., Diaz-Cinco, M. 2007. Prevalence, types, and geographical distribution of Listeria monocytogenes from a survey of retail Queso Fresco and associated cheese processing plants and dairy farms in Sonora, Mexico. Journal of Food Protection. 70(11):2596-2601. Interpretive Summary: The largest outbreak of the disease listeriosis in the United States was caused by the consumption of a soft, Hispanic-style cheese that was contaminated with the bacterial pathogen, Listeria monocytogenes (Lm). This study was conducted to determine possible sources of contamination of the pathogen on dairy farms, processing plants, and retail markets in Sonora, Mexico. In addition, it was also determined in this study if there were geographical (north, central, south) and/or seasonal effects (winter vs summer) that influenced the recovery of Lm. Finally, tests were conducted on the recovered Lm to determine if they were genetically related or not related. Of 381 total samples, Lm was recovered from 7 cheese samples (17 isolates) that were collected from 201 retail cheese samples. The pathogen was not recovered from dairy farm or processing plant samples. It was more likely to recover Lm during the summer season and from retail cheese samples in northern Sonora. Finally, testing determined that none of the 17 isolates were genetically related, indicating that there may be multiple sources of contamination. The results of this study will be helpful for processors and retailers to develop ways to control the contamination of this type of cheese with Lm.
Technical Abstract: The largest food borne listeriosis outbreak to date in North America was caused by the consumption of a soft, Hispanic-style cheese contaminated with L. monocytogenes (Lm). In this study, to determine contamination sources and isolate clonality, sampling collection for this type of cheese in the state of Sonora, Mexico was designed to address geographical (north, central, and south regions) and seasonal (summer and winter) effects, as well as harborage points (dairy farms, cheese processing plants, and retail markets). The microbiology-based Mexican Official Technique (MOT) was used to recover Lm, and pulsed field gel electrophoresis (PFGE) of genomic DNA digested with the enzyme SmaI was used to establish the relatedness of the multiple isolates retained from each positive sample. Of 381 total samples analyzed Lm was not recovered from samples tested from dairy farms or cheese processing plants. However, 7 cheese samples (17 isolates) from among 201 retail cheese samples analyzed tested positive for Lm. The distribution among the 201 cheese samples by strata was: i) 4 of 122 samples (11 isolates) tested positive in summer and 3 of 79 samples (6 isolates) tested positive in winter; and ii) 3 of 41 samples (9 isolates) in the northern, 2 of 134 samples (4 isolates) in the central, and 2 of 26 samples (4 isolates) in the southern regions tested positive for Lm. Each of the 17 isolates displayed a unique SmaI pulsotype, which highlights their heterogeneity and suggests that multiple harborage points exist. Recovery of Lm from retail cheese using MOT argues strongly for additional studies employing more highly sensitive detection methods to better determine the sources, prevalence, and types of Lm and to establish strategies to manage the potential threat of listeriosis associated with this type of cheese.