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United States Department of Agriculture

Agricultural Research Service


Location: Food Safety and Intervention Technologies

Title: Prevalence and characterization of Listeria monocytogenes, Salmonella, and Shiga-toxin producing Escherichia coli isolated from small Mexican retail makers of queso fresco

item Soto-Beltran, Marcela -
item Gerba, Charles -
item Porto-Fett, Anna
item Luchansky, John
item Chaidez-Quiroz, Cristobal -

Submitted to: International Journal of Environmental Health Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2014
Publication Date: N/A

Interpretive Summary: Queso fresco (QF) is a handmade cheese consumed and produced in Latin America and some regions of the United States. Artisanal QF has been associated as a source of pathogenic microorganisms such as Listeria monocytogenes, Salmonella, and Shiga Toxin producing E. coli. Foodborne outbreaks linked with QF consumption have occurred in the United States; however, the occurrence in Mexico is unknown due to the lack of understanding of the human health risk associated with the consumption. In a survey of QF purchased at retail establishments in Culiacan, Sinaloa, Mexico, L. monocytogenes (about 10% positive), as well as E. coli and coliforms (at least 95% positive) were detected among the 75 QF samples tested, whereas Salmonella was not detected. Knowing the incidence and characterization of pathogenic microorganisms in QF will allow the implementation of food safety regulations during its production. In addition, the use of quantitative microbial risk assessment should be conducted to estimate the risk associated with the consumption of retail Mexican-style soft cheese contaminated with pathogenic microorganisms.

Technical Abstract: Queso fresco (QF) is a handmade cheese consumed and produced in Latin America and some regions of the United States. In Mexico, QF production is associated with a microbiological risk, as it is typically manufactured using raw milk. The aims of the study were to determine the incidence and characterization of Listeria monocytogenes, Salmonella spp., and Shiga toxin-producing E. coli (STEC), in QF from retail markets of the Northwestern State of Sinaloa, Mexico, and to assess the effect of physicochemical parameters associated with Listeria monocytogenes presence. A total of 75 QF samples were obtained. L. monocytogenes, E. coli and coliforms were detected in 9.3%, 94% and 100% of the 75 QF samples, respectively. Salmonella was not detected. STEC isolates showed virulence genes stx1, stx2, eae, and H7. Microbial loads were above the maximum values recommended by the Official Mexican Standards (coliforms count =100 MPN/g, and Listeria and Salmonella must be absent in 25 g of cheese). Physicochemical parameters such as water activity (aw), moisture content, and pH played a role in Listeria prevalence in QF. Rigorous control in QF made in Culiacan, Mexico is needed to reduce the risk of foodborne pathogens.

Last Modified: 7/4/2015
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