Retail survey of Brazilian milk and Minas frescal cheese and a contaminated dairy plant to establish prevalence, relatedness, and sources of Listeria monocytogenes isolates

Abstract

A study was designed to recover Listeria monocytogenes from pasteurized milk and Minas frescal cheese (MFC) sampled at retail establishments (REs) and to identify the contamination source(s) of these products in the corresponding dairy processing plant. Fifty milk samples (9 brands) and 55 MFC samples (10 brands) were tested from REs located in Juiz de Fora, Minas Gerais, Brazil. All milk samples and 45 samples from 9 of 10 MFC brands tested negative for L. monocytogenes; however, "brand F" of MFC obtained from REs 119 and 159 tested positive. Thus, the farm/plant that produced brand F MFC was sampled; all samples from the milking parlor tested negative for L. monocytogenes, whereas several sites within the processing plant and the MFC samples tested positive. All 344 isolates recovered from retail MFC, plant F MFC, and plant F environmental samples were serotype 1/2a and displayed the same AscI or ApaI fingerprints. Since these results established that the storage coolers served as the contamination source of the MFC, plant F was closed so that corrective renovations could be made. Following renovation, samples from sites that previously tested positive for the pathogen were collected from the processing environment and from MFC on multiple visits; all tested negative for L. monocytogenes. In addition, on subsequent visits to REs 159 and 119, all MFC samples tested negative for the pathogen. Studies are ongoing to quantify the prevalence, levels, and types of L. monocytogenes in MFC and associated processing plants to lessen the likelihood of listeriosis in Brazil.

FIG. 1.

Diagram of plant F cheese processing plant. (A) Arrows indicate the flow of milk from the milking parlor to the cheese vat to form the curd, then to the sink/table to place the curd in the molds, and then to the coolers/refrigeration units for overnight storage on day 1 of production. (B) Thin arrows show the flow of cheese from the coolers/refrigeration units to the sink/table for removal from the molds and subsequent packaging on day 2 of production. Thick arrows show the flow of cheese after packaging either back to the coolers/refrigeration units for storage or for shipment from the plant to REs.

FIG. 2.

Photographs of plant F before and after renovations. (A) Coolers used for storage of MFC. (B) Cheese processing area. (C) Main entrance.

FIG. 3.

Schematic representation of AscI- and ApaI-generated PFGE restriction patterns for L. monocytogenes strains isolated from a sample of MFC and from the coolers/refrigeration units. L. monocytogenes MFS1435 (serotype 1/2a) was used as the reference standard (Lm standard).