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Forty-nine market cheeses representing 24 types and 28 brands were purchased from local supermarkets. Pieces of cheeses of approximately 1.5 × 0.5 cm were surface inoculated with log10 3.95 to 4.36 cells of a Listeria monocytogenes pool made up of five strains (Scott A, V7, RM-1, VPH1, VPH2) and placed in petri dishes. After wrapping with cellophane, the dishes were stored at 4, 8, and 30°C for up to 36 d. Of the cheeses, 36.7% supported growth equivalent to a mean inoculum increase of 1.4 log10 (range 0.21 to 3.58) in at least one storage temperature. They included soft Hispanic type (Queso Fresco, Panela Ranchero, pH 6.2–6.6), Ricotta (pH 5.9–6.1), Teleme (pH 5.9), Brie (pH 7.2–7.7), Camembert (pH 7.3), and cottage (pH 4.9–5.1) cheeses. Ricotta was the best and cottage the worst substrate for growth. Cheeses not supporting Listeria growth but causing gradual death at all temperatures include: Cotija (Hispanic hard cheese), cream, blue, Tillamook, Cracker Barrel, Monterey Jack, Swiss, Cheddar, Colby, string, Provolone, Muenster, Feta, and Kasseri with values of pH 4.3–5.6, process (American, Monterey Jack, Piedmont, pH 5.7–6.4), and Limburger (pH 7.2) cheeses. A highly significant (P<0.005) correlation of Listeria growth with cheese pH values >5.5 and absence of starter cultures during the cheese manufacturing was observed. Overall, the study demonstrated that cross-contamination of certain cheeses with L. monocytogenes originating from raw foods (meat, poultry, fish, vegetables), after opening of packages, may lead to significant growth of the pathogen during refrigerated storage.

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In 1984, we monitored 4 ranches with a total of 24 houses (15,000–20,000 birds/house) for 3 consecutive generations (January–August). On epidemiologic grounds, infection of birds did not originate at the hatcheries or the central water and feed. Considering all lots of birds, the infection rate increased from 2.3% by the 10th day to 9.5, 29.7, 47.9, 65.7, 78.6 and 81.8% by the 20th, 30th, 40th, 45th, 50th day and at slaughter times, respectively. Transmission from one generation of chickens to the next via the old litter is suspected, but not proven microbiologically. A 5-log reduction of Campylobacter jejuni was shown in experimentally inoculated litters stored at 17 and 30°C for 6 d and 8°C for 11 d. The houses remained empty for 9–29 d before being filled with new chicks. Carrier flocks contaminated the slaughterhouse equipment to such an extent that negative flocks processed afterwards resulted in contaminated meat. Lack of effective sanitation at the end of the day contributed to the contamination of meat from Campylobacter-free birds processed the next day. Feather picker drip water was positive 94% of the sampling times at levels of log10 3.4 (1.0–4.7). Scalding temperatures did not affect the level of contamination in the finished products (P>0.2). An ELISA based on heat-stable antigens was adapted for the detection of circulating antibodies. Of 56 broilers aged 50 to 68 d, only 2 (3.5%) 68 d old with log10 5.4 C. jejuni/g of feces were considered as positive. Birds considered negative harbored C jejuni in their ceca at levels of log10 2.0 to 5.4/g of feces. Five out of 6 (83%) 18 month-old hens were considered as positive. Yet, none of these birds were found carrying C. jejuni in their feathers or ceca.