No abstract

Aims: To determine whether isolates of Listeria monocytogenes differ in their ability to adsorb and form bio®lms on a food-grade stainless steel surface. Methods and Results: Strains were assessed for their ability to adsorb to a test surface over a short time period. Although some differences in numbers of bound cells were found among the strains, there were no correlations between the degree of adsorption and either the serotype or source of the strain. The ability of each strain to form a bio®lm when grown with the test surface was also assessed. With the exception of a single strain, all strains adhered as single cells and did not form bio®lms. Signi®cant differences in adherence levels were found among strains. Strains demonstrating enhanced attachment produced extracellular ®brils, whereas those which adhered poorly did not. A single strain formed a bio®lm consisting of adhered single cells and aggregates of cells. Conclusions: Signi®cant differences were found in the ability of various L. monocytogenes strains to attach to a test surface. In monoculture, the majority of strains did not form bio®lms. Signi®cance and Impact of the Study: Differences in attachment and bio®lm formation among strains provide a basis to study these characteristics in L. monocytogenes.

Various retail foods were analyzed for the presence of Listeria spp. One hundred ten samples of vegetables including lettuce, celery, tomatoes, and radishes, and 14 samples of pasteurized milk, were found to be free of L. monocytogenes. In contrast 9 of 16 (56.3%) chicken legs, 38 of 44 (86.4%) ground meats, and 6 of 30 (20%) fermented sausages contained the organism. Of 530 samples of ice cream products obtained at the manufacturing level, only 2 were positive for L. monocytogenes.

A total of 1564 fresh samples of 10 vegetable types from two different retail levels (533 samples from farmers' outdoor markets and 1031 samples from supermarkets) were surveyed for the occurrence of thermotolerant campylobacters. In samples from the outdoor markets, campylobacters were detected on six types of vegetables; the detection rates were spinach, 3.3; lettuce, 3.1; radish, 2.7; green onions, 2.5; parsley, 2.4; and potatoes, 1.6%. Campylobacter jejuni was the predominant species (88%), with the remainder being C. lari (8%) and C. coli (4%). When the outdoor market samples were thoroughly washed with chlorinated water, all were negative for campylobacters. Of the samples from supermarkets, all were negative for campylobacters whether purchased in summer or winter. These results suggest that vegetables sold at farmers' outdoor markets are produced and (or) stored under less sanitary conditions than those sold at supermarkets, and they could constitute health hazards. Therefore, vegetables (e.g., potatoes and spinach) from farmers' markets must be decontaminated by washing with chlorinated water or cooked thoroughly before consumption.

The ability of four Listeria monocytogenes strains to initiate growth in brain heart infusion broth adjusted to various pH values with either acetic, lactic, citric or hydrochloric acid was investigated. Acetic acid was the most effective inhibitor tested, since in broth adjusted with this acid a higher minimum pH was required for growth of the various strains at both 4 and 30°C, as compared with broth adjusted with the other acidulants. The minimum pH value required for the initiation of growth of L. monocytogenes ranged from 5·0 to 5·7 at 4°C, and from 4·3 to 5·2 at 30°C, depending upon the acidulant used.