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The attachment of Listeria monocytogenes isolates Scott A and Jalisco Cheese to stainless steel surfaces at 35", 21", and 10°C was investigated using scanning electron microscopy (SEM). Cells were found to adhere at all three temperatures, but cells with fibrils were observed only at 21°C. When grown at pH 8, the attachment matrix was more prevalent at 21°C than at 35°C and may also be related to the length of incubation time at 21°C. It appears that adherence may be related to the flagella and any exopolymer surrounding the cells.

Listeria monocytogenes serotype 3a and Pseudomonas fragi ATCC 4973 were examined for attachment capability and biofilm development on glass coverslips under flowing systems. Tryptic soy broth supplemented with yeast extract was the growth medium. A continuous flow slide chamber was developed for in situ observations using phase-contrast microscopy. Glass coverslips were examined by epifluorescent and scanning electron microscopy for biofilm formation. The ultrastructure of attached test organisms was examined for the presence of exopolymers using transmission electron microscopy. In pure cultures, attachment of L. monocytogenes to glass coverslips was sparse, while P. fragi accumulated on glass coverslips as a confluent layer of cells. When L. monocytogenes was grown in mixed culture with P. fragi, an exopolymer-producing microorganism, attachment and microcolony formation by L. monocytogenes was enhanced. Results suggest that under flowing conditions the presence of an exopolymer-producing microorganism may be more important than hydrophobicity, surface charge, or flagellar movement in attachment of L. monocytogenes to inert surfaces.

The growth parameters for Wisteria monocytogenes Scott A at various temperatures, pH values, water activity values, and in different carbohydrates were assessed. Growth was seen from 4°C through 45°C and at pH values from 4.7 through 9.2. Generation times were calculated for each parameter at which growth was observed. Optimum growth was defined as shortest generation time. L. monuqyfugenes grew optimally over the temper&re range 30-37°C. The optimum uH was 7.0. Growth was demonstrated in solutions of UD to 39.4% sucrose (a~ of 0.92). Carbohydrate fermentation was variable. Further studies involved examination of growth at optimal water activities (0.97) with suboptimal temperatures and pH vaIues, and at 7.0, with suboptimal temperatures and water activities. Growth was seen when these parameters were combined.

Microorganisms have been shown to adhere to food-contact surfaces and may provide a route for the contamination of processed food. To better understand this phenomenon, the effects of growth media and surface conditioning on the adherence of Pseudomonas fragi, Salmonella typhimurium and Listeria monocytogenes cells to stainless steel were studied. The microorganisms were grown in tryptic soy broth (TSB), 1% reconstituted skim milk (RSM) and RSM with 1% sucrose (RSM + S). Stainless-steel surfaces were conditioned by immersion in growth media for 1 h and then were rinsed in phosphate-buffered saline (PBS) prior to the adherence assay. After growing in each medium, cells were harvested, resuspended in PBS, and then allowed to contact the stainless steel for 30 min. Adherence was quantified by acridine orange-staining the cells and viewing under epifluorescence microscopy. Growth media had little influence on adherence to stainless steel that had not been preconditioned. P. fragi and L. monocytogenes cells adhered in the highest numbers when grown in RSM plus sucrose. S. typhimurium cells showed the highest level of adherence when grown in TSB. Analysis of variance yielded P values of less than 0.01, indicating that both growth media and surface conditioning were significant in the level of adherence observed.

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