Bacillus toyonensis (a Gram-positive bacterium) and Pseudomonas aeruginosa (a Gram-negative bacterium) isolated from the different surfaces of a dairy plant in our previous study were selected as the test bacteria for the present study. These two test bacteria were investigated in terms of their attachment on the stainless steel test surfaces in a model dairy batch system. After incubation at 5 °C and 20 °C for 6 h, 12 h, and 24 h, stainless steel plates were examined using cultural counts, profilometer, scanning electron microscopy (SEM), and fluorescent microscopy. Also, the test plates were subjected to a cleaning/disinfection procedure used in the dairy plant. Tests were employed before and after the cleaning/disinfection procedures. Cell wall characteristics and holding temperature were found to be significant for the attachment of the test bacteria to stainless steel test surfaces. In the study, the effect of the holding temperature varied depending on the type and characteristics of the bacteria. The adhesion ability of P. aeruginosa was higher than that of B. toyonensis. Increases in the holding temperature may increase the adhesion ability of the bacteria. Milk growth medium was found to be more successful in preventing the attachment ability of P. aeruginosa compared to B. toyonensis. This indicates that the chemical characteristic of the contact material may affect adhesion. The adhered bacterial cells were entirely removed by means of the cleaning/disinfection treatment. Therefore, the adhesion of bacterial cells could be explained as "initial phase of biofilm formation." It can be concluded that the microorganism cell adhesion on the surface is followed by biofilm formation, and this situation lasts for many years. These results reveal the importance of controlling biofilm formation in dairy plants from the beginning.

Department of Food Engineering Hacettepe University Beytepe Ankara Turkey

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