Effect of milk proteins on adhesion of bacteria to stainless steel surfaces

Stainless steel coupons were treated with skim milk and subsequently challe nged with individual bacterial suspensions of Staphylococcus aureus, Pseudo monas fragi, Escherichia coli, Listeria monocytogenes, and Serratia marcesc ens, The numbers of attached bacteria were determined by direct epifluoresc ence microscopy and compared with the attachment levels on clean stainless steel with two different surface finishes. Skim milk was found to reduce ad hesion of S. aureus, L. monocytogenes, and S. marcescens, P. fragi and E. c oli attached in very small numbers to the clear surfaces, making the effect of any adsorbed protein layer difficult to assess. Individual milk protein s alpha-casein, beta-casein, kappa-casein, and alpha-lactalbumin were also found to reduce the adhesion of S. aureus and L. monocytogenes, The adhesio n of bacteria to samples treated with milk dilutions up to 0.001% was inves tigated, X-ray photoelectron spectroscopy was used to determine the proport ion of nitrogen in the adsorbed films. Attached bacterial numbers were inve rsely related to the relative atomic percentage of nitrogen on the surface. A comparison of two types of stainless steel surface, a 2B and a no. 8 mir ror finish, indicated that the difference in these levels of surface roughn ess did not greatly affect bacterial attachment, and reduction in adhesion to a milk-treated surface was still observed. Cross-linking of adsorbed pro teins partially reversed the inhibition of bacterial attachment, indicating that protein chain mobility and steric exclusion may be important in this phenomenon.