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

Abstract

Stainless steel coupons were treated with skim milk and subsequently challenged with individual bacterial suspensions of Staphylococcus aureus, Pseudomonas fragi, Escherichia coli, Listeria monocytogenes, and Serratia marcescens. The numbers of attached bacteria were determined by direct epifluorescence microscopy and compared with the attachment levels on clean stainless steel with two different surface finishes. Skim milk was found to reduce adhesion of S. aureus, L. monocytogenes, and S. marcescens. P. fragi and E. coli attached in very small numbers to the clear surfaces, making the effect of any adsorbed protein layer difficult to assess. Individual milk proteins alpha-casein, beta-casein, kappa-casein, and alpha-lactalbumin were also found to reduce the adhesion of S. aureus and L. monocytogenes. The adhesion of bacteria to samples treated with milk dilutions up to 0.001% was investigated. X-ray photoelectron spectroscopy was used to determine the proportion of nitrogen in the adsorbed films. Attached bacterial numbers were inversely 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 mirror finish, indicated that the difference in these levels of surface roughness did not greatly affect bacterial attachment, and reduction in adhesion to a milk-treated surface was still observed. Cross-linking of adsorbed proteins partially reversed the inhibition of bacterial attachment, indicating that protein chain mobility and steric exclusion may be important in this phenomenon.

FIG. 1

Adhesion of a number of bacterial species to both untreated stainless steel 2B samples and samples which have been treated with skim milk (bars represent the standard error of the mean).

FIG. 2

Effect upon the adhesion of S. aureus of pretreating stainless steel samples with individual milk proteins (0.5 mM) (bars represent the standard error of the mean).

FIG. 3

Adhesion of S. aureus (A), L. monocytogenes (F6861) (B), and S. marcescens (C) to stainless steel samples with a 2B surface finish which have been pretreated with a range of milk dilutions (bars represent the standard error of the mean). An asterisk indicates that the results represent the average of 40 readings from two replicate samples.

FIG. 4

XPS analysis of stainless steel 2B surfaces indicating levels of nitrogen (N 1s) and iron (Fe 2p) following treatment with a range of milk dilutions, combined with adhesion data for S. aureus.

FIG. 5

Adhesion of S. aureus and L. monocytogenes to stainless steel surfaces with either a 2B or a no. 8 mirror surface finish, with and without surface pretreatment with undiluted milk.

FIG. 6

Effect upon the adhesion of S. aureus and L. monocytogenes of treating a preadsorbed milk layer with glutaraldehyde (glut.) for 2 h compared with that of an untreated control (bars represent the standard error of the mean). An asterisk indicates that the results represent the average of 40 readings from two replicate samples.