Abstract
Several techniques for enumerating bacteria at surfaces were investigated. Epifluorescence was chosen as being the most suitable method. There was not a great increase in adhesion between samples exposed to bacteria for 2 h and those exposed for 6 h. Sequential bacterial adhesion studies suggested that this may be due to a limiting number of suitable sites at the sample surface. Treatment of stainless steel samples with skimmed milk was shown to reduce the adhesion of several bacterial species. Individual milk proteins, alpha-, beta-, kappa-casein and alpha-lactalbumin, also reduced the adhesion of S. aureus and L. monocytogenes to stainless steel surfaces. The casein proteins were demonstrated to be more effective at reducing bacterial adhesion than the globular protein, alpha-lactalbumin. Cross-linking of mobile protein chains with glutaraldehyde caused an increase in the numbers of bacteria attaching to the surface. Several surface analysis techniques were implemented in order to investigate further the relationship between bacteria and a protein-coated surface. X-ray photoelectron spectroscopy demonstrated that kappa-casein adsorbed to the stainless steel surface in the largest amounts and alpha-lactalbumin the least. Thick layers of kappa-casein were also demonstrated at the surface using atomic force microscopy, whereas virtually no detail was observed with alpha-lactalbumin, again indicating lower adsorption. This appears to be the reason for the higher levels of bacterial adhesion at surfaces pre-treated with this protein. Atomic force microscopy of milk-treated surfaces produced images indicating a sparse background coverage with thicker protein deposits above this. Contact-mode imaging suggested that the surface of these structures comprised more loosely bound material. Combining this information with that provided by bacterial adhesion investigations and the effect of glutaraldehyde treatment upon milk, it is postulated that bacterial adhesion at a protein-covered surface is reduced as a result of steric forces generated by protein material extending into solution.