The effect of silver ion-implantation of stainless steel on bacterial adhesion and biofilm formation : a thesis presented in partial fulfilment of the requirements for a Master of Science in Microbiology at Massey University, Palmerston North, New Zealand
Biofilms comprise an assembly of microbial communities attached to a surface and enclosed in a polysaccharide matrix. Biofilms are a problem in dairy manufacturing plants where they cause biofouling of the stainless steel surface, resulting in product contamination and the need to shorten manufacturing runs for frequent cleaning. The equipment has to be thoroughly cleaned at regular intervals to remove biofilms. Clean-In-Place (CIP) procedures used in dairy manufacturing plants are not effective enough to remove all the biofilm cells as the extracellular polysaccharide (EPS), an integral part of a biofilm, reduces the penetration of cleaning agents. One possible alternative method to control biofilm growth involves surface modification of the stainless steel by implanting silver ions to prevent the attachment of viable bacteria that would otherwise form biofilms.
Stainless steel coupons, implanted with 1 × 1016 silver ions per cm2, and control stainless steel coupons were tested for the attachment of Streptococcus thermophilus and Pseudomonas fluorescens in various media for up to 30 minutes. Biofilm formation and EPS production for up to 24 hours was studied on the silver-implanted and control coupons in whole milk, skimmed milk and whey. It was found that there was higher attachment (0.49 and 0.18 log CFU per cm2 of S. thermophilus and P. fluorescens, respectively) on the stainless steel coupons than on the silver-implanted coupons in saline. In the presence of milk and whey, the difference in the attachment of bacteria on the two coupons reduced. Biofilm studies showed that the number of bacteria colonising both types of coupons was not statistically significantly different (P > 0.05). While the Live/Dead® BacLight™ Bacterial Viability stain showed that there were a few dead cells on the silver-implanted surfaces, scanning electron micrographs showed that the bacteria attached to a conditioning layer formed by the milk and whey proteins. Furthermore, both bacteria produced EPS, which, along with the conditioning film, might have masked the effect of silver ions from bacteria, resulting in similar numbers of bacteria present on the test and control coupons. Thus, due to the shielding effect of EPS and the conditioning film, the silver-implanted surfaces may be of limited practical value in the dairy industry.