The main aim of this study was to investigate the potential propagation of bacteriophage in biofilms of cheese starter cultures. It was hypothesized that the biofilms of the starter cultures may form in cheese and casein plants, perhaps in whey drain lines and become a reservoir and site of propagation of phage particles. Phage can infect and destroy the bulk starter cultures used for cheese production and this might cause a severe economic loss in the cheese manufacturing industry. Bacteriophage attacks against thermophilic dairy starter bacteria are now recognized as the main cause of slow or faulty fermentative production in dairy plants. The objective of this study was to determine whether the starter cultures are capable of producing biofilms and to investigate the ability of phage to propagate inside them. The project was started by examination of attachment and biofilm formation by S. thermophilus, a common starter bacterium used for the manufacture of cheese.

The initial experiments showed that these starter cultures have the capacity to attach and produce biofilms on stainless steel coupons at a temperature of 30ºC, which showed that they were capable of attaching to the whey effluent lines in a cheese factory and thereby become potential sites for phage propagation. An experiment was conducted by infecting an existing biofilm with phage. The results showed that phage have the capacity to penetrate an existing biofilm matrix and produce an infection, causing the number of cells to decrease sharply. The results of this study has confirmed the fact that phage not only attack planktonic cultures, but have the ability to propagate in biofilms.

The experiment was repeated, observing the fate of the cells in the biofilm over a prolonged duration. It was observed that the number of cells in the biofilm decreased with concomitant increase of phage numbers. However, over the period of observation, there remained viable cells in the biofilm. Therefore it is evident from this study that phage particles replicate within a biofilm, and thus biofilms are a potential reservoir of infection in cheese and casein plants. It is suggested that the incidence of phage in the cheese industry can be reduced by preventing the growth of biofilms in whey and casein transfer pipes.