Diversity of bacterial strains and bacteriophages in Norwegian cheese production
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- Master's theses (KBM) 
A study on diversity of bacterial strains and bacteriophages in Norwegian cheese production was assessed on samples from TINE`s cheese production. Samples were obtained from a dairy plant in Nærbø that was experienced fermentation problems. 96 bacterial strains were also isolated from another TINE's dairy plant in Storsteinnes which was producing quality cheese. Both dairies were using the same commercial starter cultures. For phage isolation bulk starter and whey samples were used to infect 96 bacterial strains. Forty phages were isolated from specific indicator strains. Using the isolated phages, phage typing of all the 96 strains was performed, and plaque formation was obtained on 80 strains. The sensitivity of bacterial strains was highly variable and isolated phages had a broad host range infecting between 4% (Ф837) and 44% (Ф878) of the strains. In addition, (GTG)5-PCR enabling the differentiation of all the bacterial strains in to 8 clusters. Furthermore, cell wall polysaccharide genes typing of bacterial strains were also carried out and 91 strains were found to have genes encoding different CWPS types, while five strains were suggested to have another type of cell surface. Our results showed that Lactococcus lactis of CWPS group B were the predominant starter culture used in the Norwegian cheese production. Finally, the effect of the isolated phages in fermentation activity of mixed starter cultures was also assessed and results showed a change in pH during milk fermentation. Though, pooled phage isolates caused inhibition in acid production, our phage isolates alone were not shown to cause a severe effect in the activities of the commercial starter culture. Therefore, further studies using single starter culture are recommended to reveal the actual impact of the isolated phages on dairy fermentation processes.