Quality typically falls when bacterial strains in starter cultures become infected by viruses called bacteriophage. This is particularly problematic in industrial cheese production, which uses “frozen batch inoculum,” according to the report. Unlike traditional “back-slopping” methods, in which samples from previous cheeses are used as starter cultures, frozen batch inoculum ensures that the bacteria, and thus the cheese product, will not vary.

However, while preventing bacterial evolution, frozen batch inoculum fails to preclude phage evolution. Thus, phage frequently gain the upper hand on the invariant bacteria.

Monitoring could offer prompt detection of quality problems, said corresponding author Helge Holo, PhD, Professor of Microbiology, Norwegian University of Life Sciences, Aas, Norway. Then, countermeasures could quickly mitigate the problems.

For example, the tool could identify strains that are most important to cheese quality. Then, those strains could be engineered to resist phage, or other strains having similar influence on taste and cheese quality, with less phage susceptibility, could substitute for those that are less resistant to phage, said Dr. Holo.

The tool the researchers developed is the use of next generation sequencing to sequence a gene called epsD. This is a protein that manufactures exopolysaccharide, an important compound made largely of sugars, that sits on the outer surfaces of cells. It is thought to be involved, among other things, in resisting phage.

In the study, the investigators isolated more than 200 strains of bacteria from three commercial starter cultures. From these, they sequenced the genomes of 95 strains. They then searched for the most variable gene that was present in all strains. The purpose was partly because such a gene could be used to determine how diverse the strains are—important information about starter cultures.

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