Microbial changes of natural milk cultures for mozzarella cheese during repeated propagation cycles

Natural milk cultures are undefined starters produced by a selective treatment (milk pasteurization, incubation at high temperature and backslopping), and used for the production of Mozzarella cheese. The objective of this study was to monitor the microbial composition (at or below the species level) and the variability of two model laboratory cultures (LC) and an artisanal culture (NMC) produced at a dairy plant over 13 reproduction cycles using culture independent and dependent methods. PCR-DGGE of V3 region of 16S RNA gene and of a fragment of the lacSZ operon of Streptococcus thermophilus proved that the cultures were dominated by S. thermophilus but other species (Lactobacillus delbrueckii, L. helveticus, Lactococcus lactis, enterococci) were present. These results were partially confirmed by culture dependent analysis. Molecular identification and typing (RAPD-PCR and lacSZ PCR-DGGE and partial sequencing of serB in S. thermophilus) confirmed NMC had the highest diversity, while the two replicate LCs diverged at some early stage. Variability in acid production activity and in aroma production was also observed, and was highest in NMC. S. thermophilus bacteriophages were detected by multiplex PCR.