Multilocus sequence typing of Propionibacterium freudenreichii

Propionibacterium freudenreichii is used as a ripening culture in Swiss cheese manufacture. This study investigates the molecular diversity and the population structure of this bacterium via multilocus sequence typing (MLST). Internal fragments of seven genes sequenced for 113 strains of different subspecies and origins allowed the resolution of 46 sequence types (STs) with occurrence frequencies ranging from 1 to 11. The core genome of the species harbours a low level of nucleotide polymorphism. In our data, single nucleotide polymorphisms account for only 2.28% of the concatenated sequences, and the average polymorphism rate in pairwise comparisons is 0.46%. The analyses reveal quantitatively comparable contributions of recombination and mutation in nucleotide changes at core genome loci along cell lineages. Remarkably, the STs exhibit little if any dairy biotope specialization. Phenotypic characterisation of the strains, based on their aptitude to use lactose and nitrate, shows that the two previously identified subspecies (freudenreichii and shermani) do not reflect the ancestral relationships in the P. freudenreichii population. The considerable phenotypic heterogeneity, found even at the ST level, suggests instead a history of recurrent switches between phenotypes.