The long-term survival of Propionibacterium freudenreichii in a context of nutrient shortage

AimsPropionibacterium freudenreichii is an actinobacterium widely used in dairy industry during the ripening process of Swiss-type cheeses and which presents probiotic properties. P. freudenreichii is reportedly a hardy bacterium, able to survive during the cheese-making process and when subjected to digestive stresses. During this study the long-term survival (LTS) of P.freudenreichii was investigated for 11days by means of phenotypic characterization in a culture medium without the addition of any nutrients. Methods and ResultsFor 11days, in a non-nutrient supplemented culture medium, eight strains were monitored by measuring their optical density, counting colony-forming units (CFU) and using LIVE/DEAD staining and microscopy observation. Under these conditions, all strains displayed high survival rates in the culture medium, their culturability reaching more than 9log(10)CFUml(-1) after 2days. After 11days, this value ranged from 78 to 82log(10)CFUml(-1) depending on the strain, and at least 50% of the P.freudenreichii population displayed an intact envelope. As lysis of part of a bacterial population may be a microbial strategy to recover nutrients, in CIRM-BIA 138 (the strain with the highest population at day 11), cell lysis was assessed by quantifying intact bacterial cells using qPCR targeting the housekeeping gene tuf. No lysis was observed. ConclusionTaken together, our results suggest that P.freudenreichii strains use a viable but nonculturable state to adapt to the LTS phase. Significance and Impact of the StudyAssessing the viability of P.freudenreichii and understanding their mechanisms for survival should be of great interest regarding their potential probiotic applications.