The strain level antagonism between chain-forming lacticaseibacillus rhamnosus LV108 and hsryfm 1301

Identifying the interaction between different strains is crucial during the fermentation, clinical trials, production and consumption of mixed fermented foods. To investigate the interaction between 2 Lacticaseibacillus strains, strain-specific primer pairs for L. rhamnosus LV108 (L2, R 2 = 99.95%, E = 0.964) and L. rhamnosus hsryfm 1301 (Y3, R 2 = 99.88%, E = 0.986) were designed and verified. The DNA extraction and propidium monoazide (PMA) treatment process were optimized so that the DNA treatment effect could be compatible for both the 2 strains. During mixed fermentation, L. rhamnosus LV108 and L. rhamnosus hsryfm 1301 were successfully counted by qPCR, and the number of the living cells was measured at strain level by PMA-qPCR. The enumeration through PMA-qPCR eliminated the adverse impact resulted from the variable lengths of the cell chain. Both L. rhamnosus LV108 and L. rhamnosus hsryfm 1301 grew weaker in the mixed cultivation. Since 4 h, the growth of L. rhamnosus LV108 began to be inhibited, which resulted in an 85% decline of the number of genomic DNA compared with the single strain cultivation. This study provides a method for measuring the ratio of strains from the same species in a mixed culture system and monitoring the interaction at the strain level.