Transcriptome-phenotype matching analysis of how nitrogen sources influence Lacticaseibacillus rhamnosus tolerance to heat stress and oxidative stress.

Background: Spray drying is the most cost-effective production method for lactic acid bacteria starters, but heat and oxidative stresses result in low survival rates. The heat stress and oxidative stress tolerance of Lacticaseibacillus rhamnosus cultured in tryptone-free MRS (NP-MRS) broth was much stronger than that in MRS or tryptone-free MRS broth supplemented with phenylalanine (Phe-MRS). Here, multiple transcriptome-phenotype matching was performed on cells cultured in NP-MRS, MRS and Phe-MRS broths to reveal the mechanism by which nitrogen sources influence L. rhamnosus tolerance to heat stress and oxidative stress.

Results: Compared with cells cultured in NP-MRS broth, 83 overlapping differentially expressed genes (DEGs) were downregulated by either tryptone or phenylalanine. The overlapping DEGs were mainly classified into carbohydrate metabolism and membrane transport pathways, which are often repressed by glucose during carbon catabolite repression (CCR). In the presence of glucose, the heat stress or oxidative stress tolerance of L. rhamnosus hsryfm 1301 was not strengthened by supplementation with secondary carbohydrates. Replacing glucose with mannose, fructose or ribose improved the heat stress and oxidative stress tolerance of L. rhamnosus hsryfm 1301 (5 to 46-fold).

Conclusions: Alleviation of CCR might be a reason for the resistance of L. rhamnosus hsryfm 1301 to heat stress and oxidative stress in a low-nitrogen environment. The survival rate of L. rhamnosus during spray drying will hopefully be improved by relieving CCR. It is a new discovery that nitrogen sources influence CCR in L. rhamnosus.