Abstract
The viable bacterial count is a crucial quality indicator for lactic acid bacteria (LAB) starters and fermented foods. Metabolic activity is an integral component of stress tolerance pathways. Lacticaseibacillus rhamnosus exhibits enhanced heat and oxidative stress tolerance in tryptone-free media. To investigate the stress tolerance mechanisms from a metabolic perspective, the heat and oxidative stress tolerance and transcriptomic changes in L. rhamnosus hsryfm 1301 and its ccpA deficient strain (ΔccpA) were analyzed under different nitrogen source conditions. Slower growth, decreased heat stress tolerance, and enhanced oxidative stress tolerance were observed in ΔccpA in MRS. Compared to the wild-type strain, 260 genes were upregulated and 55 genes were downregulated in ΔccpA, mainly including carbon source transport and metabolism genes, but no typical stress tolerance genes. The regulation of pfk, pyk, dnaK, and groEL was different from that in other lactic acid bacteria. The pathways related to acetate production were regulated solely by ccpA deletion, while dnaK, groEL, and de novo pyrimidine synthesis genes were only regulated by tryptone. Fatty acid and purine synthesis genes and glmS were co-regulated by ccpA and tryptone. The deletion of ccpA eliminated the nitrogen source-induced oxidative stress tolerance changes. It was found that ccpA in L. rhamnosus can affect both carbon and nitrogen source metabolism, altering stress tolerance.