Identification of genes essential for bile acid resistance in the probiotic Lacticaseibacillus paracasei strain Shirota.

Bile acid resistance is crucial to allow probiotic strains to survive in the gastrointestinal tract and exert health-promoting effects on their hosts. Our aim here was to determine the mechanism of this resistance via a genetic approach by identifying the genes essential for bile acid resistance in Lacticaseibacillus paracasei strain Shirota (LcS). We generated 4649 transposon-inserted lines of L. paracasei YIT 0291, which has the same genome sequence as LcS but lacks the pLY101 plasmid, and we screened them for bile-acid-sensitive mutants. The growth of 14 mutated strains was strongly inhibited by bile acid, and we identified 10 genes that could be involved in bile acid resistance. Expression of these genes was not markedly induced by bile acid, suggesting that their homeostatic expression is important for exerting bile acid resistance. Two mutants in which the transposon was independently inserted into cardiolipin synthase (cls) genes, showed strong growth inhibition. Disruption of the cls genes in LcS caused decreased cardiolipin (CL) production and the accumulation of the precursor phosphatidylglycerol in bacterial cells. These data suggest that LcS possesses several mechanisms for exerting bile acid resistance, and that homeostatic CL production is among the factors most essential for this resistance.