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The facultative anaerobic bacterium Lactobacillus casei IGM394 is used as a host for drug delivery systems, and it exhibits the same growth rate under aerobic and anaerobic conditions. L. casei strains carry several genes that facilitate oxygen and reactive oxygen species (ROS) tolerance in their genomes, but their complete functions have not been uncovered. To clarify the oxygen and ROS tolerance mechanisms of L. casei IGM394, we constructed 23 deficient mutants targeting genes that confer oxidative stress resistance. Significantly decreased growth and high H 2 O 2 accumulation were observed in the NADH peroxidase gene-mutated strain (Δ npr ) compared with the findings in the wild type. The H 2 O 2 degradation capacity of Δ npr revealed that NADH peroxidase is a major H 2 O 2 -degrading enzyme in L. casei IGM394. Interestingly, Δ ohrR , a mutant deficient in the organic hydroperoxide (OhrA) repressor, exhibited higher H 2 O 2 resistance than the wild-type strain. Increased Npr expression and H 2 O 2 degradation ability were observed in Δ ohrR , further supporting the importance of OhrA to ROS tolerance mechanisms. The other mutants did not exhibit altered growth rates, although some mutants had higher growth in the presence of oxygen. From these results, it is presumed that L. casei IGM394 has multiple oxygen tolerance mechanisms and that the loss of a single gene does not alter the growth rate because of the presence of complementary mechanisms. Contrarily, the H 2 O 2 tolerance mechanism is solely dependent on NADH peroxidase in L. casei IGM394.

NADH peroxidase plays a crucial role in consuming H<sub>2</sub>O<sub>2</sub> in <i>Lactobacillus casei</i> IGM394