Lactococcus lactis SpOx Spontaneous Mutants: a Family of Oxidative-Stress-Resistant Dairy Strains

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Abstract
Numerous industrial bacteria generate hydrogen peroxide (H 2 O 2 ), which may inhibit the growth of other bacteria in mixed ecosystems. We isolated sp ontaneous ox idative-stress-resistant (SpOx) Lactococcus lactis mutants by using a natural selection method with milk-adapted strains on dairy culture medium containing H 2 O 2 . Three SpOx mutants displayed greater H 2 O 2 resistance. One of them, SpOx3, demonstrated better behavior in different oxidative-stress situations: (i) higher long-term survival upon aeration in LM17 and milk and (ii) the ability to grow with H 2 O 2 -producing Lactobacillus delbrueckii subsp. delbrueckii strains. Furthermore, the transit kinetics of the SpOx3 mutant in the digestive tract of a human flora-associated mouse model was not affected.