Slow food for lactobacilli: characterization of 1,2-propanediol metabolism in <i>Levilactobacillus parabrevis</i> FUA3697 in sourdough

Some lactobacilli convert lactate to acetate and 1,2-propanediol in the stationary phase of growth, but the key enzyme of the pathway, lactaldehyde dehydrogenase, has not been characterized in any of the <i>Lactobacillales</i>. It was, therefore, the aim of this study to characterize lactate to 1,2-propanediol conversion in <i>Levilactobacillus parabrevis</i> FUA3697. The strain converted lactate to 1,2-propanediol during 14 days of incubation. The concentration of 1,2-propanediol was higher in media with maltose, glucose, and fructose as carbon sources when compared to arabinose; the addition of lactate increased the 1,2-propanediol formation. The 1,2-propanediol formation was higher in sourdough than in laboratory media and higher in sorghum sourdoughs than in wheat sourdoughs. The construction of a mutant strain <i>Lv. parabrevis</i> FUA3697Δ<i>aldA</i> confirmed that AldA is responsible for the conversion, as the mutant strain did not produce 1,2-propanediol. Competition experiments demonstrated that the disruption of <i>aldA</i> slightly decreased the competitiveness in wheat sourdoughs but not in sorghum sourdoughs. After incubation of the strains over 28 days at 4°C, the wild-type strain resumed growth and acidification faster than the <i>aldA</i>-deficient isogenic mutant. In summary, lactate conversion to acetate and 1,2-propanediol maintains slow metabolic activity over weeks or even months, thus increasing the ecological fitness.IMPORTANCEThe genes coding for lactate conversion to 1,2-propanediol are relatively broadly distributed in heterofermentative lactobacilli and also present in few homofermentative lactobacilli. To date, the conversion has been verified experimentally only in <i>Lentilactobacillus buchneri</i>, few other lentilactobacilli and, more recently, <i>Levilactobacillus lettrarii</i>. 1,2-Propanediol is a substrate for further conversion to propionic acid. The metabolic pathway contributes to spoilage of pickles and cider but is considered a beneficial metabolic trait in silage and sourdough fermentations. Our results document that lactobacilli using this pathway can be employed for improved activity and competitiveness of starter cultures that are distributed under refrigerated conditions.