Competition for Nitrogen Resources: An Explanation of the Effects of a Bioprotective Strain Metschnikowia pulcherrima on the Growth of Hanseniaspora Genus in Oenology.

As a biological alternative to the antimicrobial action of SO2, bioprotection has been proposed to winemakers as a means to limit or prevent grape musts microbial alteration. Competition for nitrogenous nutrients and for oxygen are often cited as potential explanations for the effectiveness of bioprotection. This study analyses the effect of a bioprotective M. pulcherrima strain on the growth of one H. valbyensis strain and one H. uvarum strain. Bioprotection efficiency was observed only against H. valbyensis inoculated at the two lowest concentrations. These results indicate a potential species-dependent efficiency of the bioprotective strain and a strong impact of the initial ratio between bioprotective and apiculate yeasts. The analysis of the consumption of nitrogen compounds revealed that leucine, isoleucine, lysine and tryptophan were consumed preferentially by all three strains. The weaker assimilation percentages of these amino acids observed in H. valbyensis at 24 h growth suggest competition with M. pulcherrima that could negatively affects the growth of the apiculate yeast in co-cultures. The slowest rate of O2 consumption of H. valbyensis strain, in comparison with M. pulcherrima, was probably not involved in the bioprotective effect. Non-targeted metabolomic analyses of M. pulcherrima and H. valbyensis co-culture indicate that the interaction between both strains particularly impact lysin and tryptophan metabolisms.

Bioprotection Efficiency of Metschnikowia Strains in Synthetic Must: Comparative Study and Metabolomic Investigation of the Mechanisms Involved.

Three Metschnikowia strains marketed as bioprotection yeasts were studied to compare their antimicrobial effect on a mixture of two Hanseniaspora yeast strains in synthetic must at 12 °C, mimicking pre-fermentative maceration by combining different approaches. The growth of the different strains was monitored, their nitrogen and oxygen requirements were characterised, and their metabolomic footprint in single and co-cultures studied. Only the M. fructicola strain and one M. pulcherrima strains colonised the must and induced the rapid decline of Hanseniaspora. The efficiency of these two strains followed different inhibition kinetics. Furthermore, the initial ratio between Metschnikowia and Hanseniaspora was an important factor to ensure optimal bioprotection. Nutrient consumption kinetics showed that apiculate yeasts competed with Metschnikowia strains for nutrient accessibility. However, this competition did not explain the observed bioprotective effect, because of the considerable nitrogen content remaining on the single and co-cultures. The antagonistic effect of Metschnikowia on Hanseniaspora probably implied another form of amensalism. For the first time, metabolomic analyses of the interaction in a bioprotection context were performed after the pre-fermentative maceration step. A specific footprint of the interaction was observed, showing the strong impact of the interaction on the metabolic modulation of the yeasts, especially on the nitrogen and vitamin pathways.