Comparative morphological characteristics of three Brettanomyces bruxellensis wine strains in the presence/absence of sulfur dioxide

High intraspecific variation of the cell surface physico-chemical and bioadhesion properties in Brettanomyces bruxellensis

Brettanomyces bruxellensis is the most damaging spoilage yeast in the wine industry because of its negative impact on the wine organoleptic qualities. The strain persistence in cellars over several years associated with recurrent wine contamination suggest specific properties to persist and survive in the environment through bioadhesion phenomena. In this work, the physico-chemical surface properties, morphology and ability to adhere to stainless steel were studied both on synthetic medium and on wine. More than 50 strains representative of the genetic diversity of the species were considered. Microscopy techniques made it possible to highlight a high morphological diversity of the cells with the presence of pseudohyphae forms for some genetic groups. Analysis of the physico-chemical properties of the cell surface reveals contrasting behaviors: most of the strains display a negative surface charge and hydrophilic behavior while the Beer 1 genetic group has a hydrophobic behavior. All strains showed bioadhesion abilities on stainless steel after only 3 h with differences in the concentration of bioadhered cells ranging from 2.2 × 10² cell/cm² to 7.6 × 10⁶ cell/cm². Finally, our results show high variability of the bioadhesion properties, the first step in the biofilm formation, according to the genetic group with the most marked bioadhesion capacity for the beer group.

Brettanomyces bruxellensis: Overview of the genetic and phenotypic diversity of an anthropized yeast

Human-associated microorganisms are ideal models to study the impact of environmental changes on species evolution and adaptation because of their small genome, short generation time, and their colonization of contrasting and ever-changing ecological niches. The yeast Brettanomyces bruxellensis is a good example of organism facing anthropogenic-driven selective pressures. It is associated with fermentation processes in which it can be considered either as a spoiler (e.g. winemaking, bioethanol production) or as a beneficial microorganism (e.g. production of specific beers, kombucha). Besides its industrial interests, noteworthy parallels and dichotomies with Saccharomyces cerevisiae propelled B. bruxellensis as a valuable complementary yeast model. In this review, we emphasize that the broad genetic and phenotypic diversity of this species is only beginning to be uncovered. Population genomic studies have revealed the co-existence of auto- and allotriploidization events with different evolutionary outcomes. The different diploid, autotriploid and allotriploid subpopulations are associated with specific fermented processes, suggesting independent adaptation events to anthropized environments. Phenotypically, B. bruxellensis is renowned for its ability to metabolize a wide variety of carbon and nitrogen sources, which may explain its ability to colonize already fermented environments showing low-nutrient contents. Several traits of interest could be related to adaptation to human activities (e.g. nitrate metabolization in bioethanol production, resistance to sulphite treatments in winemaking). However, phenotypic traits are insufficiently studied in view of the great genomic diversity of the species. Future work will have to take into account strains of varied substrates, geographical origins as well as displaying different ploidy levels to improve our understanding of an anthropized yeast's phenotypic landscape.

Shedding Light on the Formation and Structure of Kombucha Biofilm Using Two-Photon Fluorescence Microscopy

Kombucha pellicles are often used as inoculum to produce this beverage and have become a signature feature. This cellulosic biofilm produced by acetic acid bacteria (AAB) involves yeasts, which are also part of the kombucha consortia. The role of microbial interactions in the de novo formation and structure of kombucha pellicles was investigated during the 3 days following inoculation, using two-photon microscopy coupled with fluorescent staining. Aggregated yeast cells appear to serve as scaffolding to which bacterial cellulose accumulates. This initial foundation leads to a layered structure characterized by a top cellulose-rich layer and a biomass-rich sublayer. This sublayer is expected to be the microbiologically active site for cellulose production and spatial optimization of yeast–AAB metabolic interactions. The pellicles then grow in thickness while expanding their layered organization. A comparison with pellicles grown from pure AAB cultures shows differences in consistency and structure that highlight the impact of yeasts on the structure and properties of kombucha pellicles.

Prediction of Genetic Groups within Brettanomyces bruxellensis through Cell Morphology Using a Deep Learning Tool

Brettanomyces bruxellensis is described as a wine spoilage yeast with many mainly strain-dependent genetic characteristics, bestowing tolerance against environmental stresses and persistence during the winemaking process. Thus, it is essential to discriminate B. bruxellensis isolates at the strain level in order to predict their stress resistance capacities. Few predictive tools are available to reveal intraspecific diversity within B. bruxellensis species; also, they require expertise and can be expensive. In this study, a Random Amplified Polymorphic DNA (RAPD) adapted PCR method was used with three different primers to discriminate 74 different B. bruxellensis isolates. High correlation between the results of this method using the primer OPA-09 and those of a previous microsatellite analysis was obtained, allowing us to cluster the isolates among four genetic groups more quickly and cheaply than microsatellite analysis. To make analysis even faster, we further investigated the correlation suggested in a previous study between genetic groups and cell polymorphism using the analysis of optical microscopy images via deep learning. A Convolutional Neural Network (CNN) was trained to predict the genetic group of B. bruxellensis isolates with 96.6% accuracy. These methods make intraspecific discrimination among B. bruxellensis species faster, simpler and less costly. These results open up very promising new perspectives in oenology for the study of microbial ecosystems.

Biodiversity among Brettanomyces bruxellensis Strains Isolated from Different Wine Regions of Chile: Key Factors Revealed about its Tolerance to Sulphite

Brettanomyces bruxellensis is regarded as the main spoilage microorganism in the wine industry, owing to its production of off-flavours. It is difficult to eradicate owing to its high tolerance of adverse environmental conditions, such as low nutrient availability, low pH, and high levels of ethanol and SO2. In this study, the production of volatile phenols and the growth kinetics of isolates from various regions of Chile were evaluated under stressful conditions. Through randomly amplified polymorphic DNA (RAPD) analysis, 15 strains were identified. These were grown in the presence of p-coumaric acid, a natural antimicrobial and the main precursor of off-flavours, and molecular sulfur dioxide (mSO2), an antimicrobial synthetic used in the wine industry. When both compounds were used simultaneously, there were clear signs of an improvement in the fitness of most of the isolates, which showed an antagonistic interaction in which p-coumaric acid mitigates the effects of SO2. Fourteen strains were able to produce 4-vinylphenol, which showed signs of phenylacrylic acid decarboxylase activity, and most of them produced 4-ethylphenol as a result of active vinylphenol reductase. These results demonstrate for the first time the serious implications of using p-coumaric acid, not only for the production of off-flavours, but also for its protective action against the toxic effects of SO2.

Sulfur dioxide response of Brettanomyces bruxellensis strains isolated from Greek wine

Brettanomyces bruxellensis is the most common spoilage wine yeast which can provoke great economic damage to the wine industry due to the production of undesirable odors. The capacity of the species to adapt in various environmental conditions offers a selective advantage that is reflected by intraspecific variability at genotypic and phenotypic level. In this study, microsatellite analysis of 22 strains isolated from Greek wine revealed the existence of distinct genetic subgroups that are correlated with their geographical origin. The response of these strains to increasing levels of sulfur dioxide confirmed the presence of both sensitive and tolerant strains, which belong to distinguished genetic clusters. The genetic categorization of B. bruxellensis strains could be used by the winemakers as a diagnostic tool regarding sulfur dioxide sensitivity.

Starter cultures as biocontrol strategy to prevent Brettanomyces bruxellensis proliferation in wine

Brettanomyces bruxellensis is a common and significant wine spoilage microorganism. B. bruxellensis strains generally detain the molecular basis to produce compounds that are detrimental for the organoleptic quality of the wine, including some classes of volatile phenols that derive from the sequential bioconversion of specific hydroxycinnamic acids such as ferulate and p-coumarate. Although B. bruxellensis can be detected at any stage of the winemaking process, it is typically isolated at the end of the alcoholic fermentation (AF), before the staring of the spontaneous malolactic fermentation (MLF) or during barrel aging. For this reason, the endemic diffusion of B. bruxellensis leads to consistent economic losses in the wine industry. Considering the interest in reducing sulfur dioxide use during winemaking, in recent years, biological alternatives, such as the use of tailored selected yeast and bacterial strains inoculated to promote AF and MLF, are actively sought as biocontrol agents to avoid the “Bretta” character in wines. Here, we review the importance of dedicated characterization and selection of starter cultures for AF and MLF in wine, in order to reduce or prevent both growth of B. bruxellensis and its production of volatile phenols in the matrix.