Microbial vintage

It always has been a very important questions to some - What makes a great wine? Answers to this question have been - to say the least - diverse: the right soil, a good grape, a competent winegrower, etc. Well, it looks like we have to add one more item to the list - good bacteria.

On their journey from the vineyard to the wine bottle, grapes are transformed to wine through microbial activity, with indisputable consequences for wine quality parameters. Wine grapes harbor a wide range of microbes originating from the surrounding environment, many of which are recognized for their role in grapevine health and wine quality. However, determinants of regional wine characteristics have not been identified, but are frequently assumed to stem from viticultural or geological factors alone.

Researchers from the University of California, Davis, analyzed grape samples from eight California wineries and found that microbes factor into the way wines taste from region to region. In total they collected 273 grape must samples. Must is actually a term for wine grapes that have been plucked from the stem and crushed together. The reason why the researchers decided to use must, and not just individual grapes, was because individual grapes - even from the same vineyard - can have different microbial communities on them. Using must was a rather simple way of pooling the samples.

The study used a high-throughput, short-amplicon sequencing approach (Illumina platform) with primers for the V4 domain of bacterial 16S rRNA and the internal transcribed spacer (ITS) DNA Barcode to identify the types of bacterial and fungal species in each set of must. 

They found that different bacterial compositions were present in different regions, potentially giving grapes a distinctive taste or in the worst case - ruining them. Lactic acid bacteria, for instance, were found to be common in the Napa Valley. Lactic acid bacteria can lead to the spoiling of wine, but they are also capable to turn malic acid into lactic acid, which gives a Zinfandel wine a distinct taste. Species composition differences between regions might also contribute to differences of taste between two Zinfandel wines.    

This metabarcoding approach makes it possible to ascertain which specific microbes and combinations of microbes are more or less important for the quality of a given wine in a given vineyard over time. The consistency of the microbial diversity over time may ultimately contribute to the quality of a wine and the reputation of vineyards. However, whether these regionally different microbiota actually directly modulate wine sensory qualities must be experimentally tested, as do pretty much all putative features of wine terroir.

We demonstrate that grape-associated microbial biogeography is nonrandomly associated with regional, varietal, and climatic factors across multiscale viticultural zones. This poses a paradigm shift in our understanding of food and agricultural systems beyond grape and wine production, wherein patterning of whole microbial communities associated with agricultural products may associate with downstream quality characteristics. Elucidating the relationship between production region, climate, and microbial patterns may enhance biological control within these systems, improving the supply, consumer acceptance, and economic value of important agricultural commodities.