Determining the core bacterial and fungal genera in table olive fermentations

Table olives are among the most ancient and important fermented foods of the Mediterranean basin. Their production is still strongly related to traditional practices, and the lack of thermal treatments, the reliance on natural contamination and selective factors (NaCl, pH, occurrence of phenolics, etc.) determine the dynamics of the microbial community. Lactic acid bacteria (LAB) and yeasts have a pivotal role in table olive microbial communities, but several halophilic and alkalophilic microorganisms may also contribute, positively or negatively, to the quality and safety of this fermented vegetable food. We have used metataxonomic data extracted from the FoodMicrobionet database to provide quantitative insights on the structure of bacterial and fungal microbial communities of table olives and to identify core genera in different trade preparations. Celerinatantimonas and Lactiplantibacillus were the most prevalent genera among bacteria, followed by several LAB, halophilic and alkalophilic lactic acid bacteria (HALAB) and Gram negatives, including non-halophilic species. Similarly, 3 fungal genera (Pichia, Candida, and Wickerhamomyces) were the most abundant and prevalent among fungi. The distribution of both bacteria and fungi varied significantly in different olive varieties, among olives, brines and contact surfaces or materials, and at different production stages, and no clear grouping related to the combination of ripeness and trade preparation was found, although HALAB were characteristically abundant in Spanish style green olives. Addition of starter cultures affected the composition and dynamics of microbial communities to a variable extent.