Phylogenetic and functional characterization of tannin-degrading bacteria isolated during composting of olive mill waste

Composting of olive mill waste (OMW) can be a convenient way to remediate and valorise this by-product. Microorganisms play a key role in the organic matter transformation that occurs during OMW composting thanks to their vast array of enzymatic activities. Microbial tannases, in particular, are involved in the degradation of tannins, naturally occurring polyphenolic compounds that are abundant in OMW. These enzymes are of great commercial importance for their use in food and beverage processing and for the production of fine-chemicals, such as antioxidants. We have investigated the biodiversity of mesophilic and thermophilic bacterial strains isolated during composting and characterized their capacity of degrading tannins. Amplified rDNA restriction analysis (ARDRA) allowed the clustering of all the isolates in several operational taxonomic units (OTUs) while sequencing of the 16S rRNA gene provided their phylogenetic identification. Among the isolates, several different species were identified, belonging to the Phyla Proteobacteria, Actinobacteria and Firmicutes. Bacterial tannase activity was assessed incubating the isolates with methylgallate as reaction substrate followed by the observation of colour variation from green to brown and by absorbance measurement at 440 nm. Among the mesophilic isolates tested, six showed a moderate to strong tannase activity. On the contrary, no tannase activity was found among the thermophilic isolates tested. The strains that showed tannase activity belonged to four different OTUs, suggesting the presence of, at least, four different species of bacteria capable of tannins degradation. Interestingly, isolates belonging to the same OTU often showed a great variability in tannase activity suggesting that different bacterial populations, even belonging to the same species, can have different roles in the transformation of the organic matter that takes place during the OMW composting process. This work highlights the importance of combining phylogenetic and functional studies to better exploit the biotechnological potential of environmental microorganisms.