Characterization of four olive-mill-wastewater indigenous bacterial strains capable of aerobically degrading hydroxylated and methoxylated monocyclic aromatic compounds

Seven aerobic bacterial strains capable of degrading several of the monocyclic aromatic compounds occurring in the phenolic fraction of olive-mill wastewaters (OMWs) were isolated from an Italian OMW. The results of the 16S rDNA restriction analysis evidenced that these strains are distributed among four different groups. One strain of each group was taxonomically characterized by sequencing the amplified 16S rDNA, and the four strains were assigned to the genera Comamonas (strain AV1A), Ralstonia (strain AV5BG), Pseudomonas (strain AV2A) and Sphingomonas (strain AV6C). The four strains, when checked for the ability to degrade nine monocyclic aromatic compounds abundant in OMWs, were found to significantly metabolize five to eight of them, both as resting cells and growing cells. Specific enzyme analyses of the same selected strains showed: (1) the occurrence of O-demethylating activities towards four methoxylated mono-aromatic acids in three of the four studied strains (strains AV1A, AV5BG and AV6C), (2) ring-cleavage activity towards protocatechuic acid in all of the strains, and (3) a ring-cleavage activity towards catechol in strain AV6C. The isolates described here exhibit a biodegradation potential towards monocyclic aromatic compounds of OMWs that is markedly broader and higher than that displayed by other aerobic bacteria described previously. These features make them excellent candidates for removing the low-molecular-weight phenolic compounds persisting in the effluent following anaerobic digestion of OMWs.