Evaluation of the biocide activity of tomatine-rich extracts from tomato cannery residues against fungi and bacteria

Synthetic pesticides are discouraged for their environmental and health impacts, making research into alternatives essential. Several solutions of vegetal origin are being evaluated. The use of residual biomass from the agri-food system is particularly suitable due to its abundance and often unexplored potential. This study focuses on characterizing and assessing the activity of extracts obtained from wastes of the tomato cannery industry (including green fruit, stems, and leaves), which are rich in steroidal glycoalkaloids such as α-tomatine and tomatidine in different proportion. The antimicrobial activity of these extracts was tested on three bacterial strains belonging to the Escherichia coli (EC), Xanthomonas campestris (XC), and Bacillus pumilus (BP) species, as well as the phytopathogenic fungus Botrytis cinerea (BC). In particular, the mechanism of action of the extracts in relation to their surfactant properties was investigated, with the effect of the analytical standard serving as a reference. Both extracts showed strong inhibition of bacterial and fungal growth in vitro, with values reaching 100 %.

The inhibitory effect was mainly due to the presence of α-tomatine in the extracts, which reached its aggregated state of micelle at the critical micelle concentration (CMC). Tomatidine, although known for its biocidal properties, did not contribute significantly due to its limited solubility. However, exceptions to this pattern were observed for extract rich in tomatidine, which exhibited efficacy at doses below the CMC. A possible explanation could be the enhanced solubility of tomatidine (which corresponds to enhanced bioactivity) in the presence of surfactant secreted by BP or as a consequence of the interaction between tomatidine and α-tomatine at the pre-micellar state for BC. In vivo assays with BC showed a reduction in symptoms comparable to that of a commercial fungicide available for organic agriculture, particularly at low concentrations. The relative content of α-tomatidine and tomatidine in the extracts modulated their bioactivity. An excess of tomatidine relative to α-tomatine led to a decrease in biocidal effect due to the chemical interactions among these species.