ANTIMICROBIAL ACTIVITY FROM POLYPEPTIDE-RICH EXTRACTS OF THE SEAGRASS POSIDONIA OCEANICA

The present study was carried out to assess the antibacterial, antifungal and antibiofilm properties of polypeptide-rich extracts isolated from green leaves and rhizomes of Mediterranean seagrass Posidonia oceanica (L. Delile) (Posidoniaceae). The seagrass was collected, washed with freshwater, grinded with liquid nitrogen in order to obtain fine powders that were exposed to extraction by acetic acid and antiproteases. The crude extracts isolated from leaves and rhizomes of P. oceanica were subjected to microbiological assays to evaluate the antibacterial, antifungal and antibiofilm activity of polypeptide fraction against two reference bacterial strains Staphylococcus aureus ATCC 25923 and Pseudomonas aeruginosa ATCC 15442, and the fungus Candida albicans ATCC 10231. The antimicrobial and antifungal activity of the extracts were evaluated starting from 50% v/v concentration of each sample and the results are expressed in terms of Minimum inhibitory concentration (MIC), with the values detailed in percentage v/v and in concentration g/ml of protein content. The most interesting result has been deduced from rhizomes that showed a MIC of 12.5% v/v, corresponding to 3.37 g/ml of protein content, against three selected pathogens compared to the sample from the leaves that revealed a MIC of 25% v/v, corresponding to a protein concentration of 4.25 g/ml. The crude extracts isolated from P. oceanica were also active to combat the biofilm formation at sub-MIC concentration. The inhibition was evident with crude extracts from rhizomes in S. aureus ATCC 25923 at IC50 of 0.54 g/ml, compared to a value of IC50 of 0.74g/ml with the extracts from leaves. Moreover, the polypeptide fraction of leaves of P. oceanica was also able to inhibit the biofilm formation in C. albicans ATCC 10231 at IC50 = 0.58 g/ml. Peptide fractions displaying antimicrobial activity were further investigated by by High-Pressure Liquid Chromatography/nano-Electrospray Ionization tandem Mass Spectrometry (RP-HPLC/nESI-MS/MS) and database search. Database search allowed the characterization of fourteen peptides, one of them is related to a Viridiplantae-derived protein, whereas the others are attributable to bacterial proteins present in the investigated database. Moreover, most of identified peptides showed similarities with already described antimicrobial peptides (AMPs) from bacteria, animals and plants.