Drinking water is currently a scarce world resource, the preparation of which requires complex treatments that include clarification of suspended particles and disinfection. Seed extracts of Moringa oleifera Lam., a tropical tree, have been proposed as an environment-friendly alternative, due to their traditional use for the clarification of drinking water. However, the precise nature of the active components of the extract and whether they may be produced in recombinant form are unknown. Here we show that recombinant or synthetic forms of a cationic seed polypeptide mediate efficient sedimentation of suspended mineral particles and bacteria. Unexpectedly, the polypeptide was also found to possesses a bactericidal activity capable of disinfecting heavily contaminated water. Furthermore, the polypeptide has been shown to efficiently kill several pathogenic bacteria, including antibiotic-resistant isolates of Staphylococcus, Streptococcus, and Legionella species. Thus, this polypeptide displays the unprecedented feature of combining water purification and disinfectant properties. Identification of an active principle derived from the seed extracts points to a range of potential for drinking water treatment or skin and mucosal disinfection in clinical settings.

Crushed seeds of the Moringa oleifera tree have been used traditionally as natural flocculants to clarify drinking water. We previously showed that one of the seed peptides mediates both the sedimentation of suspended particles such as bacterial cells and a direct bactericidal activity, raising the possibility that the two activities might be related. In this study, the conformational modeling of the peptide was coupled to a functional analysis of synthetic derivatives. This indicated that partly overlapping structural determinants mediate the sedimentation and antibacterial activities. Sedimentation requires a positively charged, glutaminerich portion of the peptide that aggregates bacterial cells. The bactericidal activity was localized to a sequence prone to form a helix-loop-helix structural motif. Amino acid substitution showed that the bactericidal activity requires hydrophobic proline residues within the protruding loop. Vital dye staining indicated that treatment with peptides containing this motif results in bacterial membrane damage. Assembly of multiple copies of this structural motif into a branched peptide enhanced antibacterial activity, since low concentrations effectively kill bacteria such as Pseudomonas aeruginosa and Streptococcus pyogenes without displaying a toxic effect on human red blood cells. This study thus identifies a synthetic peptide with potent antibacterial activity against specific human pathogens. It also suggests partly distinct molecular mechanisms for each activity. Sedimentation may result from coupled flocculation and coagulation effects, while the bactericidal activity would require bacterial membrane destabilization by a hydrophobic loop.Diverse communities in the world traditionally use different natural agents from animal or vegetal sources as raw water additives to produce drinking water. Systematic studies have shown that among the different plant-derived materials tested, Moringa oleifera seeds seem to be one of the most effective primary water treatments (17, 28). Water-soluble proteins released from the crushed seed kernels function as natural flocculating agents, which have been proposed to bind and crosslink particles suspended in a colloidal structure, forming larger sedimenting particles (17,18,29). Microorganisms are generally attached to solid particles in raw water samples, and treatment employing Moringa seed powder can remove over 90% of the bacterial load (35). Unexpectedly, a study on the sedimentation effect of the seed-derived peptide termed Flo indicated also that it mediates bacterial disinfection, being able to kill antibiotic-resistant bacteria, including several human pathogens (62).The cationic nature of the Flo polypeptide is characteristic of a wider group of cationic peptide antibiotics that are usually less than 10 kDa in size and display an overall net positive charge. Cationic antimicrobial peptides (AMPs) make part of the innate immunity response, which is the first line of defense against pathogens. Most of the AMPs are expressed constitu...

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