MYTILUS GALLOPROVINCIALIS PROTAMINE-LIKE PROTEINS ARE NEW BACTERICIDAL MOLECULES ACTIVE ALSO AGAINST ANTIBIOTIC RESISTANT BACTERIA

The discovery of antibiotics with novel mechanisms of action is a critical issue to overcome the serious problem of growing numbers of bacteria resistant to conventional antibiotics. Sperm nuclear basic proteins are the chromosomal proteins that are found associated with DNA in sperm nuclei at the end of spermiogenesis. Protamine-like proteins are one of the three types of Sperm Nuclear Basic Proteins, and represent a structurally and functionally intermediate group of proteins between the histone and protamine type. Protamine-like proteins represent the major acid-soluble protein components of the mussel Mytilus galloprovincialis sperm chromatin and consist of the protamine-like proteins PL-II, PL-III and PL-IV. The aim of this study was to investigate the antibacterial activity of these proteins since, to date, there are reports on bactericidal activity of protamines and histones, but not on protamine like proteins. We tested the bactericidal activity of these proteins against Gram-negative bacteria: Proteus mirabilis, Proteus vulgaris, Pseudomonas aeruginosa, Salmonella typhmurium, Enterobacter aerogenes, Enterobacter cloacae, and Escherichia coli as well as on Gram-positive bacteria: Enterococcus faecalis and two different strains of Staphylococcus aureus. The results show that Mytilus galloprovincialis protamine-like proteins exhibited bactericidal activity against all bacterial strains tested with different minimum bactericidal concentration values, ranging from 15.7 to 250 µg/mL and also on the clinical isolates of the same bacterial species. Interestingly, these proteins were active against some bacterial strains tested that are resistant to conventional antibiotics. For their possible therapeutic use, we investigated the tossicity of these proteins. We found that these proteins showed very low toxicity as judged by red blood cell lysis and viability MTT assays and seem to act both at the membrane level and within the bacterial cell. Antibacterial proteins have a potential as alternative treatments to standard antibiotic therapies but oral administration would most likely result in the proteins being degraded in the digestive system. In order to analyze this aspect we generated an in vitro model of gastrointestinal digestion of PL-proteins and tested the bactericidal activity of the product obtained on a Gram-positive and a Gram-negative strain. We obtained the same results with respect to undigested protamine-like proteins on the Gram-positive bacterium. In conclusion, this work presents the first evidence obtained for Mytilus galloprovincialis of bactericidal activity of protamine-like-proteins.