10656 | Chem. Commun., 2016, 52, 10656--10659 This journal is © The Royal Society of Chemistry 2016 Cite this: Chem. Commun., 2016, 52, 10656 Phosphonium pillar[5]arenes as a new class of efficient biofilm inhibitors: importance of charge cooperativity and the pillar platform† Roymon Joseph,‡ Dana Kaizerman,‡ Ido M. Herzog, Maya Hadar, Mark Feldman, Micha Fridman and Yoram Cohen* Biofilm formation, which frequently occurs in microbial infections and often reduces the efficacy of antibiotics, also perturbs many industrial and domestic processes. We found that a new class of water soluble pillar[5]arenes bearing phosphonium moieties (1, 2) and their respective ammonium analogues (3, 4) inhibit biofilm formation with IC 50 values in the range of 0.67–1.66 lM. These compounds have no antimicrobial activity, do not damage red blood cell membranes, and do not affect mammalian cell viability in culture. Comparison of the antibiofilm activities of the phosphonium- decorated pillar[5]arene derivatives 1 and 2 with their respective ammonium counterparts 3 and 4 and their monomers 5 and 6, demonstrate that while positive charges, charge cooperativity and the pillararene platform are essential for the observed antibiofilm activity the nature of the charges is not. According to the reports from the National Institutes of Health, about 65% of infections treated in the developed world involve microbial biofilms. 1 Biofilm-associated diseases in humans include lung infections, ear infections, urinary and gastroin- testinal tract infections, chronic and burn wound infections, nosocomial, catheter-related, and dental infections. 2 The for- mation of biofilms on biomedical devices, surgical implants, urinary tract catheters, and contact lenses, dramatically increases the chances of introducing persistent infections into the human body. 2 Biofilm growth also has detrimental effects in industrial and domestic domains resulting in high costs associated with cleaning and maintenance. 3 In addition, bacteria in biofilms are significantly more resistant to antibiotics than are bacteria grown in suspension. 4 Since prevention of biofilm formation could dramatically reduce effects of infectious diseases and the cost of industrial processes there is a great demand for molecules that will effectively inhibit biofilm formation. 5 Among various interactions responsible for the formation of biofilms, electrostatic interactions are considered as one of the earliest forces influencing the adherence of bacterial cells to surfaces. 6 The outer surfaces of biofilms consist of an anionic matrix, and disruption of this matrix is thought to be an effective approach for preventing biofilm formation. Cationic amphiphiles therefore appear to be attractive candidates to inhibit early-stage biofilm formation by preventing adhesion of the bacteria to a surface. 7 In recent years a few inhibitors of biofilm formation based on cationic amphiphiles have been reported. 8 Among the cationic amphiphiles derived from quaternary ammonium and phos- phonium salts, the later ones display increased antimicrobial properties compared to their ammonium counterparts. 9 For example, Endo and co-workers demonstrated the antimicrobial properties of a series of phosphonium salts against 11 strains of microorganisms including methicillin-resistant Staphylococcus aureus (MRSA). 9a In recent years, compounds with phosphonium moieties have been used in various biomedical applications and in water treatment including for antifouling purposes. 10 Most of the aforementioned studies report use of phosphonium salts as biocides. The exception is a recent report by Ferna ´ndez and co-workers describing the antifouling properties of several alkyl- triphenylphosphonium salts and their abilities to act as non-toxic quorum sensing disruptors. 11 Indeed, Melander and co-workers pointed out that it is extremely important to develop antibiofilm agents operating via non-biocidal mechanisms for several reasons, the most important one is avoiding resistance development. 12 Very recently, we found that ammonium and methyl imida- zolium cationic pillar[n]arenes are effective inhibitors of biofilm formation by several strains of Gram-positive bacteria. Interest- ingly, we observed that this new class of antibiofilm agents shows no antimicrobial activity and no effect on bacterial growth and causes no damage to red blood cells or toxicity to human cells in culture. 13 Therefore in the present work, we prepared a series of phosphonium and ammonium decorated pillar[5]arenes (1–4, Scheme 1) and their respective monomers (5 and 6, Scheme 1) and studied their anti-biofilm activity with the aim of evaluating the School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel. E-mail: ycohen@post.tau.ac.il † Electronic supplementary information (ESI) available: Experimental procedures, spectroscopic characterization data, and detailed protocols for the biological assays. See DOI: 10.1039/c6cc05170g ‡ These authors contributed equally. Received 21st June 2016, Accepted 2nd August 2016 DOI: 10.1039/c6cc05170g www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 09 August 2016. 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