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Mechanisms of bacterial resistance, namely, quorum sensing (QS), biofilm formation, and efflux pumps, have been identified as critical biological processes in MDR bacteria. Following previous reports on the activity of phenothiazines against mechanisms of bacterial resistance, in this work we focus on the synthesis of xanthene derivatives aiming to discover phenothiazine bioisosteres with improved activity. Four compounds were obtained from the conjugation of xanthydrol with sulfonamides and aniline and were fully characterized. Their antibacterial activity was assessed considering their minimum inhibitory concentration (MIC) against Gram-positive and Gram-negative strains, efflux pump inhibition, influence on biofilm formation and quorum-sensing (QS) inhibition. It was observed that the MIC of all the tested compounds was above 64 \u00b5g\/mL The four 9-xanthenyl derivatives obtained, particularly the xanthene sulfonamide derivatives 3b and 3c, showed promising results on QS inhibition with a reduction of pigment production of 48 and 41 mm, and on biofilm formation with a reduction of 78 and 79%, respectively.<\/jats:p>","DOI":"10.3390\/ddc1010004","type":"journal-article","created":{"date-parts":[[2022,12,7]],"date-time":"2022-12-07T05:50:52Z","timestamp":1670392252000},"page":"29-42","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":8,"title":["Xanthene Derivatives Targeting Bacterial Efflux Pumps, Quorum-Sensing, and Biofilm Formation"],"prefix":"10.3390","volume":"1","author":[{"given":"Miguel","family":"Maia","sequence":"first","affiliation":[{"name":"CIIMAR\u2014Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Novo Edif\u00edcio do Terminal de Cruzeiros do Porto de Leix\u00f5es, Avenida General Norton de Matos, S\/N, 4450-208 Matosinhos, Portugal"},{"name":"Laboratory of Organic and Pharmaceutical Chemistry, Faculty of Pharmacy, University of Porto, Rua de Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4201-2774","authenticated-orcid":false,"given":"Fernando","family":"Dur\u00e3es","sequence":"additional","affiliation":[{"name":"CIIMAR\u2014Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Novo Edif\u00edcio do Terminal de Cruzeiros do Porto de Leix\u00f5es, Avenida General Norton de Matos, S\/N, 4450-208 Matosinhos, Portugal"},{"name":"Laboratory of Organic and Pharmaceutical Chemistry, Faculty of Pharmacy, University of Porto, Rua de Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4077-6060","authenticated-orcid":false,"given":"Diana I. S. P.","family":"Resende","sequence":"additional","affiliation":[{"name":"CIIMAR\u2014Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Novo Edif\u00edcio do Terminal de Cruzeiros do Porto de Leix\u00f5es, Avenida General Norton de Matos, S\/N, 4450-208 Matosinhos, Portugal"},{"name":"Laboratory of Organic and Pharmaceutical Chemistry, Faculty of Pharmacy, University of Porto, Rua de Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8638-5815","authenticated-orcid":false,"given":"Nikoletta","family":"Szemer\u00e9di","sequence":"additional","affiliation":[{"name":"Department of Medical Microbiology, Albert Szent-Gy\u00f6rgyi Health Center and Faculty of Medicine, University of Szeged, Semmelweis utca 6, 6725 Szeged, Hungary"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8352-6539","authenticated-orcid":false,"given":"Lu\u00eds","family":"Gales","sequence":"additional","affiliation":[{"name":"ICBAS\u2014Institute of Biomedical Sciences Abel Salazar, Universidade do Porto, Rua de Jorge Viterbo Ferreira 228, 4050-313 Porto, Portugal"},{"name":"Bioengineering & Synthetic Microbiology, I3S\u2014Instituto de Investiga\u00e7\u00e3o e Inova\u00e7\u00e3o em Sa\u00fade, University of Porto, Rua Alfredo Allen, 208, 4200-135 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6115-8811","authenticated-orcid":false,"given":"Paulo","family":"Martins-da-Costa","sequence":"additional","affiliation":[{"name":"CIIMAR\u2014Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Novo Edif\u00edcio do Terminal de Cruzeiros do Porto de Leix\u00f5es, Avenida General Norton de Matos, S\/N, 4450-208 Matosinhos, Portugal"},{"name":"ICBAS\u2014Institute of Biomedical Sciences Abel Salazar, Universidade do Porto, Rua de Jorge Viterbo Ferreira 228, 4050-313 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4676-1409","authenticated-orcid":false,"given":"Madalena","family":"Pinto","sequence":"additional","affiliation":[{"name":"CIIMAR\u2014Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Novo Edif\u00edcio do Terminal de Cruzeiros do Porto de Leix\u00f5es, Avenida General Norton de Matos, S\/N, 4450-208 Matosinhos, Portugal"},{"name":"Laboratory of Organic and Pharmaceutical Chemistry, Faculty of Pharmacy, University of Porto, Rua de Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8085-0950","authenticated-orcid":false,"given":"Gabriella","family":"Spengler","sequence":"additional","affiliation":[{"name":"Department of Medical Microbiology, Albert Szent-Gy\u00f6rgyi Health Center and Faculty of Medicine, University of Szeged, Semmelweis utca 6, 6725 Szeged, Hungary"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5397-4672","authenticated-orcid":false,"given":"Em\u00edlia","family":"Sousa","sequence":"additional","affiliation":[{"name":"CIIMAR\u2014Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Novo Edif\u00edcio do Terminal de Cruzeiros do Porto de Leix\u00f5es, Avenida General Norton de Matos, S\/N, 4450-208 Matosinhos, Portugal"},{"name":"Laboratory of Organic and Pharmaceutical Chemistry, Faculty of Pharmacy, University of Porto, Rua de Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2022,12,6]]},"reference":[{"key":"ref_1","unstructured":"O\u2019Neill, J. 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