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antibiotics Article Effective Photodynamic Inactivation of 26 Escherichia coli Strains with Different Antibiotic Susceptibility Profiles: A Planktonic and Biofilm Study Òscar Gulías 1,*, Giselle McKenzie 1 , Miquel Bayó 2, Montserrat Agut 1,* and Santi Nonell 1,* 1 2 Received: 4 December 2019; Accepted: 21 February 2020; Published: 25 February 2020 Institut Químic de Sarrià, Universitat Ramon Llull, 08017 Barcelona, Spain; giselle.mckenzie@icloud.com Laboratori d’anàlisis M. Bayó, 08221 Terrassa, Spain; miquel@analisis-bayo.cat * Correspondence: oscar.gulias@iqs.url.edu (Ò.G.); montserrat.agut@iqs.url.edu (M.A.); santi.nonell@iqs.url.edu (S.N.) Abstract: The emergence of multidrug-resistant bacteria is a growing problem and alternative therapies are being sought to effectively address this issue. The aim of this study is to assess a range of Escherichia coli strains’ susceptibility to Methylene Blue-mediated antimicrobial photodynamic therapy and determine if this is affected by their antibiotic-resistance profile. Two reference and twenty-four uropathogenic clinical E. coli strains were used in this study. All were tested in vitro for antimicrobial susceptibility against sixteen antibiotics. Strains underwent photodynamic treatments using the photosensitizer Methylene Blue with red light and tested in both planktonic and biofilm state. It was found that reference strain ATCC 25922 was susceptible to all tested antibiotics whereas reference strain ATCC 35218 showed resistance only to Ampicillin. With the exception of strains number 16 and 22, all of the isolated strains were multidrug-resistant according to the criteria established by the European Centre for Disease Prevention and Control and the Centre for Disease Control and Prevention, where acquired non-susceptibility to at least one agent in three or more antimicrobial categories is outlined. Photodynamic therapy induced more than 3 log10 colony-forming units’ reduction to all strains in planktonic state. Whereas when tested in biofilm state, two and a half times the original dose of methylene blue was necessary to cause a 3 log10 antimicrobial effect. There were statistically significant differences in susceptibility among the strains tested in both the planktonic and biofilm experiments. Nevertheless, antimicrobial photodynamic therapy could inactivate all multidrug-resistant strains in the planktonic and biofilm state. Keywords: antimicrobial photodynamic therapy; Escherichia coli; uropathogenic strains; planktonic state; biofilm; Methylene Blue 1. Introduction Since the discovery of penicillin by Fleming in 1928, many different families of antibiotics were approved and are currently being used to treat a great variety of infections [1]. However, their therapeutic potential has been threatened by the emergence of increasingly resistant bacterial strains, first observed by Abraham and Chain in 1940. Antimicrobial resistance (AMR) is an increasingly serious threat that endangers human and animal health [2]. It has been estimated that more than 670,000 cases of infections with multidrug-resistant bacteria occurred in the EU in 2015, which caused about 33,000 deaths [3]. Four pathogens have been identified of major concern: cephalosporin-resistant Escherichia coli, methicillin-resistant Staphylococcus aureus (MRSA), carbapenem-resistant Pseudomonas aeruginosa and cephalosporin-resistant Klebsiella pneumoniae [3]. There is a gap between the burden of Antibiotics 2020, 9, 98; doi:10.3390/antibiotics9030098 www.mdpi.com/journal/antibioticsPDF Image | Effective Photodynamic Inactivation of 26 E coli Strains
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