polymers Article Development of Antibacterial Protective Coatings Active against MSSA and MRSA on Biodegradable Polymers Iva Rezi´ c 1, * , Mislav Majdak 1 , Vanja Ljolji´ c Bili´ c 2 , Ivan Pokrovac 2 , Lela Martinaga 1 , Maja Somogyi Škoc 3 and Ivan Kosalec 2, *   Citation: Rezi´ c, I.; Majdak, M.; Ljolji´ c Bili´ c, V.; Pokrovac, I.; Martinaga, L.; Somogyi Škoc, M.; Kosalec, I. Development of Antibacterial Protective Coatings Active against MSSA and MRSA on Biodegradable Polymers. Polymers 2021, 13, 659. https://doi.org/10.3390/ polym13040659 Academic Editor: Antonio Zuorro Received: 8 February 2021 Accepted: 19 February 2021 Published: 23 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Applied Chemistry, Faculty of Textile Technology, University of Zagreb, 10000 Zagreb, Croatia; mmajdak@ttf.hr (M.M.); lela.martinaga@ttf.hr (L.M.) 2 Faculty of Pharmacy and Biochemistry, Institute for Microbiology University of Zagreb, 10000 Zagreb, Croatia; vljoljic@pharma.hr(V.L.B.); ivan.pokrovac.fbf@gmail.com (I.P.) 3 Department of Material Testing, Faculty of Textile Technology, University of Zagreb, 10000 Zagreb, Croatia; maja.somogyi@ttf.hr * Correspondence: iva_rezic@net.hr (I.R.); ikosalec@pharma.hr (I.K.) Abstract: In this work the in vitro antimicrobial activity of colloidal solutions of nine different commercially available nanoparticles were investigated against Staphylococcus aureus strains, both methicillin-sensitive (MSSA) and methicillin-resistant (MRSA). Research covered antimicrobial in- vestigation of different metal and metal-oxide nanoparticles, including Ag 10 nm, Ag 40 nm, Al 2 O 3 100 nm, Au 20 nm, Pt 4 nm, TiO 2 100 nm, Y 2 O 3 100 nm, ZnO 100 nm and ZrO 2 100 nm nanoparticles. Such materials were foreseen to be applied as coatings on 3D-printed biodegradable polymers: i.e., catheters, disposable materials, hospital bedding items, disposable antimicrobial linings and ban- dages for chronic wounds. Therefore, the antimicrobial activity of the nanoparticles was determined by agar well diffusion assays and serial microdilution broth assays. In addition, the chromatographic characterization of elements present in trace amounts was performed as a method for tracing the nanoparticles. Moreover, the potential of preparing the rough surface of biodegradable polymers for coating with antimicrobial nanoparticles was tested by 3D-printing fused deposition methodology. The in vitro results have shown that particular nanoparticles provided powerful antimicrobial effects against MSSA and MRSA strains, and can be easily applied on different biopolymers. Keywords: antibacterial coating; Staphylococcus aureus; MRSA; nanoparticles; 3D printing 1. Introduction By the year 2050, more people could die from the infections caused by antibiotic- resistant bacteria than from AIDS, tuberculosis and viral hepatitis combined together. Only in Europe 25,000 deaths per year and costs of over €1.5 billion are the consequences of the activity of resistant microorganisms. Moreover, the estimations show that in the next 30 years more people could die from the infections caused by antibiotic-resistant bacteria than from cancer [1]. Despite the reductions in the incidence of infections since 2005, Staphylococcus aureus infections still account for a significant amount of morbidity and mor- tality in the United States [2]. Antimicrobially-active biodegradable polymers that contain metal and metal oxide nanoparticles present in the outer layer of coatings can have impor- tant applications in medicine and medical materials. Therefore, such materials might have potential usage against methicillin-resistant S. aureus (MRSA). Moreover, polymer materials that are not produced by classical fiber production and spinning technology, but by other processes (such as additive technology or 3D printing) can easily be functionalized with metal nanoparticles with the goal to obtain antimicrobial properties. Such materials can be used as catheters, consumables, dressings and wound dressings. In addition, would be possible to apply them in the production of protective masks and protective suits and to answer to the most prominent problems of today’s medical materials [3]. Polymers 2021, 13, 659. https://doi.org/10.3390/polym13040659 https://www.mdpi.com/journal/polymers