polymers Review Polymeric Nanoparticles for Antimicrobial Therapies: An up-to-date Overview Vera Alexandra Spirescu 1 , Cristina Chircov 1 , Alexandru Mihai Grumezescu 1,2, * and Ecaterina Andronescu 1   Citation: Spirescu, V.A.; Chircov, C.; Grumezescu, A.M.; Andronescu, E. Polymeric Nanoparticles for Antimicrobial Therapies: An up-to-date Overview. Polymers 2021, 13, 724. https://doi.org/10.3390/ polym13050724 Academic Editor: Mohammad Arjmand Received: 15 February 2021 Accepted: 24 February 2021 Published: 27 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 Science and Engineering of Oxide Materials and Nanomaterials, University Politehnica of Bucharest, 011061 Bucharest, Romania; veraspirescu@gmail.com (V.A.S.); cristina.chircov@yahoo.com (C.C.); ecaterina.andronescu@upb.ro (E.A.) 2 Research Institute of the University of Bucharest—ICUB, University of Bucharest, 050657 Bucharest, Romania * Correspondence: grumezescu@yahoo.com; Tel.: +40-21-318-1000 Abstract: Despite the many advancements in the pharmaceutical and medical fields and the develop- ment of numerous antimicrobial drugs aimed to suppress and destroy pathogenic microorganisms, infectious diseases still represent a major health threat affecting millions of lives daily. In addition to the limitations of antimicrobial drugs associated with low transportation rate, water solubility, oral bioavailability and stability, inefficient drug targeting, considerable toxicity, and limited patient compliance, the major cause for their inefficiency is the antimicrobial resistance of microorganisms. In this context, the risk of a pre-antibiotic era is a real possibility. For this reason, the research focus has shifted toward the discovery and development of novel and alternative antimicrobial agents that could overcome the challenges associated with conventional drugs. Nanotechnology is a possible alternative, as there is significant evidence of the broad-spectrum antimicrobial activity of nanomaterials and nanoparticles in particular. Moreover, owing to their considerable advantages regarding their efficient cargo dissolving, entrapment, encapsulation, or surface attachment, the possibility of forming antimicrobial groups for specific targeting and destruction, biocompatibility and biodegradability, low toxicity, and synergistic therapy, polymeric nanoparticles have received considerable attention as potential antimicrobial drug delivery agents. In this context, the aim of this paper is to provide an up-to-date overview of the most recent studies investigating polymeric nanoparticles designed for antimicrobial therapies, describing both their targeting strategies and their effects. Keywords: polymeric nanoparticles; antimicrobial therapy; medical field; antimicrobial resistance; toxicity; limited patient compliance; nanotechnology; nanomaterials; up-to-date overview; target- ing strategies 1. Introduction Microorganisms are an essential part of human existence, being responsible for numer- ous and diverse processes, including nitrogen fixation, vitamin production, photosynthesis, and organic matter decomposition. However, the delicate balance between microorgan- isms and the immune system may shift in favor of microorganisms, thus causing immune deficiencies [1]. Therefore, diseases caused by pathogenic microorganisms, such as bacteria, viruses, fungi, parasites, protozoa, or algae, can be directly or indirectly (vector-borne) transmitted from one individual to another, which is termed as infectious diseases [2–5]. In antiquity, around half of the individuals died before reaching sexual maturity, while in late medieval times, one-third of babies died in their infancy, which was mostly due to infectious diseases [6]. Thus, the discovery of antimicrobials, including antibacterial, antiviral, antifungal, and antiparasitic or anthelmintic drugs, has greatly impacted the global health system, as the infection-related mortality was significantly reduced [4,7,8]. Specifically, they have allowed the early treatment of infections without identifying the pathogen, consequently bringing novel possibilities for modern medicine, such as surgery, Polymers 2021, 13, 724. https://doi.org/10.3390/polym13050724 https://www.mdpi.com/journal/polymers