**1. Introduction**

Antibiotic resistance (AR) has for some decades been rapidly increasing and represents, nowadays, one of the world's greatest challenges affecting animal and human health. The emergence of AR can be attributed not only to inappropriate antimicrobial prescriptions in humans but also to the overuse of antibiotics in animal breeding and agriculture [1,2].

Therefore, the identification of new anti-infective therapies is urgently needed. Over the past years, numerous efforts have been made to identify new molecules or new methods that could overcome the growing microbial resistance. In this perspective, antimicrobial peptides (AMPs) may represent potential candidates for the treatment of infections caused by multiresistant microorganisms. Inspired by this research area, our group at the "Polytechnic University of Marche" decided to evaluate the effectiveness of selected new AMPs over the last 20 years.

AMPs are oligopeptides usually composed of 12–50 cationic and hydrophobic amino acids with a positive net charge, representing essential components of innate immunity [3–5]. AMPs display broad-spectrum activity against a wide variety of pathogens, such as yeast, fungi, viruses, and bacteria [6,7].

In particular, many AMPs kill pathogens by interacting with negatively charged bacterial cell membranes; this leads to a change in their electrochemical potential, which generates cell membrane damage. Although several AMPs may also kill pathogens indirectly by modulating host immune responses [8–11], AMPs can also act with other different mechanisms of action, such as inhibiting communication between pathogens. These aspects will be described in the specific paragraphs that follow. Additionally, some AMPs show synergistic interactions with conventional molecules, contributing to the decrease in the

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selection of antibiotic-resistant bacteria and allowing us to restore sensitivity of conventional treatments [12,13]. The aim of this article is to summarise the 20 year experience of our research. Initially, our team included researchers from the Polytechnic University of Marche affiliated with the Clinic of Infectious Diseases, the Clinic of Surgery, the INRCA Experimental Animal Models for Aging Unit, and the Department of Inorganic Chemistry at the Medical University of Gdansk (Poland). Our scientific production was initially focused on the antimicrobial properties of AMPs in medical device infection, biofilm, and sepsis. With the arrival of researchers from dermatology and the Institute of Pathological Anatomy and Histology, our interest shifted to bacterial and fungal skin diseases. Our main goal was to provide new treatments to overcome multiresistant infections, particularly those caused by methicillin-resistant *Staphylococcus aureus* (MRSA) in chronic and surgical skin wounds, including burns. We studied the effects of AMPs not only for treating resistant bacteria-related infections in the skin but also for wound healing.

Simultaneously, we also continued to evaluate the potential and spectrum of action of selected AMPs against fungi and Gram-positive and Gram-negative bacteria. In the beginning, the experimental model was in vitro on colonies of resistant microorganisms taken from patients and then switched, in the case of effective AMPs, to the in vivo animal model. As our research progressed, we decided to assess the potential impact on wound healing by considering histological features and immunohistochemical markers that could quantify the action of AMPs vs. conventional antibiotics, such as vascular endothelium growth factor (VEGF), matrix metalloproteinases (MMP) expression, growth factors, or their receptors.

In the following paragraphs, we summarised the main areas of research performed, indicating the methods used and the results obtained.

#### **2. AMPs and Biofilm in Medical Devices**

Biofilm is characterised by bacterial cells that adhere to a substratum, interface, or each other and are embedded in a matrix of substances produced by the cells themselves. Biofilm often tends to develop on medical devices, in particular long-term silicone catheters such as the central venous catheter (CVC). The biofilm protects bacteria from antimicrobial therapy, leading to frequent failure of conventional antibiotic therapy [14–16].

Due to this issue, in the last few years different novel drug technologies have been studied, including antimicrobial peptides.
