**Preface to "Focus on Antibiotics – New Challenges and Steps Forward in Discovery and Development"**

The design of new antibiotics is still considered the primary weapon in fighting against the growing bacterial resistance to known antibiotics. The Special Issue "Focus on Antibiotics – New Challenges and Steps Forward in Discovery and Development" covers a wide range of topics briefly presented below.

A review article was focused on new representatives of antibacterial quinolones; a new generation (the fifth) associated with higher potency, and a broad spectrum of activity arises. Additionally, the tetracycline antibiotic class has acquired new valuable members with excellent development potential. Another review article comprises the advantages and disadvantages of Ru (II/III) frameworks as antimicrobial agents. In addition, some aspects regarding the relationship between their chemical structure and mechanism of action, cellular localisation, and/or metabolism of the ruthenium complexes in bacterial and eukaryotic cells are also discussed. Synthesis and characterization of two Co(III) complexes with diamine chelate ligands were also presented; the complexes were sequentially tested against Gram-positive and Gram-negative bacteria. Nanoparticles are very attractive in multiple medical applications. Gold nanoparticles (GNPs) were synthesised by using ceftriaxone antibiotic. Titanium dioxide nanoparticles proved to induce inhibitory effects against planktonic cells and biofilms of human oral cavity isolates. *Lepidium sativum* L. and silver nanoparticles (AgNPs) were successfully combined and tested against ten reference strains of pathogens. In addition, another study targeted the characterisation and genome analysis of *Arthrobacter bangladeshi* sp. nov., applied for the green synthesis of silver nanoparticles and their antibacterial efficacy against two drug-resistant pathogens. One review has been focused on *Myxobacterial* species and assesses their ability to produce bioactive compounds. A recently validated drug target in *Mycobacterium tuberculosis* is an essential phosphopantetheinyl transferase. Newly identified tools will facilitate finding new anti-mycobacterial drug leads and guide the design of phosphopantetheinyl transferase inhibitors based on structure-activity studies. Antimicrobial peptides are very promising in their activity against drug-resistant species. Thus, a novel PapMA analogue was designed with increased anti-carbapenem-resistant *Acinetobacter baumannii* activity.

We are grateful to the authors for their contributions and hope this Special Issue will stimulate further research and discussion on antibiotics. We also thank the reviewers for their time and expertise, and the editorial team for supporting this Special Issue. Finally, we hope this Special Issue will serve as a valuable resource for researchers, clinicians, and policymakers working towards a future where antibiotics remain vital in our fight against infectious diseases.

> **Aura Rusu, Valentina Uivarosi, and Corneliu Tanase** *Editors*
