Acinetobacter baumannii (A. baumannii) is an opportunistic pathogen associated with healthcare-related infections and is of particular concern due to its high level of antibiotic resistance and its ability to form biofilms. The global emergence of carbapenem-resistant A. baumannii highlights the urgent need for alternative therapeutic strategies. This study investigated the antibacterial and antibiofilm activities of two scorpion venom-derived peptides, pantinin-1 and pantinin-2, against a reference strain and a clinical isolate of A. baumannii. We found that both peptides, in the non-cytotoxic concentration range, have strong bactericidal activity, showing a minimum inhibitory concentration (MIC) of 6.25 μM and 12.5 μM for pantinin 1 and 2, respectively. Scanning electron microscopy (SEM) analysis showed that the peptides cause extensive damage to the bacterial membrane. Furthermore, both peptides showed potent antibiofilm activity, inhibiting adhesion and maturation, arresting biofilm expansion, and reducing the expression of key biofilm-associated genes (bap, pgaA, and smpA). Altogether, these findings indicate that pantinin-1 and pantinin-2 act through a dual mechanism, combining bactericidal and antivirulence activities. Their strong efficacy at low micromolar concentrations, together with low cytotoxicity, underscores their potential as innovative therapeutic candidates against infections caused by carbapenem-resistant, biofilm-forming A. baumannii.

Background: Lung infections represent a major cause of morbidity and mortality in patients with cystic fibrosis (CF) and are predominantly associated with chronic infection by Pseudomonas aeruginosa. The clinical management of CF lung disease is increasingly compromised by the emergence of multidrug-resistant strains, biofilm formation, and the expression of multiple virulence determinants. Antimicrobial peptides (AMPs), evolutionarily conserved effectors of innate immunity, have emerged as promising therapeutic candidates due to their ability to exert both bactericidal and anti-virulence activities. In this study, we investigated the antimicrobial and mechanistic effects of Ranalexin-1G, an AMP derived from the skin secretion of Rana grylio, against P. aeruginosa, for which its antibacterial activity has not previously been reported in the literature. Methods: Antibacterial activity was determined against the reference strain and three clinical isolates of P. aeruginosa by broth microdilution assays, time-kill kinetics and anti-biofilm assays, while peptide-mediated modulation of virulence was evaluated through transcriptional analysis of key virulence-associated genes by RT-PCR. The impact of Ranalexin-1G on host–pathogen interactions was further assessed using bacterial invasion assays in human bronchial epithelial cells (BEAS-2B). Results: Ranalexin-1G exerted a rapid bactericidal effect within 6 h at non-cytotoxic concentrations and displayed modest anti-biofilm effects, with greater efficacy in inhibiting biofilm formation. Mechanistically, peptide treatment resulted in a reduction in the expression of selected genes involved in biofilm formation and virulence, including those associated with alginate biosynthesis and type III secretion system-mediated cytotoxicity. Consistently, Ranalexin-1G markedly impaired bacterial invasion of epithelial cells, indicating interference with early host–pathogen interaction processes. Notably, the peptide displayed robust antimicrobial activity against multidrug-resistant P. aeruginosa clinical isolates from CF patients. Conclusions: Collectively, these findings suggest that Ranalexin-1G acts through a dual mechanism involving direct bactericidal activity and modulation of selected virulence pathways, supporting further investigation of its potential as an anti-virulence and host-directed approach for the treatment of chronic P. aeruginosa infections in cystic fibrosis.