In Vitro and In Vivo Antibacterial and Antibiofilm Activity of Zinc Sulfate (ZnSO4) and Carvacrol (CV) Alone and in Combination with Antibiotics Against Pseudomonas aeruginosa

<b>Background/Objectives</b>: Biofilm-embedded bacteria, such as <i>Pseudomonas aeruginosa (P. aeruginosa)</i>, are highly resistant to antibiotics, making their treatment challenging. Plant-based natural compounds (PBCs) and metal(loid)-based antimicrobials (MBAs) are promising alternatives. This study evaluated the minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and synergistic effects of zinc sulfate (ZnSO₄), carvacrol (CV), and antibiotics (ciprofloxacin [CIP], tobramycin [TOB], and azithromycin [AZM]) against <i>P. aeruginosa</i> PAO1. <b>Methods</b>: The MIC and MBC of ZnSO₄, CV, and antibiotics were determined using a 96-well plate method. Cytotoxicity was assessed via MTT assay. Fractional inhibitory concentration (FIC), fractional bactericidal concentration (FBC), minimal biofilm inhibition concentration (MBIC), and minimum biofilm eradication concentration (MBEC) indices were calculated for each combination of agents. Checkerboard assays identified interactions, and the effectiveness of combinations was further evaluated in a mouse chronic lung infection model with treatments delivered intratracheally, intraperitoneally, and orally. <b>Results</b>: TOB had the lowest MIC and MBC values, proving most effective against <i>P. aeruginosa</i> PAO1. Strong synergy was observed with CV + ZnSO₄ (CV + Zn) combined with CIP, CV with CIP, and CV + Zn with TOB, as indicated by low FIC indices. CV + Zn with TOB and CV with TOB had low FBC indices, while CV + Zn with AZM showed antagonism. In vivo, intratracheal TOB + CV + Zn reduced lung inflammation and tissue involvement, yielding the best histopathological outcomes. The MIC of CIP and TOB was reduced 5-fold and 4-fold, respectively, when combined with CV + Zn. <b>Conclusions</b>: CV + Zn demonstrated strong synergistic effects with antibiotics and effectively managed <i>P. aeruginosa</i> lung infections in mice. These findings highlight its potential as an innovative therapy for biofilm-associated infections.