Synthesis and Evaluation of Different Nanoparticles in the Enhancement of Nalidixic acid Antimicrobial Activity

Abstract

Background: Antimicrobial resistance (AMR) represents one of the most pressing global public health challenges of the twenty-first century, rendering many conventional antibiotic regimens increasingly ineffective against priority pathogens such as Pseudomonas aeruginosa and Staphylococcus aureus. Nalidixic Acid (NA), a firstgeneration quinolone antibiotic and WHO Essential Medicine, possesses wellcharacterised inhibitory activity against bacterial DNA gyrase and topoisomerase IV, but its clinical utility is constrained by poor aqueous solubility, rapid dissolution, and susceptibility to efflux pump-mediated resistance. This study aimed to address these limitations through the design, synthesis, and comparative evaluation of six structurally distinct nanocarrier systems for the delivery of NA, with a dual focus on improving physicochemical properties and preserving antimicrobial efficacy. Methods: Six nanoparticle formulations were prepared: three phospholipid-based liposomal systems (Soy PC:Cholesterol, Egg PC:Cholesterol, and DPPC:Cholesterol at a 7:3 molar ratio), one niosomal system (Span 60:Cholesterol at a 50:50 molar ratio), and two polyphenolic nanocarriers self-assembled from freeze-dried extracts of Moringa oleifera and Quercus coccifera. All formulations were characterised by dynamic light scattering (DLS) for hydrodynamic diameter, polydispersity index (PDI), and zeta potential, and by UV-Vis spectrophotometry for encapsulation efficiency (EE%). In vitro drug release was assessed by dialysis bag diffusion over 120 hours under physiological sink conditions. Antimicrobial activity was evaluated by broth microdilution minimum inhibitory concentration (MIC) assay against reference strains of P. aeruginosa (ATCC 27853) and S. aureus (ATCC 25923). Results: Nanoencapsulation consistently improved the physicochemical profile of NA. Drug loading reduced mean PDI by 28.6% across all formulations, indicating enhanced colloidal homogeneity. Particle sizes ranged from 96.1 nm (Moringa nanocarriers) to 217.6 nm (DPPC:Chol liposomes), with zeta potentials between −16.4 and −29.1 mV. Encapsulation efficiencies were high across all systems (78–90%), with Moringa nanocarriers achieving the highest EE (approximately 90%). All nanocarrier systems substantially attenuated the burst release of free NA (approximately 98% at 12 hours), with initial 12-hour release values of 25–45% and 120-hour cumulative releases of 65–90%. The Quercus coccifera nanocarriers provided the most sustained release profile (25% at 12 hours; 65% at 120 hours), while the niosomal system achieved nearcomplete preservation of NA antimicrobial potency (MIC 3.3 mM vs. 3.0 mM for free NA against P. aeruginosa), uniquely combining sustained release with maintained bacteriostatic activity. Discussion: These findings demonstrate that nanoencapsulation is an effective strategy for simultaneously improving the physicochemical properties and therapeutic performance of NA. The niosomal and polyphenolic nanocarrier platforms emerged as the most promising candidates for translational development, offering the dual advantages of controlled drug release and maintenance of antimicrobial activity against clinically relevant pathogens. This study provides a rigorous comparative evidence base for nanocarrier selection in antibiotic repurposing strategies targeting the global AMR crisis