Nanostructured Lipid Carriers Co-Loaded with Doxycycline, Gentamicin, and Thymol for Enhanced Intracellular Antibacterial Activity Against Brucella melitensis.

BACKGROUND

Brucellosis is a zoonotic infection caused by Brucella spp. which persist within macrophages and complicate treatment. This study developed nanostructured lipid carriers (NLCs) co-loaded with doxycycline, gentamicin, and thymol to enhance intracellular delivery and antibacterial activity against Brucella melitensis.

METHODS

Drug-loaded NLCs were prepared using high-shear homogenization followed by sonication. The formulations were characterized in terms of particle size, polydispersity index (PDI), zeta potential, morphology by FE-SEM, encapsulation efficiency, and in vitro drug release profile. Cytotoxicity was assessed using the MTT assay on J774A.1 macrophage. Antibacterial activity and intracellular efficacy against Brucella melitensis were evaluated using microbiological and cell-based assays.

RESULTS

The optimized formulation showed a size of 398 ± 30 nm, PDI 0.35, and zeta potential -17.5 mV, with 97% encapsulation efficiency and stable morphology over nine months. The system exhibited sustained release up to 95% at 80 h, in contrast to rapid diffusion from free drugs. Intracellular studies showed that Gen-Dox-Thy@NLC achieved a 3.4-log reduction in bacterial load, demonstrating significantly higher efficacy (~1.6-log greater reduction) compared with free drugs (p < 0.05), while maintaining macrophage viability above 85%.

CONCLUSION

The co-loaded NLC formulation provided controlled drug release, improved intracellular delivery, and enhanced antibacterial activity against Brucella melitensis. These findings suggest that Gen-Dox-Thy@NLC may serve as a promising candidate for brucellosis treatment and warrants further in vivo studies.