Synthesis of terbium-labelled tetracycline-loaded calcium phosphate nanoparticle and its mode of action on multi-drug-resistant pathogenic bacteria Escherichia coli and Salmonella kentucky

This study dealt with synthesis of a luminescent nano-form of tetracycline, characterization of its important physico-chemical properties, and molecular mechanism of its antibacterial action on tetracycline-resistant bacterial species. Nanonization was done by entrapping tetracycline (Tet) molecules within calcium phosphate nanoparticles (CPNPs) and doping them with fluorescent terbium (Tb) ions. To characterize the particles, techniques like AFM, SEM, TEM, DLS, absorption-fluorescence-FTIR spectrometry and dialysis were used and to investigate their antibacterial potency and mechanism of action, techniques of agar plating, Ni²⁺-NTA chromatography, absorption-fluorescence-CD spectroscopy, gel electrophoresis and NS-TEM were used. Tet-Tb-CPNPs, prepared as colloidal suspension, were highly mono-dispersed, moderately stable, spherical in shape, ∼30 nm in size and ∼220 kDa in MW; entrapment efficiency of tetracycline within the nanocomposite particles was about 55 % and its release from them was sustained, bringing out above 95 % of entrapped tetracycline over seven days. The bactericidal concentration of Tet-Tb-CPNP on diarrhoea-causing MDR (including tetracycline) bacteria E. coli and S. kentucky was about 40-45 μg/mL. Binding of Tet-Tb-CPNPs with bacterial ribosome resulted in disruption and degradation of ribosomal proteins and RNAs; such ribosomal degradation was the root cause of its antibacterial action. Moreover, the nanonized tetracycline had no significant toxicity on human neuroblastoma SH-SY5Y cells at its antibacterial dose. Therefore, further pharmacological and clinical investigations are utmost important before commercializing Tet-Tb-CPNP as a potential nano-antibiotic.