In vitro antimicrobial efficacy of laser-synthesized silver nanoparticles against antibiotic-resistant Escherichia coli isolated from dairy cattle wastewater

Background and Aim: Poorly managed dairy farm wastewater is a significant reservoir of antibiotic-resistant bacteria, particularly Escherichia coli, contributing to the environmental spread of antimicrobial resistance (AMR) and posing risks to animal and public health. Conventional wastewater treatment systems are often insufficient to inactivate these resistant organisms. Silver nanoparticles (AgNPs), especially those synthesized by pulsed laser ablation (PLA) in liquid, offer a high-purity, chemical-free nanomaterial with promising antimicrobial properties. This study aimed to evaluate the in vitro antimicrobial efficacy of laser-synthesized AgNPs against antibiotic-resistant E. coli isolated from dairy cattle wastewater within a One Health framework. Materials and Methods: Wastewater samples were collected aseptically from 50 smallholder dairy farms in East Java, Indonesia. E. coli isolates were identified using standard cultural, morphological, Gram staining, and biochemical (Indole, methyl red, Voges–Proskauer, citrate) methods. Antibiotic resistance was screened using the Kirby–Bauer disk diffusion method against streptomycin, erythromycin, penicillin, and tetracycline. AgNPs were synthesized via PLA in polyvinylpyrrolidone medium and characterized using transmission electron microscopy, ultraviolet–visible spectroscopy, and Fourier transform infrared spectroscopy. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of AgNPs were determined by broth microdilution and agar subculture methods, respectively, across concentrations ranging from 0.195 to 100 ppm. Statistical analysis was performed using one-way analysis of variance followed by Tukey’s post hoc test at a significance level of p < 0.05. Results: PLA successfully produced monodisperse AgNPs with a mean diameter of 11.62 ± 1.8 nm and a characteristic surface plasmon resonance peak at 418 nm, confirming high-purity and stability. Twenty antibiotic-resistant E. coli isolates were evaluated. MIC values ranged from 37.5 to 100 ppm, with erythromycin-resistant isolates showing the lowest MICs (45.0 ± 10.5 ppm) and streptomycin-resistant isolates the highest (75.0 ± 33.3 ppm). Most isolates (75%) exhibited MBC values >100 ppm, indicating predominantly bacteriostatic activity at the tested concentrations. No statistically significant differences in MIC values were observed among resistance groups (p > 0.05). A concentration of 62.5 ppm was identified as the most effective inhibitory dose across resistance profiles. Conclusion: Laser-synthesized AgNPs demonstrated consistent in vitro inhibitory activity against antibiotic-resistant E. coli from dairy wastewater, with an optimal MIC of approximately 62.5 ppm. These findings highlight the potential application of AgNPs as a supplementary control strategy in dairy waste management and AMR mitigation, supporting an integrated One Health approach.