MIC and MBC of silver nanoparticles in antibiotic-resistant Salmonella sp. from dairy cattle waste

Salmonella sp. is an important pathogen often found in dairy farm waste and poses health risks to humans and the environment. The increasing resistance of Salmonella sp. to various antibiotics due to uncontrolled use of antibiotics in the livestock industry is a serious infection control challenge.

This study aimed to evaluate the antimicrobial activity of Silver Nanoparticles (AgNPs) against antibiotic-resistant Salmonella sp isolates originating from dairy farm waste.

Silver nanoparticles with a concentration of 100 ppm (≈100 µg·ml⁻¹) were synthesized using the Nd:YAG laser method with polyvinylpyrrolidone (PVP) as a stabilizer and tested against five Salmonella sp. isolates resistant to penicillin, streptomycin, erythromycin, tetracycline, and oxytetracycline. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) tests were performed using the microdilution method.

The colloidal AgNPs inhibited visible growth of all tested Salmonella sp. isolates at 100 ppm, with the penicillin-resistant isolate having the lowest MIC value (50 ppm). However, bactericidal activity was observed only for the penicillin-resistant isolate at 100 ppm; MBCs for other isolates exceeded 100 ppm. These findings indicate that AgNP is effective as a bacteriostatic agent against antibiotic-resistant Salmonella sp., but its bactericidal activity is not optimal.

Factors, such as PVP stabilization and low silver ion (Ag⁺) release, likely limit the ability to kill bacteria. Therefore, the optimization of AgNP formulations to increase Ag⁺ release and penetration into bacterial membranes, as well as further studies on environmental safety, are needed before their widespread application in livestock waste treatment.