Role of Antibiotic Bound Microplastics on Pathogenic Microbes: A in Vitro Antimicrobial Assay By Well-Diffusion Technique

The objectives of the study is to assess the sorption behavior of three broad spectrum antibiotics; Ciprofloxacin (CPX) and Tetracycline (TC) to polyethylene microplastic (PEMP), and to observe the influence of antibiotic-sorbed PEMP on some pathogenic microbes at 1 g/L solid solution mixture under in vitro conditions. Antibiotic concentrations, 100 mg/L, were spiked to PEMP solid solution and kept for shaking at 100 rpm for 12 h. Two Gram negative (Escherichia coli-ATCC 25922, Klebsiella pneumonia-ATCC 13883) and Gram positive pathogenic bacteria (Staphylococcus aureus-ATCC 25923, Bacillus cereus-ATCC 11778) were used for the study. Sterile Mueller Hinton agar medium was used (20 ml/plate) for well-diffusion assay with positive and negative controls. Culture plates were incubated at 37 ^oC for 24 h and zones of inhibition were measured. Only a limited adsorption of antibiotics was observed by the PEMP; 28 and 13 mg/g for CPX and TC respectively. The TC and CPX loaded PEMP showed antibacterial activity against S. aureus, B. cereus and E. coli. In the case of B. cereus, a bacteriostatic effect was demonstrated by TC loaded PEMP. None of the antibiotics loaded PEMP indicated any inhibitory action against the K. pneumonia at the concentration tested although the positive control (TC) showed a bactericidal activity. The results confirm the role of PEMP as a vector to transport antibiotics to control common pathogenic bacteria in the aquatic environment. Further studies will be conducted to understand the effect of different environmental conditions and minimum inhibitory concentrations on pathogenic and common microbes.