Fate and Transport of Antibiotics in the Soil-Plant System

Antibiotics can enter the environment through the application of biosolids and the use of treated wastewater effluent for irrigation. In the agro-environment, low levels of antibiotics may contribute to an increased risk of selection for antimicrobial resistance in the soil, as well as alter overall microbial biomass and the relative abundance of different microbial groups. Alterations of the soil microbiome is likely to impact plant health as a result of the symbiotic relationship between plants and soil microbes. Furthermore, as a result of common evolutionary ancestry, many of the same receptors and processes that antibiotics target in microbes are conserved in plants. As a result, a direct phytotoxic effect from antibiotic exposure is possible. The aim of the study is to evaluate fate and transport of antibiotics in the soil-plant system. An environmentally relevant mixture of antibiotic compounds was derived. A synthetic wastewater effluent combined with the antibiotic mixture was used for irrigation in a 14-week mesocosm study with barley (Hordeum vulgare) as the model crop. The transport of antibiotics in the soil plant system was monitored via targeted analytical measurement of antibiotic concentrations in soil pore water and mesocosm leachate using HPLC-QqQ-MS. Findings show that some of the antibiotics (i.e. tetracycline, oxytetracycline, ciprofloxacin, amoxicillin and penicillin v) are undetectable in both the soil pore water and the leachate. Risk quotients were derived for each antibiotic from the measured concentrations using recently published predicted no effect concentrations (PNECs) for the selection of antimicrobial resistance. Metronidazole was the only antibiotic found to exceed the PNEC at environmental concentrations, suggesting that it could be posing a risk of inducing antimicrobial resistance in the agricultural environment. Laboratory degradation experiments were used to determine antibiotic stability, with possible degradants being identified using non-targeted high mass accuracy mass spectrometry. It was discovered that oxytetracycline, metronidazole and amoxicillin do not degrade via hydrolysis but did via photolysis. Further experiments investigated the photolytic degradation products using novel gas phase UV spectroscopic techniques, with the aim of being used in future work to identify stable degradation products in the soil pore water and leachate. Results from this work seek to provide new insights into the fate and transport of human use antibiotics in the agricultural environment.