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.