Abundance and transferability of antibiotic resistance as related to the fate of sulfadiazine in maize rhizosphere and bulk soil

Veterinary antibiotics entering agricultural land with manure pose the risk of spreading antibiotic resistance. The fate of sulfadiazine (SDZ) introduced via manure and its effect on resistance gene levels in the rhizosphere were compared with that in bulk soil. Maize plants were grown for 9 weeks in soil fertilized with manure either from SDZ-treated pigs (SDZ treatment) or from untreated pigs (control). CaCl2-extractable concentrations of SDZ dissipated faster in the rhizosphere than in bulk soil, but SDZ remained detectable over the whole time. For bulk soil, the abundance of sul1 and sul2 relative to 16S rRNA gene copies was higher in the SDZ treatment than in the control, as revealed by quantitative PCR on days 14 and 63. In the rhizosphere, sampled on day 63, the relative sul gene abundances were also significantly increased in the SDZ treatment. The accumulated SDZ exposure (until day 63) of the bacteria significantly correlated with the log relative abundance of sul1 and sul2, so that these resistance genes were less abundant in the rhizosphere than in bulk soil. Plasmids conferring SDZ resistance, which were exogenously captured in Escherichia coli, mainly belonged to the LowGC group and carried a heterogeneous load of resistances to different classes of antibiotics.