Long term exposure of agricultural soil to veterinary antibiotics changes the population structure of symbiotic nitrogen-fixing bacteria occupying nodules of soybeans (Glycine max)

Antibiotics can be entrained onto agricultural land through the application of organic wastes or wastewater for irrigation. In order to evaluate the impacts on soil microorganisms of exposure to antibiotics (ATB), replicated plots treated each year with a mixture of sul- famethazine, chlortetracycline and tylosin at concentrations ranging from 0.1 to 10 mg/kg, were initiated in 1999 at the AAFC research farm in London ON. Plots were seeded with soybeans. Soybean nodules were collected in 2012, after 14 annual ATB treatments. The bradyrhizobia strains were characterized by serology and molecular methods.The sensitivity of isolates to the 3 ATB was also evaluated. Based on molecular characterization the collec- tion of 278 bradyrhizobia was very diverse. The distribution of isolates in RS fingerprint groups was significantly different in the 3 soils treated with antibiotics compared to control soil, with an increase in proportion of strains belonging to RS types a, b, c and q. This result confirms those based on serotyping: strains belonging to <em>B. liaoningense</em> serogroup 135 are more abundant in soils having received antibiotics (compared to control). Using RS fingerprinting results, Shannon diversity indexes were computed, T-tests revealed that Shannon diversity indexes were significantly different between control soil and soil having received the lower dose of antibiotics. Overall, the present study indicates that long term treatment with veterinary antibiotics i.e. tylosin, chlortetracycline and sulfamethazine alters the composition of Bradyrhizobial populations occupying soybean nodules. The sensitivity of bradyrhizobia to the three antibiotics was not associated with the treatment from which they were recovered.