Monday, November 13, 2006 - 1:45 PM

Impacts of Biosolids-Borne Antibiotics on Soil Microbial Communities.

Mary Stromberger, Dept. of Soil & Crop Sciences, Colorado State University, Fort Collins, CO 80523-1170 and Kiera Coffin, Colorado State University, Dept. Soil and Crop Sciences, Fort Collins, CO 80523.

Land application is a beneficial reuse of biosolids because it adds organic matter and plant nutrients to soil. However, an emerging concern is the potential for biosolids to introduce antibiotics and antibiotic-resistance genes to soil and water environments.  In a previous study, we found that 1) samples of aerobically and anaerobically-digested biosolids contained antibiotics within each of three antibiotic classes (macrolides, sulfonamides, and tetracyclines), 2) tetracyclines (and oxytetracycline specifically) were present in the greatest concentrations, and 3) biosolids samples also contained significant populations of oxytetracycline-resistant bacteria.  In a field study, we examined the antibiotic resistance patterns of bacterial isolated from agricultural soils, cropped to winter wheat, with and without a history of biosolids amendment.  When biosolids were surface applied at agronomic rates, the percentage of bacterial isolates resistant to oxytetracyline was similar between control (nonamended) plots and plots receiving biosolids.  However, when biosolids were applied at relatively high rates (5 tons acre-1) and incorporated into soil (20 cm depth), the percentage of bacterial isolates resistant to oxytetracycline was greater in biosolids-amended plots compared to nonamended control plots.  Under these conditions, 60% of the culturable soil bacteria were resistant to 1 ppm oxytetracyline compared to 27% in nonamended plots, and 4% of bacteria were resistant to 10 ppm oxytetracyline in the biosolids-amended plots compared to <1% in control plots.  Representative isolates were identified and tested against other antibiotics by the Kirby-Bauer antibiotic sensitivity test to determine if long-term application of biosolids alters soil microbial antibiotic resistance patterns.   Different results between the two study sites may be explained by lower application rates and/or photodegradation of oxytetracycline at the first site.   Our research demonstrates that biosolids are a source of both antibiotics and antibiotic-resistant organisms, but the potential for biosolids to affect the antibiotic resistance of soil organisms may depend on application practices.