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Fate and Transfer of Antibiotic-Resistance Genes Excreted by Farm Animals
YEAR: 2003

The use of antibiotic agents in fields other than human treatment has grown rapidly over the past few decades. Currently, the veterinary use of antibiotics accounts for approximately 40-50% of all antibiotics produced, and only 20% of that amount is employed in disease treatment, with the rest serving a sub-therapeutic role in growth promotion and disease prevention. Unfortunately, due to the poor uptake mechanisms of the treated animals, most of the antibiotics administered are excreted, unaltered, into the soil surrounding feed lots. Recently, it has been observed that microbial strains have developed antibiotic resistance as an effect of high dosages of antibiotic agents in the soil. Some effort has gone into identifying the phenomena of antibiotic resistance development around feed lots, specifically the detection of the identities of the microorganisms present and the tet-determinants responsible for the resistance. The interest of the current project lies in the detection and monitoring of the development of resistant strains as well as the specific genes responsible for the antibiotic resistance. Flow through columns (30 cm) have been packed with previously unexposed (to antibiotics) soil, as to mimic the natural conditions of the runoff water underlying feed lots. Currently, two columns are operating in a continuous manner, with one receiving a solution consisting of synthetic ground water and acetate, and the second, enriched with tetracycline. The concentrations of both acetate and tetracycline are monitored at both the entrance and exit of the columns, and profiles of these columns along their length are also monitored. Early results point out that tetracycline is being degraded upon contact with the column soil, and only about 5% of the original amount is detectable at the column exit. Most probable number (MPN) enumeration technique was used to monitor changes on the total heterotrophic population and antibiotic resistant strain populations of the two columns. PCR detection of tet-genes of the isolated resistant strains was performed but the targeted determinants were not detected in these microorganisms, suggesting the presence of tet-determinants coding for the efflux pump excretion mechanisms. Preliminary results suggest that sustained TC exposure decreases the concentrations of total heterotrophs and increases the fraction of the microorganisms that are resistant to the antibiotic. Whether discontinuing TC exposure results in the rapid loss of tet-resistance in the exposed microorganisms remains to be determined.