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This study investigated the short-term impact of irrigation with Amoxicillin solutions on the presence of the amoxicillin-resistance trait among culturable soil heterotrophic aerobic bacteria. The microcosm experimental design consisted of 15 days of incubation of 10 g soil samples irrigated daily with distilled water containing increasing doses of amoxicillin (0, 0.1, 1, 10, 100, 1000 µg g-1 of soil day-1). The hypothesis was that continuous daily addition of antibiotics would increase the proportion of antibiotic-resistant bacteria in soils. After the incubation period, the total and antibiotic resistance heterotrophic aerobic bacteria communities were assessed through serial dilution of soil suspensions, followed by agar plate culture enumeration, isolation, identification and microscopy observation. The presence of antibiotic-resistant bacteria was also evaluated directly on treated wastewater used for field irrigation before this microcosm study to assess the amoxicillin-resistant bacteria bioaugmentation hypothesis. Results indicated that the Amoxicillin resistance was widespread among bacteria present in both treated wastewater used for irrigation and in the receiving soil. A microcosm experiment was attempted as a ‘proof of concept’ to demonstrate that irrigation with treated wastewater containing antibiotics would exert selective pressure and promote the proliferation of antibiotic resistance bacteria. Unexpectedly, the results from the microcosm incubations indicated the daily addition of amoxicillin did not increase bacterial antibiotic resistance trait abundance in soils, which even significantly decreased for all tested doses. The antibiotic-resistant species identified among the isolates were Pseudomonas mosselii, P. otitidis, P. mendocina, P. flavescens, Stenotrophomnas maltophilia, Bacillus thuringiensis, Aeromonas veronii, Candida parapsilosis, Streptomyces violaceoruber and Microbacterium barkeri.
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