Integrated fish farming practices, in which livestock manure is added directly to fish ponds as fertilizer, are likely to increase the antimicrobial resistance of bacteria in associated ponds, report researchers from the Royal Veterinary and Agricultural University in Denmark, the Danish Veterinary Institute, and the Aquatic Animal Health Research Institute at Kasetsart University in Thailand.

Integrated fish farming produces high yields inexpensively because manure stimulates the growth of photosynthetic organisms, which serve as food for the farmed fish. As a result, farmers spend little, if any, money on supplemental fish food. However, because livestock is often fed growth promoters, such as antibiotics, which concentrate in their manure, antibiotics are often transferred into the ponds with the manure.

To determine whether antibiotics in manure are leading to an increase in the number of antibiotic-resistant bacteria in integrated fish farms, Andreas Petersen and colleagues tracked indicator organisms at chicken–fish, duck–fish, pig–fish, and four control fish farms. Within two months on the integrated farms, the researchers saw the resistance to such antimicrobials as oxytetracycline and sulfamethoxazole increase from less than 5% to 100%, and to 80% for ciprofloxacin. To their knowledge, this is the first study to examine antimicrobial properties in fish ponds treated with manure. Although the human health implications are not directly addressed in the study, resistance traits can easily transfer between a wide variety of aquatic environmental bacteria and potentially to humans through the food chain. (Appl. Environ. Microbiol. 2002, 68, 6036–6042)