Antimicrobial resistance of intestinal Aeromonas spp. and Enterococcus spp. in fish cultured in integrated broiler-fish farms in Thailand

摘要

Integrated fish farming in Southeast Asia combines intensive husbandry with extensive aquaculture. The feed of the livestock, most importantly chicken and pigs, is supplemented with various antimicrobials for growth promoting and prophylactic and therapeutic treatment. Animal manure is excreted into the ponds and fertilizes the water and this supports the growth of photosynthetic organisms. Antimicrobials and antimicrobial residues enter the pond and may establish a selective pressure in favour of antimicrobial-resistant bacteria. Intestinal samples were taken from fish collected from eight integrated broiler-fish farms and nine control farms culturing fish only. Levels of resistance to chloramphenicol (CHL), ciprofloxacin (CIP), erythromycin (ERY), oxytetracycline (OTC), streptomycin (STR) and sulfamethoxazole (SMZ) were determined in two groups of indicator organisms, i.e. Aeromonas spp. and Enterococcus spp. Resistance was determined by the disk diffusion method and for Aeromonas spp. also by plate counts on agar supplemented with antimicrobials. Differences in levels of antimicrobial resistance were analysed by logistic regression with each individual farm included as random effect. For Enterococcus spp., significant higher levels of resistance to chloramphenicol (8%), erythromycin (91%, odds ratio (OR) 35), oxytetracycline (75%, OR 14), and streptomycin (72%, OR 31) were found among fish intestinal samples from integrated farms compared to Enterococcus spp. derived from control farms (0%, 23%, 16%, and 31%, respectively). Resistance among the Aeromonas spp. was low (< 20%) and insignificant between the different farm types, mainly because of a high variation in resistance. However, resistance to oxytetracycline in Aeromonas spp. isolated in integrated farms was 37% but not significant (P= 0.053) compared to control farms (13%). Resistance determination based on plate counting did not reveal any significant differences between Aeromonas spp. in fish intestinal samples from integrated and control farms. The results indicated that the practice of integrated fish farming may lead to higher levels of antimicrobial-resistant bacteria in the intestine of the fish. However, they also showed that the use of two groups of different indicator organisms gave different outcomes, thus stressing the importance of using different indicators when assessing antimicrobial resistance in the aquatic environment.