Pond aquaculture: potential reservoirs for antibiotic resistance

Increasing attention has been given to antibiotic usage in aquaculture, with their addition to the environment being a potential source for the proliferation of antibiotic resistance genes (ARGs). These genes could then be spread to human pathogens via horizontal gene transfer. Antibiotic resistance is exacerbated by anthropogenic activity in fields such as medicine, animal farming, and waste treatment. Thus, adding to growing fears over how we may be able to treat future infectious diseases.

Although the aquaculture industry is widespread, a vast amount of aquaculture production is carried out in southeastern Asia, with China accounting for 71% of aquaculture production globally. Agriculture-aquaculture is common practice in China with terrestrial farmland being in close association with aquaculture ponds. Animal manure is directed into the pond system promoting phytoplankton proliferation to feed the fish stock. The manure often contains antimicrobial growth promoters which have not been fully digested by farmland animals. This leads to the spread of antibiotic resistance and supports antibiotic-resistant bacterial communities.

The study presented here aimed to investigate the concentrations of antibiotics, the abundance of ARGs and the bacterial communities within the water and sediments of aquaculture ponds in Guangdong, China. This is the first study to use culture-independent methods to address these aims.

Paired water and sediment samples were taken from four different ponds. Antibiotic (tetracyclines, sulphonamides and (fluoro)quinolones) concentrations were analysed by ultra-performance liquid chromatography electrospray tandem mass spectrometry. DNA was extracted from the water and sediment samples, DNA was amplified by PCR to determine the presence of ARGs followed by a qPCR reaction to quantify the abundance of ARGs. Finally, the bacterial community composition was analysed by amplifying 16s rRNA by PCR, the amplicons were then sequenced using Ion Torrent to allow for bacterial identification.

Antibiotic concentrations in sediments reached as high as 446 μg kg^-1 and 98.6 ng L^-1 in water samples. Although some of the antibiotic concentrations were below bacterial inhibition they are high enough to allow the development and maintenance of antibiotic resistance. A greater proportion of antibiotics were found in sediment samples compared to water samples, indicating sediments are reservoirs for antibiotics and are potential hotspots for the development of antibiotic resistance. ARG abundances were also found to be much higher than those found in pristine environments. The bacterial communities were dominated by the Bacteroidetes, Proteobacteria, and Firmicutes. Alarmingly, opportunistic human pathogens such as Clostridium and Escherichia were identified in all sediment and water samples, with Escherichia being a potential vector for horizontal gene transfer of ARGs to other human pathogens. However, many sequences were unidentifiable showing there is still more to know about these communities while also highlighting a possible downside to a metagenomic approach to identifying community compositions.

There is without doubt, that anthropogenic activities within aquiculture are mediating the development of antibiotic resistance and supporting potentially pathogenic bacterial communities. Better management practices need to be put into place, such as the use of fertilizers free from animal waste contaminated with antibiotics.

Paper reviewed:

Xiong, W., Sun, Y., Zhang, T., Ding, X., Li, Y., Wang, M., & Zeng, Z. (2015). Antibiotics, antibiotic resistance genes, and bacterial community composition in fresh water aquaculture environment in China. Environmental Microbiology, 70, 425-432.