Aquaculture can promote the presence and spread of antibiotic-resistant enterococci in marine sediments.

No antibiotics used but antibiotic resistance genes were activated.

   The use of antibiotics has favoured the spread of antibiotic-resistant (AR) microorganisms in the environment and it can become an issue for the future ability to treat infectious diseases. In the group of AR bacteria there are also fecal and human intestinal species such as Enterococci (also used as fecal indicator bacteria in water quality monitoring) that are able to transmit AR genes to other bacteria by horizontal gene transfer mechanisms thus increasing the diffusion of resistance genes in the marine environment.

   In this work the authors focus the research on the distribution of antibiotic resistant enterococci in sediments of aquaculture sites. Why this location? Fish-farms are places where AR can be favoured by large use of antibiotics. Even without continuous antibiotic administration, in these places resistant bacteria can survive and grow in sheltered environments with good supply of nutrients and organic matter (such as in sediment of fish-farm ponds). They chose two sampling sites inside a fish-farm pond (Station 1, near the fish feeding area; Station 2 far from the feeding area) and one sampling site (Station 3 in the water supply channel) as control, outside and upstream of the pond. This fish-farm pond contain 14000 seabream and seabass and it is located in central Italy (Varano Lagoon) near my home town.

   To investigate the presence of Enterococcus in the sediments and to screen the occurrence of resistant genes, the authors used molecular tools (qPCR for counting cells, PCR for detecting the presence of specific AR genes) and culture techniques (with/without adding antibiotics in culture medium). They analysed the sediment grains sizes and the quantity of organic matter finding St. 1 and St. 2  dominated by silt-clay fraction of grains (93% and 89%) and high organic matter content (28,3 mg/g and 24,3 mg/g), whereas St. 3 presented less silt-clay fraction (73%) and less organic matter (13,4 mg/g). They also analyse the presence of AR genes directly from sediment sample finding very low number or nothing at all and this result is in accordance with not-used antibiotics declared by the fish-farm owner.

  They analysed the resistance for the antibiotics (tetracycline TET, erythromycin ERY, ampicillin AMP and gentamycin CN) counting the number of bacteria before and after incubation with antibiotic-added culture medium (so no-antibiotics is the reference). Fig 1. 

Figure 1. Abundance of benthic enterococci in (A) St. 1, (B) St. 2 and (C) St.3 control site.  The number was determined by qPCR before (No antibiotics) and after incubation with antibiotic-supplemented  culture medium. (*) Not detectable. The results are expressed as number of cell/g of wet sediment.

   From the Fig 1. is easy to see that the number of enterococci is quite similar in the three stations before adding antibiotics. This situation change significantly after the antibiotics treatment showing clearly the absence of resistant enterococci in the St 3. Significant differences were found between different antibiotics: AMP and CN increase from 4-time to 11-time the number of cell in both St. 1 and St. 2; TET and ERY decrease in both stations. An interesting result is that although AR genes were not found before antibiotic treatment, after that the AR genes were amplified, allowing the enterococci to grow. The authors hypothesized that difference between inside and outside the pond can be caused by many factors and possible sources of resistant enterococci for example wild bird populations that also feed on the fish pond more than outside and could be an additional load of resistant bacteria. Also the high amount and supply of organic matter and fecal material from the fishes can contribute to keep the benthic enterococci more metabolically active thus allowing them to quickly adapt to the antibiotic exposure.

   In conclusion this work show that aquaculture (even despite direct use of antibiotic, as the owner said) can represent a reservoir for AR genes in enterococci population that under different environment condition can be activated. Another interesting point in my opinion is the different effect of different antibiotics opening a putative selection effect between different strains. Considering also the potential of horizontal gene transfer, fish-farm can contribute to spread of these antibiotics resistances to other species. So should be interesting search AR genes in other marine sediment bacteria. In this article there were provided more detailed information of the specific AR genes detected (i.e. tet(M), tet(L), tet(O), erm(B), mef, ecc…..) but I tried to simplify.

Di Cesare, A., Luna, G. M., Vignaroli, C., Pasquaroli, S., Tota, S., Paroncini, P., & Biavasco, F. (2013). Aquaculture can promote the presence and spread of antibiotic-resistant enterococci in marine sediments. PloS one, 8(4), e62838.