The incidence of antibiotic resistant bacterial strains in Italian fish farms

The increase in intensive large-scale fish aquaculture has been linked to the development of antibiotic resistance within some marine microbial communities. In Italy, a number of environmentally different areas in the coastal North Adriatic region have been established as semi-intensive and extensive fish farms. These farms normally operate either with productive management of the coastal environment (mostly lagoons) in marine and brackish water using extensive and semi-intensive methods, or by intensively farming commercially valuable fish and shellfish species in tanks and cages.

Several environmental hazards can arise from these techniques, as the intensive conditions can result in the spread of infectious diseases in the farmed fish and shellfish species, which are caused by several types of Vibrio, Aeromonas and Photobacterium. The subsequent use of the antibiotics tetracycline (TET) and oxytetracycline (OXTET), flumequine (FLU), trimethoprim (TIM) and the association of trimethoprim with sulfadiazide (TIM-SUL), which have been linked to the induction of antimicrobial resistance (AR) in marine bacteria by horizontal gene transfer and mobile genetic elements. Pollution in coastal areas causes significant problems in the marine environment, and can result in contaminated water that used for drinking, irrigation and recreation, and hence reduce health quality.

Fig. 1. Different incidences of antibiotic resistance strains in water, sediment and biofilm samples obtained from the aquaculture centres.

This study measured the global incidence of antimicrobial resistance and the frequency of AR bacteria to either individual or multiple antibiotics commonly used in Italian fish farms at a number of different sites within each centre that are distributed along the Adriatic Sea between the Venetian Lagoon and the Gargano area in Puglia (Italy), as well as in a coastal site in the Veneto region. Samples were taken from sediment, biofilms and water from inlet water (W­_in) and outlet (W_out) entering and leaving the fish tank/ farm at each aquaculture centre.

Under the selective pressure of the antibiotics used in fish farms, AR marine bacterial strains can persist and form an environmental reservoir directly involved in the seafood chain. AR bacteria exist in natural environments where antibiotics affect bacterial metabolism, so comparisons were done between bacterial strains from coastal aquaculture centres and those from an area 2 km away from the coastline. This meant the data from the more intensively- and recently-treated aquaculture centres could be compared to a baseline AR incidence. There was a higher level of susceptibility to FLU, as the resistance frequency was <1% (0.3%), compared to TET and OXTET, which averaged out at 20% resistance (but ranged between 10 to 50%).

Veneto I is a fish farm located in a small “valle di pesca”, (valley/ brackish lake) in the north of the Adriatic region, where only experimental techniques to produce new fish species are used. This experimental aquaculture approach seems to be effective, as it is the centre with the lowest global incidence of AR strains (24%). Veneto II is also a brackish lake where the fish are farmed in a semi-intensive culture. In comparison with the first site within the same area, there was a global incidence of 45% of AR strains to at least one antibiotic, but the most significant result that has been found for Veneto II is the 18% of multi-resistant strains in the centre. The authors suggest that this large difference is because there is a more frequent use of antibiotics in the second farm, whereas the first one had no records of usage in at least a year.

Gargano I is an open-sea site located 2km away from the Puglia coastline that sporadically used antibiotics for intensively-farmed fish in 35 cages up to 2009-2010, and the AR resistance incidence was similar to other centres, but the level of multi-resistance was similar to that in the coastal sites. Finally, Gargano II is located in a lagoon in the Gargano peninsula area. Horses and sheep were being extensively bred in the surrounding area, and were also being given TET; therefore this external source of TET-resistant bacteria could be why 50% of strains were resistant to the antibiotic. This site was found to have the highest incidence of AR, with 79% of strains showing resistance to at least one antibiotic and a third with multiple resistances, but individually were still all susceptible to TEM, TEM-SUL and FLU, which are only used in aquaculture.

Further research could look into more detail at how the community composition is altered by the different aquaculture methods and the antibiotics used (i.e. which microbial species are dominant when a certain antibiotic is introduced). In addition, they could’ve also investigated whether some strains of the same bacteria are resistant to at least one of the antibiotics at one site, but still susceptible to the same ones at other sites, because some antibiotics have been used for longer periods of time/ higher frequencies (i.e. so that random mutations and/ or horizontal gene transfer can occur).

Labella, A., Gennari, M, Ghidini, Trento, I., Manfrin, A., Berrego, J.J., and Lleo, M.M. (2013) High incidence of antibiotic multi-resistant bacteria in coastal areas dedicated to fish farming. Marine Pollution Bulletin, 70: 197-203