Sediment bioelectrochemical system with potential application against vibriosis in aquaculture

Abstract

Aquaculture can suffer from heavy losses caused by vibriosis, which denotes diseases caused by Vibrio bacteria. Due to disadvantages of available measures, mostly involving the use of antibiotics, to control these bacteria, novel measures are still needed. Therefore, aiming at an innovative solution for that issue in a long term, we investigated the inhibitory effects of the sediment bioelectrochemical system (SBES) on a V. harveyi strain (Vh) and a V. parahaemolyticus strain (Vp). SBESs were installed in test pond models that were lab-scale plastic tanks containing artificial brackish water, while the resembling systems not having SBESs were used as the controls. Once the SBES stably generated electricity (at ca. 0.5 mA), the tests with the Vibrio strains were conducted by: (i) supplementing the Vibrio cell suspensions directly into the tank waters, (ii) immersing the Vibrio cells suspensions each in an isolated axenic space into the tank environments, and (iii) adding the filtered tank waters after taken out of the tanks to the Vibrio cells. Based on plate enumeration to evaluate the inhibitory effects of such treatments on the Vibrio strains, we discovered that almost 100% of both Vh and Vp cells could not grow after 5 min in contact with the SBES water while more than 80% of them could still grow after 1–2 h in contact with the water from the control tanks. Paralleling tests showed that the SBES water was also toxic to an Escherichia coli strain but only slightly inhibited a Bacillus pumilus strain and a Lactobacillus plantarum strain, reducing their growths by only about 60% and 30%, respectively. Further tests revealed that the effects were not due to suspected proteinacous components or metal contents or peroxides in the SBES water. On the other hand, applying a redox potential of 20 mV (vs. Ag/AgCl), which was the anode potential measured in the SBES, onto a graphite rod immersed in Vh cell suspension led to the growth inhibition to 90% of the cells, although the unpoised graphite rod also seemed toxic, inhibiting ca. 60% of Vh cells. A similar experiment with titanium (known to be non-toxic to bacteria), instead of graphite, clearly showed the vibrio-inhibiting effect of the applied redox potential. Altogether, the results directly evidence the inhibitory effect of the SBES on V. harveyi and V. parahaemolyticus. This effect, together with the mild effect on the beneficial bacilli, implies that SBES integration in aquaculture ponds can be a promising novel technological option that allow controlling vibriosis in-situ while minimizing the uses of antibiotics or other chemicals, toward more sustainable aquaculture practices.