Abstract
Bacterial infectious diseases cause a huge economic loss in aquaculture. Active biological control that uses bacterivorous organisms to remove pathogens is an ecologically friendly approach for the cultural system to counteract the bacterial infection. The ciliate is one of the main predators of bacteria in aquatic ecosystems, but whether it can be effectively adopted to protect aquaculture organisms from bacterial pathogens still remains to be investigated. In this study, we optimized the culturing method for a marine ciliate Strombidium sp. NTOU1 and analyzed its bacterivorous properties. Strombidium sp. NTOU1 could feed on a variety of bacteria including pathogenic species. By controlling the amount of frozen bacteria Erwinia spp. in the medium, the ciliate grew to the maximum density within 4 days and could reach 1.2 x 105 cells/mL after the suction filtration enrichment. Ingested bacteria were observed in the food vacuole of the ciliate, and the average bacterial clearance rate of a single NTOU1 cell was ~300 cells/hr. In the challenge trial which grouper larvae were exposed to an extreme environment containing a high density of the pathogen Vibrio campellii, only 33% of the grouper larvae could survive after 5 days. However, preincubating with Strombidium sp. NTOU1 for an hour resulted their survival rate to rise to 93%. Together, our results demonstrated that Strombidium sp. NTOU1 has the potential to become a biological control species to actively remove pathogens in aquaculture. In addition, the technical improvement to culture Strombidium sp. NTOU1 provides an advantage for this ciliate in the future academic research or biotechnological application.
Highlights
As global capture fisheries resources are gradually depleted, aquaculture has become an important source of high-quality animal proteins to meet the need of the world’s continuously growing population (Goldburg and Naylor, 2005; Brander, 2007)
NTOU1 could be cultured in the laboratory using the unboiled rice medium made of rice grains and artificial seawater (ASW) (Lee, 2015; Supplementary Figure S1)
At the initial bacterial concentration of 2.00×107 CFU/mL, all the bacteria we tested were able to sustain the culture of Strombidium sp., but the growth rate of ciliates was significantly higher when they were fed with the Erwinia spp. than in other conditions (Figure 1). These results demonstrated that Strombidium sp. could utilize a variety of bacterial species as the food source, and the Erwinia spp. isolated from the rice medium is preferential in our culturing condition
Summary
As global capture fisheries resources are gradually depleted, aquaculture has become an important source of high-quality animal proteins to meet the need of the world’s continuously growing population (Goldburg and Naylor, 2005; Brander, 2007). Antibiotics are still the most effective way to fight bacterial infectious diseases, but their excessive use in aquaculture has become a serious problem (Cabello, 2006; Defoirdt et al, 2011). The abuse of antibiotics induces the resistance of bacterial pathogens in the environment, indirectly causing the prevalence of super bacteria, which in turn threatens human health (Sapkota et al, 2008; Heuer et al, 2009). Antibiotics harm probiotics in the environment and the intestine of animals, leading to an imbalance in microbial ecology that contributes to the instability of the aquaculture system (Martinez, 2011). Alternative methods, which can effectively control the infectious disease and maintain the ecological balance of the culture, are urgently on demand (Pérez-Sánchez et al, 2018)
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