Intensive aquaculture systems come with obstacles of degraded water quality and disease outbreak. Such issues are managed by water exchange, which is accounted for an enormous amount of water drainage and subsequent cost and environmental impacts [1, 2].

Biofloc technology (BFT) functionality depends on the heterotrophic bacterial activity which their dominant abundance allows these bacteria to manipulate the abundance of harmful bacteria (cyanobacteria and vibrio). At the beginning stage of BFT maturation, the heterotrophic bacterial abundance is not enough to control the growth of harmful bacteria (for example, vibrio). After 3-4 months of BFT running system, a load of suspended solids is created. This solids’ accumulation affects animal health and product quality in terms of the gills blocked function and flesh off-flavor (cyanobacteria), but suspended solids elimination causes a sudden change in microbial composition and nutrients concentration, leading to decreased heterotrophic and nitrifying bacterial loads and facilitating pathogenic bacterial growth (vibrio) [3].

Pathogenic abundance can be manipulated through carbon to nitrogen ratio (C/N) [4], carbon source (starch, seaweed, starch and seaweed mix, the data under publication in Aquaculture journal), dietary ingredients and their effects on host immune response, intestinal bacterial composition, and BFT heterotrophic bacterial activity. Since non-starch polysaccharides (lignin, cellulose, and hemicellulose) in grains [5], amino acids in hydrolysate food’s by-products, and protein source (yeast, macroalgae, insect meal) are dietary factors affecting host immune response and intestinal bacterial composition [6]. In addition, dietary digestibility affects the level of solid waste in an aquaculture system [7], for example, pectin and cellulose decrease the dry matter digestibility compared to starch [8] which generate a high solid waste in water column [9-11].

Varity of dietary and carbon sources’ compositions in BFT make it difficult to generalize one strategy for BFT disease control at early (low heterotrophic bacterial abundance) and late (high cyanobacterial and vibrio abundances) BFT maturation stages. Since different nutrients and bioactive molecules are included. In a bacterial community, the balance among different bacterial groups is formed as result of competition on space and nutrients, in addition to the quorum sensing factor’s level, and even virulence factor’s level [12-14]. In a study (under revision by Aquaculture journal), BFT showed a variety in its bioactive molecules’ composition related to the bacterial cell activity or bacterial virulence factors. Some of these bioactive molecules were associated with a lower vibrio abundance, for example, irisflorentin and probucol, they are anti agents against the bacterial virulence factor of LPS, were associated with a lower vibrio abundance. In human, manipulating the availability of virulence factor (pyoverdine) feeds back on a decreased pathogenic bacterial load, but with high level of elimination, the host immune system further overreacted since a minimum level of virulence factor is required for the immune strength [15]. In this context, a new concept of disease resistance could be raised in application of aquaculture disease control, that chelating the virulence factors could be a new strategy in disease control.

Recently, the bile acids’ product was introduced as new feed additives in the animal health industry and mainly as emulsifying agents which basically increasing the lipid digestibility. In fact, bile acid could offer a specific action in disease control, since bile acid shows a dialysis effect on the pathogenic bacterial cell wall [16], chelation reaction with virulence factors [17-21], catabolizing reaction on virulence factor [22], and even an interaction with host immune system mediating the side effect of virulence factors on host immunity [23-28]. Most of these results need to be investigated for practical applications in aquaculture whether as feed or water additives.

References

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