No abstract

A complex and dynamic community of microorganisms, play important roles within the fish gastrointestinal (GI) tract. Of the bacteria colonizing the GI tract, are lactic acid bacteria (LAB) generally considered as favorable microorganism due to their abilities to stimulating host GI development, digestive function, mucosal tolerance, stimulating immune response, and improved disease resistance. In early finfish studies, were culture-dependent methods used to enumerate bacterial population levels within the GI tract. However, due to limitations by using culture methods, culture-independent techniques have been used during the last decade. These investigations have revealed the presence of Lactobacillus, Lactococcus, Leuconostoc, Enterococcus, Streptococcus, Carnobacterium, Weissella, and Pediococcus as indigenous species. Numerous strains of LAB isolated from finfish are able to produce antibacterial substances toward different potential fish pathogenic bacteria as well as human pathogens. LAB are revealed be the most promising bacterial genera as probiotic in aquaculture. During the decade numerous investigations are performed on evaluation of probiotic properties of different genus and species of LAB. Except limited contradictory reports, most of administered strains displayed beneficial effects on both, growth—and reproductive performance, immune responses and disease resistance of finfish. This eventually led to industrial scale up and introduction LAB-based commercial probiotics. Pathogenic LAB belonging to the genera Streptococcus, Enterococcus, Lactobacillus, Carnobacterium, and Lactococcus have been detected from ascites, kidney, liver, heart, and spleen of several finfish species. These pathogenic bacteria will be addressed in present review which includes their impacts on finfish aquaculture, possible routes for treatment. Finfish share many common structures and functions of the immune system with warm-blooded animals, although apparent differences exist. This similarity in the immune system may result in many shared LAB effects between finfish and land animals. LAB-fed fish show an increase in innate immune activities leading to disease resistances: neutrophil activity, lysozyme secretion, phagocytosis, and production of pro-inflammatory cytokines (IL-1β, IL-6, IL-8, and TNF-α). However, some LAB strains preferentially induces IL-10 instead, a potent anti-inflammatory cytokine. These results indicate that LAB may vary in their immunological effects depending on the species and hosts. So far, the immunological studies using LAB have been focused on their effects on innate immunity. However, these studies need to be further extended by investigating their involvement in the modulation of adaptive immunity. The present review paper focuses on recent findings in the field of isolation and detection of LAB, their administration as probiotic in aquaculture and their interaction with fish immune responses. Furthermore, the mode of action of probiotics on finfish are discussed.

Probiotics administration in aquafeed is known to increase feed consumption and absorption due to their capacity to release a wide range of digestive enzymes and nutrients which can participate in digestion process and feed utilization, along with the absorption of diet components led to an increase in host's health and well-being. Furthermore, probiotics improve gut maturation, prevention of intestinal disorders, predigestion of antinutrient factors found in the feed ingredients, gut microbiota, disease resistance against pathogens and metabolism. The beneficial immune effects of probiotics are well established in finfish. However, in comparison, similar studies are less abundant in the shellfish. In this review, the discussions will mainly focus on studies reported the last 2 years. In recent studies, native probiotic bacteria were isolated and fed back to their hosts. Although beneficial effects were demonstrated, some studies showed adverse effects when treated with a high concentration. This adverse effect may be due to the imbalance of the gut microbiota caused by the replenished commensal probiotics. Probiotics revealed greatest effect on the shrimp digestive system particularly in the larval and early post-larval stages, and stimulate the production of endogenous enzymes in shrimp and contribute with improved the enzyme activities in the gut, as well as disease resistance.

Chitin consists of b-1,4-linked N-acetylglucosamine residues and is estimated as the second most abundant biomass in the world after cellulose. However, relatively little chitin is utilized as a material for industrial, agricultural and medical applications and aquacultural purposes. Chitin may be useful as a constitutive material in formulated fish feed, and the interesting effects in fish merit further evaluation. There is evidence that fish and aquatic animals harbour a gut bacterial community that is distinctly different from that reported in the surrounding habitat or in the diet. Thus, the gut environment provides a specific niche, and bacterial activity in the gut is not merely a continuum of that observed in the environment. Today, it is well accepted that the gut microbiota in fish are modulated by dietary manipulations. But to what extent can dietary chitin and krill (chitin-rich) modulate the intestinal microbiota of fish and how do these dietary components affect the immune system? These questions will be discussed in the present review.

Soybean meal (SBM) is one of the most commonly used vegetable ingredient to replace fish meal in fish diets. However, SBM is limiting in some essential amino acids and contains numerous antinutritional factors and antigens that can affect intestinal microbiota and innate immune system in several finfish species and crustaceans and compromise health. The impact of SBM on health and gut microbiota of aquatic animals is not only affected by SBM in general, but also on the degree of treatment of the meal and exposure. Recently, many studies are actively seeking ways to complement or balance those adverse responses induced by high inclusion of SBM in aquaculture diets. These include advanced processing and mixture of feed with other feed components to balance antinutritional factors. The impact of dietary soybean oil on gut microbiota has also been investigated but to a lesser extent than SBM. As the gastrointestinal tract has been suggested as one of the major routes of infection in finfish species and crustaceans, the effect of soybean products on the gut microbiota is important to investigate. Several studies have focus on supplementation of SBM on the adverse responses of the innate immune system as immunological mechanisms are likely involved in the underlying pathology. However, the precise cause of the inflammatory process has not yet been clarified, even though some investigations have suggested that alcohol‐soluble antinutritional factors, especially soy saponins, are potential causative factors. Possible interactions between soybean products and innate immune system in several finfish species and crustaceans are discussed.