Dr. Ranjeeta Kumari and Dr. M. Rosalind George
It is one of the most important tasks for fish farmers to produce healthy cultured aquatic species and make it a sustainable operation. To achieve healthy stocks, it is needed to enhance the immunological responses of aquatic species under culture conditions. Thus, it would be very important to know the effect of immunostimulants on the activity of fish mucus.
In recent years, there were several studies on the effects of functional substances and micronutrients on mucus production and health status of fish. Those like probiotic bacteria, oligosaccharide, β-glucan, etc. have been tested on mucus production (Dawood et al.2016; 2015a; 2015b; Hoseinifar et al., 2015; Rodriguez-Estrada et al., 2013).
Fish are always in direct exposure with the aquatic environment; therefore, they are permanently in contact with various external hazards causing elements (e.g. aerobic and anaerobic bacteria, viruses, parasites, pollutants). To protect itself from various pathogenic microorganisms, the fish body has the first line of barrier between the ﬁsh and the environment i.e. mucus.
The mucosal surfaces of fish, include, skin, gill and gut, that act as the first line of defence against a broad spectrum of pathogens. Gill, skin and gastrointestinal tract of fish have been demonstrated and proposed to be the portal of entry for many microbial pathogens. Probiotic bacteria have been used in aquaculture to improve health of the fish, water quality and growth of farmed fish successfully (Verschuere et al., 2000). Most of the workshave been focused on protection of fish against infectious diseases (Nikoskelainen et al., 2001; Robertson et al., 2000). Varying results are obtained when selected probiotic bacteria were used to protect against different bacterial pathogens.The probiotic bacteria might be able to protect the host from pathogens by blocking the integumental attachment sites (adhesion receptors).
The good adhesion ability to mucosal surfaces is a prerequisite for bacterial colonisation, and has been well demonstrated for Lactobacillusrhamnosus (ATCC 53103) to fish mucus (Nikoskelainen et al., 2001) as well as to human mucus (Miettinen et al., 1996). Among the different extracellular compounds responsible for these processes, proteins secreted and released into the environment by probiotic bacteria (extracellular proteins) might mediate certain interactions, since they would be able to interact directly with mucosal cells, such as epithelial and immune cells.
It has been illustrated theoretically, that it is possible that certain probiotic bacteria which secrete extracellular proteins might also reach the gut mucosa, acting as molecular effectors responsible for downstream responses in mucosalcells (Sánchez et al., 2008).Extracellular proteins can be divided into two groups. The first group is composed of proteins that contain a signal peptide, which is located in the N-terminal part of the sequence to direct signal between protein to protein export machinery. The second group includes the surface-associated proteins that are basically shed from the bacteria due to the normal turnover of the cell wall. A study on freshwater swordtail indicated that Lactobacillus acidophilus as feed supplement was effective in enhancing antibacterial activity of skin mucus, and the skin mucus protein level and alkaline phosphatase activity were higher in Lactobacillus fed groups(Hoseinifar et al., 2015).
It is now possible to modulate the number of goblet cells, the composition of the mucus, and its lysozyme content by oral administration of immunostimulants. This, in turn, will lead to better mucosal vaccines for aquaculture and serve as a better-models for the study of vertebrate mucosal immunity.Production of epidermal mucus was enhanced by intake of lactic acid bacteria in Atlantic salmon (Salinas et al., 2008) and mannan oligosaccharide in sea bass (Torrecillas et al., 2011).
Increased mucus production in rainbow trout-fed diet-containing inactivated Enterococcus faecalis or mannan oligosaccharide or a combination of both was reported (Rodriguez-Estrada et al., 2013). Mucus secretion of red sea bream fed with all diet-containing of heat-killed Lactobacillus plantarum (HK-LP), was found to be higher than that fed with a HK-LP-free diet (Dawood et al., 2015). It was demonstrated that mucus secretion of red sea bream was significantly affected by either HK-LP or β-glucan, but they did not affect mucus bactericidal activity. Further mucus lysozyme activity significantly increased with increased HK-LP levels without β-glucan supplement while with β-glucan it did not change.
It was found that the lysozyme activity in mucus of red sea bream was affected by the concentrations of inactivated Pediococcuspentosaceus, and the lysozyme activity was significantly higher in fish fed with P.pentosaceusdiet than in fish fed with the basal diet(Dawood et al., 2016). Furthermore, mucus was significantly secreted more in Pediococcuspentosaceus-fed groups.
Micronutrients for effective mucus secretion
Other than probiotic bacteria, some micronutrients like vitamins have been found to be effective for mucus secretion. In red sea bream, a diet containing 325 ppm vitamin C increased the lysozyme activity of skin mucus compared to that of vitamin C-free diet (Ren et al., 2008). Similarly, Japanese eel fed with diets containing 762 ppm vitamin C showed significantly higher lysozyme activity and bactericidal activity of mucus than with 32 ppm vitamin C(Ren et al., 2007). Lactoferrin as acts as a functional supplement, has been tested to improve the health status of aquatic animals. Furthermore, the bactericidal activity of mucus was further enhanced with the addition of dietary lactoferrin. It was found on Caspian roach that dietary vitamin C significantly elevated skin mucus alkaline phosphatase, protein levels, and antimicrobial activity compared to a vitamin C-free group (Roosta et al., 2014). Mucus secretion was found to be significantly increased in spotted grouper when fed with diets containing lactoferrin from 400 to 1200 ppm compared to that of fish fed with a lactoferrin-free diet and concluded that oral lactoferrin administration could be an effective method to improve natural barriers of finfish(Yokoyama et al., 2006).
In conclusion, since the mucus plays a key role as the first-defense line for protection of aquatic animals, controlled secretion of mucus is very important for them to improve the survival, particularly for aquaculture species.The components of mucus will be able to impart strong innate immune mechanism against any stress/diseases. Although several dietary materials induce the increase of mucus secretion in aquatic animals, their effects on parasites and many such pathogens are still under investigation.
(The authors are Assistant Professors and Professor & Head in the Department of Fish Pathology and Health Management, FC&RI, Tuticorin, TNJFU, Tamil Nadu respectively. Views expressed are personal.)