Antibacterial Activity Of Skin Mucus Of Exotic Fishes

Emerging and re-emerging infectious diseases are the leading cause of mortality worldwide. Although, antibiotics are the important weapon in fighting these bacterial infections but microbes are genetically able to acquire resistance against antibiotics. With the development of antibiotic resistance in many of diseases causing microbial strains, discovery of new antibiotic constituents especially from natural resources has become the cause of concern for everyone. Amongst natural resources, use of therapeutics obtained/derived from animals is an important alternative and fish appear to be one of that animal resource. Fishes represents the largest class of vertebrates and forms the minor and major links between food webs of the aquatic ecosystems which harbor a wide assay and large number of pathogens. According to Laidler, Treasurer, Grant and Cox (1999), in intensive aquaculture system, to reduce production cost, fishes are kept in high densities and possibilities for exposure to microbes, throughout the production cycle is very high. Many of these microbes are pathogenic causing serious damage to the fish and aquaculture industry.

Therefore, fishes had to evolve the complex innate defense system which contributes to their survival. Blackstock and Pickering (1982) stated that fish skin mucus is the key component of fish innate immune system and lying at the interface between fish and the aqueous environment, combats pathogens from the moment of their first contact. Nagashima, Kikuchi, Shimakuro and Shiomi (2003) demonstrated that Fish skin mucus is constantly secreted and replaced, hence wash off the microbes or other particles entrapped in it which helps to prevent the stable colonization of potential infectious microorganisms. Pearson and Brownlee (2005) reported that mucus is secreted by goblet cells. It is slimy secretion, consisting of mucins and combination of other substances such as inorganic salts immunoglobulin, proteins and lipids suspended in water giving it characteristic lubricating properties which help in choking of pathogens. According to Aggarwal, Banerjee and Mittal (1979), skin mucus is a complex fluid and its composition varies throughout the epithelial surface of fish.

Also Zuchelkowski, Lantz and Hinton (1981) and Ellis (2001) reported that composition and rate of mucus secretion vary from species to species and has been observed to change in response to microbial exposure or to environmental fluctuations such as hyperosmolarity and pH. Besides this, Shephard (1994); Jones (2001); Fast, Sims, Burka, Mustafa and Rass (2002b); Alvarez-Pellitero (2008) and Guardiola, Guesta, Abellan, Meseguer and Esteban (2014) demonstrated that mucus also acts as a storehouse for innate immune components such as lectins, pentraxines, lysozymes, proteolytic enzymes, alkaline phosphatase, C-reactive protein, complement and antimicrobial peptides (AMPs) as well as immunoglobulins which provide antibacterial strength to fish skin mucus.

In recent years, antibacterial properties of skin mucus from many fishes have been investigated against several human and fish pathogenic bacteria by many workers viz. Austin and MacIntosh (1988); Hellio, Pons, Beaupoil, Bourgougnon and Gal (2002); Kupulakshmi, Prakash, Gunasekaran, Manimegalai and Sarojini (2008); Subramanian, Ross and Mackinnon(2008); Subramanian, Mackinnon and Ross (2009); Wei, Xavier and Marimuthu (2010); Bragadeeswaran and Thangaraj (2011); Kumari, Nigam, Mitial and Mitial (2011); Vennila, Kanchana, Arumugam, Vijalakshami and Balasubramanian (2011) which supported the hypothesis that the epidermal mucus plays a protective role and should be effective against fish and human pathogenic bacteria as well. But a very little information is available on the antimicrobial activity of the fresh water fishes of different habits and habitats.

Each species has its own habits, living in different kinds of water, eat food, which may influence the amount of mucus secretion and presence of different components of fish skin mucus with in the species or between the species and can be helpful in providing variety of immune response and components. Thus, the present study was focused on analyzing the antibacterial effect of skin mucus of three carps species viz. Cetenopharyngodon idella, Hypophthalmichthys nobilis and C. carpio which are exotic to India and cultured in polyculture with indigenous carps.