Bioprospecting of novel antimicrobial metabolitesfrom Bacillus subtilis MBTDCMFRI Ba37 andPseudomonas aeruginosa MBTDCMFRI Ps04 oftropical estuarine habitats of Cochin, India and itsapplication in fish health management

Aquaculture is an important economic activity the world over and about 90% of the global production is contributed by Asian countries. Global aquaculture has a persistent goal to maximize the production with optimal profit. Even though the aquaculture is growing at a rapid rate, the practice of aquaculture faces many challenges in its developing path. Intense cultivation in high densities significantly affect the environment by issues of used - up farm water discharges and heavy waste accumulation through hyper nutrification due to excessive feeding and high dietary nutrient composition (Liao and Mayo 1974; Boyd 1985). This has brought stress to the rearing environment and diseases to the cultured species and subsequently decreased the overall productivity due to mass mortality which has led to significant loss to the industry (Irie et al. 2005; Cruz et al. 2012; FAO 2012). Thus, it is necessary to face the challenges and find solutions in order to make aquaculture sustainable. Disease is the result of interaction between the host, pathogen and external environment. At the onset of disease, the harmonious interplay between pathogens and non-pathogens is disturbed result in unhealthy host and thereby infection occurs (Verschuere et al. 2000; Schulze et al. 2006; Zhou et al. 2009; Nemutanzhela et al. 2014). Bacteria, fungi, viruses and parasites are the major pathogens affecting aquaculture industry. Among different aquaculture pathogens, bacteria play a major role. It can survive and flourish independently in any aquatic environment. The bacterial fish infections are mainly from the species of Aeromonas, Vibrio, Flavobacterium, Edwardsiella, Yersinia, Pseudomonas, Streptococcus, Renibacterium, Piscirickettsia, Mycobacterium etc. The common bacterial disease symptoms are external reddening and haemorrhage in the peritoneum, body wall and viscera, generically referred to as haemorrhagic septicaemia. The progress of disease leads to ulcerative lesions and mortality of untreated cultured animals (Pridgeon and Klesius 2012). Among the bacterial diseases, vibriosis is caused by a major bacterial fish pathogen Vibrio spp. which is ubiquitous in all aqua cultured species (Jayaprakash et al. 2005; Thompson et al. 2010). In tropical countries, penaeid shrimp culture is highly affected by V. harveyi which causes their mass mortality (Austin & Zhang 2006). Another Vibrio species which affects the shrimp culture are V. alginolyticus, V. damsela, V. parahaemolyticus, V. vulnificus, V. splendidus and V. penaeicida (Saulnier et al. 2000).