The use and benefits of Bacillus based biological agents in aquaculture

Abstract

Global shortages in seafood resources have driven the growth of aquaculture as an economic activity, predominantly in developing countries. As a consequence of space and resource constraints, traditional aquaculture has been intensified into reticulated systems with high stocking densities of the cultured species. This results in an artificial environment that has a propensity for supporting the growth of pathogenic bacteria and the accumulation of waste metabolites in aquaculture systems. The indiscriminate release of spent aquaculture wastes into surrounding environments is also problematic. The outbreak of disease in aquaculture systems, caused by bacterial pathogens, is a complex phenomenon associated with stressful environmental conditions such as poor water quality and can ultimately result in mass mortality and significant loss to the industry. The main cause of poor water quality is waste accumulation through hyper-nutrification resulting from excessive feeding rates and high nutrient dietary composition, both of which are common phenomena in intensive aquaculture systems. High levels of nitrogenous and phosphorous waste accumulation predispose fish to infestation by parasites and pathogens and also pose a threat to the environment. Selection for certain characteristics by breeders has also in some cases reduced the vigour in breeding lines, making fish less hardy and more susceptible to disease. Of particular importance is the prevalence of bacterial disease, which results in damage and often leads to death of fish. Gram-negative bacteria such as Aeromonas hydrophila are amongst the main pathogenic micro-organisms responsible for bacterial disease. Conventional methods of dealing with disease include the use of chemicals and antibiotics, which alter natural microbial populations, damage the environment and increase resistance and virulence of pathogenic micro-organisms. Useful micro-organisms play a number of roles in pond culture, particularly with respect to productivity, nutrient cycling, nutrition of the cultured animals, water quality, disease control and environmental impact of effluents. Bacterial additives demonstrate the potential to improve water quality and reduce pathogen load and mortality, and have thus emerged in modern day aquaculture as alternatives to chemicals and antibiotics. Many bacterial strains have also demonstrated a significant algaecidal effect, which is advantageous in aquaculture systems through reduction of algal growth and hence algal blooms which can destabilise these systems. Biological agents such as Gram-positive Bacillus spp. offer an attractive solution to the challenges facing modern aquaculture. Advantages of this genus include the ability to grow rapidly, tolerate a wide range of physiological conditions and the ability to sporulate. The robust spores of Bacillus spp. are also amenable to simple and cost effective production processes and the end products are stable for long periods.