Characterisation of the role of an alkaline protease from Vibrio midae SY9 in enhancing the growth rate of cultured abalone fed a probiotic-supplemented feed

Abstract

Abstract South Africa has a rapidly developing abalone aquaculture industry based largely on the cultivation of Haliotis midae . The prophylactic use of probiotic microorganisms has been shown to enhance the health, growth and nutritional status of a variety of cultured aquaculture species. Furthermore, research has shown that H. midae fed a high protein artificial diet supplemented with the proteolytic marine bacterium Vibrio midae SY9 have enhanced digestive protease activity levels, protein digestion and absorption, and growth. The predominant V. midae SY9 extracellular alkaline protease, VmproA, has been previously shown to be present in the crop/stomach and intestinal regions of H. midae fed a diet supplemented with V. midae SY9. Therefore, VmproA may be responsible for the probiotic properties of the bacterium in farmed abalone. In this study, the role of VmproA on abalone growth was investigated using the mutant strain V. midae SY9Mut2, which is unable to produce VmproA. Over the course of a 180 day laboratory-based growth trial the growth rate of small (28 mm) abalone improved by 6.5 and 14.5% following dietary supplementation with V. midae SY9 and V. midae SY9Mut2, respectively, in comparison to abalone fed a basal diet. In situ protease activity within the H. midae digestive tract was significantly increased in animals fed the V. midae SY9 supplemented diet compared to abalone fed either the basal feed or the V. midae SY9Mut2 supplemented diet. Thus, VmproA does not appear to be the factor responsible for increased growth of probiotic-supplemented abalone. Statement of relevance The manuscript addresses the mechanism of how probiotics function in a farmed abalone species. This is important since probiotic supplementation of feed is proving successful in increasing the growth of these molluscs, and yet very little is known regarding the underlying relationship between the host animal and the probiont.