Infectious diseases have been estimated to cost the global aquaculture industry billions of dollars annually1,2. With concerns over emerging resistance and residues of antibiotics in food3 many such chemicals are now being banned and environmentally friendly alternatives are being sought. Probiotics influence the composition of the gut microbiota and confer health benefits to their host4,5 and are one of several alternative approaches gaining significant popularity in aquaculture. Whilst primarily used to manage bacterial disease, there is also some evidence that probiotics can provide protection against parasites4 and viruses6. Probiotics can inhibit the growth of pathogens in the gut through the excretion of antagonistic substances including bacterocins6,7; prevent pathogen adhesion in the gut through competition of space and nutrients3 and by modulating the immune system5. Some probiotics have been reported to improve growth and feed utilisation efficiency5 and others can also improve water quality2,8,9, which confers indirect benefits to host healthsup>6.