Probiosis, although not a new concept, has only recently begun to receive an increasing level of scientific interest. Probiotics are generally defined as ‘live microbial feed supplements that can benefit the host by improving its intestinal balance’. Probiotics fall under two broad classifications, those for animal use and those for human use. Probiotics used in animal feed are considered as alternatives to antibiotics (and therefore used as growth promoters). In 2000 Denmark banned the use of antibiotics as growth promoters in its pig industry and 2006 is the date proposed for a complete ban of antibiotics in animal feed within Europe . A viable alternative to antibiotics would therefore be an important venture and for this reason the development of new probiotic products that could be licensed for animal use is receiving considerable interest. However, the transfer of antibiotic resistance traits between bacterial species is a cause for concern where large quantities of bacteria would be given to animals. In Europe it is estimated that to licence a new probiotic product for use in animal feed requires upwards of 1.4 million Euros . Probiotics for human use, on the other hand, are subject to minimal restrictions (at least as novel foods or as dietary supplements) and come in many different forms. In supermarkets they are often sold as dairy-type products containing ‘live bacteria’ and in health food shops as capsules or tablets composed of lyophilized preparations of bacteria which promote ‘a healthy gut’, etc. Finally, on the internet some products are being sold as quasi-medicinal products which can be used for oral bacteriotherapy of gastrointestinal disorders (normally diarrhoea).
Currently, there is no universal ‘class’ of probiotic bacterium although the most common types available are lactic acid bacteria (e.g., Lactobacillus spp.). These bacteria are found normally in the gastrointestinal tract (GIT) of humans and animals and there is the vague notion that the use of indigenous or commensal microorganisms is somehow restoring the natural microflora to the gut. A second class comprises those that are not normally found in the GIT. For example, Saccharomyces boulardii has been shown to be effective in preventing the recurrence of Clostridium difficile-induced pseudomembranous colitis  as well as the antagonistic action of Escherichia coli. S. boulardii products are currently being marketed for human use. Within this group of allochthonous probiotic microbes are the spore-forming bacteria, normally members of the genus Bacillus. Here, the product is used in the spore form and thus can be stored indefinitely ‘on the shelf’. The use of spore-based products raises a number of questions though. Since the bacterial species being used are not considered resident members of the gastrointestinal microflora how do they exert a beneficial effect? Because the natural life cycle of spore formers involves germination of the spore, proliferation and then re-sporulation when nutrients are exhausted, the logical question is whether it is the germinated spore (that is the vegetative cell) that produces the probiotic effect or is it the spore itself? If the former model is correct then it would suggest that probiotics show a unified mode of action involving the action of a live bacterium within the GIT. This review, based on published studies, will present the case that spore-forming bacteria can survive and, indeed, proliferate within the GIT of animals, Although it is unlikely that they are true commensals, a case will be made that many spore formers exhibit a unique dual life cycle of growth and survival in both the environment and within the gut of animals and it is this bimodal life cycle that could provide the basis of their probiotic effect.
This review will focus exclusively on the use of spore-forming bacteria as probiotics for human and animal use. For conciseness, with a few exceptions, this review will only report on studies relating to species used in existing commercial formulations and will cover their use in humans and animals, as well as in aquaculture. Finally, it should be mentioned that this review expands on two excellent reviews in this field [6,7].