Abstract
Bacillus are Gram-positive, rod-shaped, aerobic or facultative anaerobic spore-forming bacteria with most species being saprophytes. In the form of endospore, they are resistant to some aggressive physical and chemical conditions with various species existing unusual physiological characters, enabling them to survive in different environments including fresh and marine waters, sediments, desert sands, hot springs, and Arctic soils. The genus of Bacillus includes thermophilic, psychrophilic, acidophilic, alkaliphilic, halotolerant, and halophilic representatives, which are capable of growing at wide ranges of environmental conditions such as temperature, pH, and salinity. The physiological life habitat of Bacillus bacteria is remarkable. They are able to degrade many substrates derived from animal and plant sources including cellulose, starch, proteins, agar, and hydrocarbons. These spore-forming bacteria are antibiotic producers, heterotrophic nitrifiers, denitrifiers, nitrogen fixers, iron precipitators, selenium oxidizers, oxidizers, manganese reducers, facultative chemolithotrophs, and acidophilic and alkalophilic bacteria, as well as having psychrophilic and thermophilic features. A better understanding of the cellular structure and life cycle biology of these spore-forming bacteria definitely results in a better understanding of the biological and physiological functions of these microorganisms, especially when they are used in the form of probiotics. This is very important due to an increased application of these spore-forming microorganisms as probiotics in different animals including cultured aquatic animal species. This chapter deals with the biology and life cycle of Bacillus spp. as nowadays some of them are widely used in aquaculture sector either in the feed or in the rearing water. Therefore, study of the biology of these bacteria will assist us in a better understanding of their mode of action and the mechanisms when are used as the probiotics in aquaculture sector.