ABSTRACT
The genus Bacillus comprises an expansive and diverse set of bacteria distinguished by a common feature: the ability to make dormant endospores aerobically when challenged with unfavorable growth conditions. A photograph of sporulating cells of Bacillus subtilis is shown in Figure 27.1a. In 1989, Ruth Gordon authored a chapter for the Practical Handbook of Microbiology (CRC Press) that reviewed the systematics of the genus Bacillus (Gordon 1989). The various species of Bacillus were divided into three groups based on the morphologies of the sporangium (swollen or nonswollen) and the mature spore (spherical, cylindrical, or ellipsoidal) along with physiological and biochemical test results (such as catalase production and starch hydrolysis) and growth requirements (such as salinity, temperature, and pH). The species within these groups were highly heterogeneous. Since that time, however, there have been several major reclassications of species within Bacillus, a reassessment necessitated by the introduction of more sophisticated analytical methods that rely on comparison at the genome level and by the constant inux of novel species isolated from unusual environments. Bacillus now represents just a small part of a larger taxonomical consortium of endospore-producing bacteria referred to as Bacillus sensu lato. Consequently, it is no longer possible in a review such as this one to describe each Bacillus species. On the contrary, the goal of this work is to provide the reader with an overarching summary of Bacillus sensu lato, describing in broad brushstrokes the common characteristics of its current members. However, the review will also attempt to focus on those key species, especially B. subtilis, which play an increasingly important role in industry, medicine, and basic science. This two-tiered approach should provide the investigator with a greater appreciation of a fascinating and useful group of bacteria.
