Taxonomy, Ecology, and Cellulolytic Properties of the Genus Bacillus and Related Genera

Abstract

Bacteria of the genus Bacillus and related genera (e.g., Paenibacillus, Alicyclobacillus or Brevibacillus) belong to the phylum Firmicutes. Taxonomically, it is a diverse group of bacteria that, to date, has not been well described phylogenetically. The group consists of aerobic and relatively anaerobic bacteria, capable of spore-forming. Bacillus spp. and related genera are widely distributed in the environment, with a particular role in soil. Their abundance in the agricultural environment depends mainly on fertilization, but can also depend on soil cultivated methods, meaning whether the plants are grown in monoculture or rotation systems. The highest abundance of the phylum Firmicutes is usually recorded in soil fertilized with manure. Due to the great abundance of cellulose in the environment, one of the most important physiological groups among these spore-forming bacteria are cellulolytic bacteria. Three key cellulases produced by Bacillus spp. and related genera are required for complete cellulose degradation and include endoglucanases, exoglucanases, and β-glucosidases. Due to probable independent evolution, cellulases are encoded by hundreds of genes, which results in a large structural diversity of these enzymes. The microbial degradation of cellulose depends on its type and environmental conditions such as pH, temperature, and various substances including metal ions. In addition, Bacillus spp. are among a few bacteria capable of producing multi-enzymatic protein complexes called cellulosomes. In conclusion, the taxonomy of Bacillus spp. and related bacteria needs to be reorganized based on, among other things, additional genetic markers. Also, the ecology of soil bacteria of the genus Bacillus requires additions, especially in the identification of physical and chemical parameters affecting the occurrence of the group of bacteria. Finally, it is worth adding that despite many spore-forming strains well-studied for cellulolytic activity, still few are used in industry, for instance for biodegradation or bioconversion of lignocellulosic waste into biogas or biofuel. Therefore, research aimed at optimizing the cellulolytic properties of spore-forming bacteria is needed for more efficient commercialization.