Beneficial and detrimental spore-formers: a world of diversity

Spore-forming bacteria are major players in life on Earth, and they are fascinating, as they adapt to many ecological niches. The spores are able to resist high temperatures, to dry out and to recover and germinate when outgrowth conditions become available. Spore-formers are mainly Gram-positive firmicute bacteria with low GC content; the Bacillus and Clostridia genera include prominent species. Bacillus subtilis is one of the most thoroughly studied bacteria, serving as a model to study sporulation and the regulatory networks controlling this differentiation system. B. subtilis is also a bacterium widely used for producing enzymes and metabolites of industrial importance. Among the spore-forming firmicutes, the Bacillus cereus group presents the most remarkable diversity. Frequently designated as “the Good, the Bad and the Ugly”, this bacterial group covers eight closely related species. They include the mammal pathogen Bacillus anthracis, responsible for anthrax disease, the insect pathogen Bacillus thuringiensis, used worldwide as a bioinsecticide to control pests of agricultural, forest and medical importance, and B. cereus senso stricto, an important food contaminant responsible for gastrointestinal disorders and, occasionally, more serious systemic diseases in immunecompromised individuals. The specific mammal toxicity of B. anthracis, the insecticidal activity of B. thuringiensis and the emetic activity of some B. cereus strains are due to plasmid genes. However, in addition to these specific traits, all these bacteria share chromosomal genes conferring the ability to colonize and to grow in abiotic and biotic environments. Interestingly, and in sharp contrast to these pathogens, a few B. cereus strains are used as probiotics in livestock animals and even, formerly, for humans, indicating the versatility of this group of bacteria. Among the pathogenic Clostridia, Clostridium botulinum and Clostridium perfringens are important sources of foodborne poisonings and infections, and the emergent opportunistic human pathogen Clostridium difficile is responsible for severe nosocomial intestinal infections. Due to the high resistance of the spores, to their adhesion properties and to their capacity to form biofilms, these anaerobic Clostridia and facultative anaerobic Bacilli are difficult to eliminate and have a direct economic impact upon the food industry and human health.This special issue of Research in Microbiology contains eight articles covering three main topics highlighted during the first BSPIT (Bactéries sporulantes pathogènes ou d'intérêt technologique) meeting which was held in France (Paris, July 6th and 7th, 2015) and hosted by the SFM (Société Française de Microbiologie), and which was mainly attended by French-speaking scientists.