The genus Bacillus: from pathogens to beneficial microbes

The genus Bacillus is a phenotypically large, heterogeneous collection of spore-forming Gram-positive rods that can be isolated from a great variety of environments. With the exception of B. anthracis and B. cereus, the large majority of these bacteria display poor pathogenicity in humans and are most often considered as contaminants in clinical practice. Nevertheless, occasional infections due to other Bacillus species have been reported in recent years. B. cereus mainly causes food-poisoning diseases, but it is also increasingly associated with serious human local and systemic infections. The pathogenic potential of B. cereus is due to the secretion of virulence proteins and to the presence of flagella, which play a crucial role in bacterial adhesion to surfaces and in swimming and swarming motility. The signal recognition particle (SRP)-GTPase FlhF has been shown to regulate protein secretion, flagellar formation, and flagellum-dependent motility modes in B. cereus, thus suggesting its key role in B. cereus pathogenicity. For their wide range of physiological characteristics and ability to produce a multitude of enzymes, antibiotics, and metabolites, Bacillus species are used in many medical, pharmaceutical, agricultural, and industrial processes. Bacillus spores, particularly those of the GRAS (generally recognized as safe) species B. clausii and B. coagulans, have a long history of consumption and safe use as probiotics, live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Probiotic products containing spores for human or animal use are commercialized in several countries, being widespread in Australia, Europe, Asia, USA, and South America. Chapter 1 is a general introduction to the genus Bacillus, B. cereus and its virulence, and to the role of FlhF in the regulation of flagella and virulence. The application of Bacillus strains as probiotics is also introduced. In chapter 2, we evaluated the prevalence of Bacillus spp. in clinical cultures and the application of matrix-assisted laser desorption-ionization time of flight mass spectrometry (MALDI-TOF MS) as a diagnostic technique for the identification of these bacteria. Antibiotic susceptibility and virulence potential of the clinical isolates are also reported. In chapter 3, the effects of FlhF depletion on swarming, protein secretion and pathogenicity of B. cereus are analyzed. In chapter 4, we gained more insight on the role of FlhF as SRP-GTPase in B. cereus. In particular, the ability of FlhF to self-dimerize and interact with the L2 component of the toxin hemolysin BL are reported. In chapter 5, the compositional quality and potential gastrointestinal behavior of three Bacillus-based probiotic formulations was analyzed in comparison with other top formulations commercialized in Italy. Acid resistance and bile tolerance were investigated. Chapter 6 includes a general, updated discussion and the conclusions on the overall results