Virulence-Associated Mobile Elements in Bacilli and Clostridia

This chapter focuses on (i) species that induce human diseases, (ii) species that are able to produce toxins, and (iii) the association of appropriate virulence factors with possible mobile elements. With reference to bacilli, the chapter discusses mainly Bacillus anthracis and B. cereus. A section on clostridia focuses on Clostridium perfringens, neurotoxin-producing clostridia, and species capable of producing large clostridial cytotoxins (LCTs). The chapter talks about the contribution of the genetic mobility of virulence genes to the evolution of pathogenic bacilli and clostridia. B. anthracis strains produce a tripartite protein toxin, comprising PA (protective antigen), EF (edema factor) and LF (lethal factor). The main scientific interest is focused on the toxin plasmid (pXO1) and the capsule plasmid (pXO2), since they were recognized at an early stage as carrying the major virulence factors of B. anthracis. Under laboratory conditions, gene transfer has been achieved by transduction, conjugation, or conduction and has been reported to occur between B. anthracis, B. cereus, and B. thuringiensis strains. With virulence factors that are known to be mobile, the significance of the association of mobile genetic elements with virulence genes for the evolution of the pathogenic clostridia and bacilli becomes clear. The extreme flexibility of the bacteria toward the loss and acquisition of virulence factors could help pathogenic bacteria adapt to the excessive pressure of selection to which they are subjected during the complex interactions between pathogens and their hosts.