ABSTRACT Bacillus anthracis, the anthrax agent, is a member of the Bacillus cereus sensu lato group, which includes invasive pathogens of mammals or insects as well as nonpathogenic environmental strains. Clade 1 members of the B. cereus s.l. group can exchange virulence-plasmids via horizontal transfer with B. anthracis to cause anthrax-like disease too. The natural route of infection is ingestion, often by grazing animals or, in humans, by the consumption of spore-contaminated food (gastrointestinal, GI anthrax). The proposal aims to determine the mechanisms of B. anthracis spore invasion across intestinal epithelia. The central hypothesis is that spores of B. anthracis and anthrax-causing strains germinate in the intestinal tract of infected animals. Our working model postulates that vegetative bacilli secrete several enzymes with mucin-binding and mucin-degrading activities to penetrate the thick mucin layer. Next, the adhesins BslA and BslB mediate uptake of B. anthracis into intestinal cells. BslA and BslB are two of 22 S-layer associated proteins (BSLs) that form the surface (S)- protein layer. BSLs associate via S-layer homology (SLH)-domains with secondary cell wall polysaccharide (SCWP), a peptidoglycan linked carbohydrate polymer. The integrity of the S-layer and SCWP is important for the activity of BSLs. Further, the bslA gene is located on the mobilizable pXO1 virulence plasmid. Thus, we propose that surface-protein layers contribute broadly to the pathogenesis of anthrax-causing organisms. Here, we will use mice and intestinal organoids to model GI anthrax and test our predictions. We propose to study the pathway for the synthesis of SCWP that is required to display the surface-protein layer and is essential for growth. We expect that the elucidation of the assembly pathway of SCWP will serve as a paradigm for the study of other complex polymers of Gram-positive bacteria and reveal new targets of antibacterial drugs. We anticipate that unravelling intestinal mucosa interactions with anthrax and anthrax-like pathogens will collectively advance the field of infectious diseases research.