Interactions of antimicrobial peptide chrysophsin-3 with Bacillus anthracis in sporulated, germinated, and vegetative states.

Bacillus anthracis spores contain on their surface multilayered protein coats that provide barrier properties, mechanical strength, and elasticity that aid in protecting the sporulated state and preventing germination, outgrowth, and transition into the virulent vegetative bacterial state. In this work, the antimicrobial peptide (AMP) chrysophsin-3 was tested against B. anthracis in each of the three distinct metabolic states (sporulated, germinated, and vegetative) for its bacteria-killing activity and its ability to modify the surface nanomechanical properties. Our results provide the first demonstration that chrysophsin-3 killed B. anthracis even in its sporulated state while more killing was observed for germinated and vegetative states. The elasticity of vegetative B. anthracis increased from 12 ± 6 to 84 ± 17 MPa after exposure to 0.22 mM chrysophsin-3. An increase in cellular spring constant was also observed for chrysophsin-3-treated vegetative B. anthracis. Atomic force microscopy images suggested that the changes in mechanical properties of vegetative B. anthracis after chrysophsin-3 treatment are due to loss of water content and cellular material from the cell, possibly caused by the disruption of the cell membrane by the AMP. In contrast, sporulated and germinated B. anthracis retained their innate mechanical properties. Our data indicate that chrysophsin-3 can penetrate the spore coat of B. anthracis spores and kill them without causing any significant mechanical changes on the spore surface. These results reveal a yet unrecognized role for chrysophsin-3 in the killing of B. anthracis spores without the need for complete germination or release of spore coats.