Can Effector Molecules of Innate Immunity Target Microbial Motility?

Abstract

Abstract Antimicrobial peptides are key effector molecules throughout nature responsible in part for the defense against microbial pathogens and influencing the composition of colonizing microbes. Defensins are the major antimicrobial peptides produced by Paneth cells in the mammalian small intestine and are essential for maintaining homeostasis between host and microbe. While mature defensin peptides typically have microbicidal activity, human alpha defensin 6 (HD6) lacks such activity yet provides protection from invasion by bacterial pathogens. Using a transgenic mouse model expressing HD6 in Paneth cells, we reported the importance of HD6 in preventing the translocation of pathogenic Salmonella Typhimurium (STM) across the small intestinal epithelium (doi:10.1126/science.1218831); however, the mechanism underlying the inhibition of invasion is not fully understood. We hypothesize HD6 binds to extracellular bacterial protein structures, such as flagella, and inhibits swimming motility through a process of direct binding and self-assembly. Using an agar based motility assay, we show STM radially swim at a slower rate when in the presence of HD6. In ongoing studies using live-microscopy to visualize GFP-expressing STM in the presence of HD6, we observe a decrease in STM motility in a concentration dependent manner with some STM appearing to be completely immobilized, but without changes in bacterial viability. Unexpectedly, it appears the immobilization observed is influenced more by the expression of fimbriae than by the expression of flagella. With these results in mind, we plan to expand our investigation to elucidate key molecular determinants and include other microbes to test the broader relevance of this activity of HD6.