MORPHOLOGY OF DEFENSIN-TREATED STAPHYLOCOCCUS-AUREUS

Defensins are a family of broad-spectrum antimicrobial peptides found abundantly in the cytoplasmic granules of mammalian neutrophils and Pa neth cells of the small intestine. Defensins are known to form ion cha nnels on the membranes of target cells. These channel formations and t he cytotoxicity of defensins are intimately linked. We showed the morp hological effects of defensins on the cytoplasmic membranes of Staphyl ococcus aureus by transmission electron microscopy. S. aureus exposed to defensins developed characteristic mesosome-like structures but did not show remarkable changes in cell walls. Defensins induced such str uctural changes not only at high concentration but also at low concent rations that were not bactericidal. We also showed that increasing the concentration of NaCl in the reaction mixture completely inhibited th e occurrence of membranous changes of target cells exposed to defensin s. These findings are, to our knowledge, the first report of morpholog ical changes in gram-positive bacteria treated with defensins. Our res ults indicate that the first effect of defensins in S. aureus is to da mage cytoplasmic membranes directly; they also support previous report s that the cell membrane is the principal target of defensins.