GsMTx-4, a polypeptide from the spider Grammostola spatulata, is an inhibitor of mechanosensitive channels. It is known to interact with lipid membranes, suggesting it partitions into the membrane to alter the channel gating, but the effect of the membrane charge on GsMTx-4 activity remains unknown. In this study, we found that GsMTx-4 more effectively interacts with anionic lipids than zwitter-ionic ones. The effect of GsMTx-4 on negatively charged membranes was similar to that of the antimicrobial peptide melittin, which led us to assess GsMTx-4's antimicrobial activity. Interestingly, we found that, in contrast to other neurotoxins, GsMTx-4 exhibited antimicrobial properties and was more active against Gram-positive than Gram-negative bacteria. These results suggest that GsMTx-4 exerts its antimicrobial effect by altering the packing of the membrane and/or inhibiting mechanosensitive channels. These findings could point the way towards a new class of antimicrobial peptides. (c) 2005 Elsevier Inc. All rights reserved.