Unveiling Peptide-Induced Structural Damage in Bacteria and Candida albicans Through Atomic Force Microscopy

Antimicrobial peptides (AMPs) have attracted considerable attention as alternatives to conventional antibiotics due to their ability to kill microbes with reduced toxicity against eukaryotic cells. Herein, we investigated three membranotropic peptides (MAP, VP22, and IDRY) which exhibit physicochemical properties that include high positive charge and amphipathic character, similar to those observed in AMPs, suggesting potential activity against microorganisms. We evaluated the antimicrobial activity of these candidates against clinically relevant Gram-positive and Gram-negative bacteria (E. coli, S. aureus, and S. enterica), as well as the yeast C.albicans, and investigated peptide-induced structural changes on the surface using Atomic Force Microscopy (AFM). The minimum inhibitory concentration (MIC) was determined by the broth microdilution method. Following the MIC determination, microbial cells were incubated with the peptides, fixed with glutaraldehyde, and analyzed by AFM to obtain 3D topography images of the cell surface. MAP exhibited antimicrobial activity against E. coli, S. aureus, and S. enterica, with a MIC ≥ 120μM. IDRY showed a MIC  ≥ 250μM for E. coli and ≥ 500μm for S. aureus and S. enterica. VP22 did not exhibit detectable antibacterial activity against the bacterial strains. In contrast, all three peptides were active against C. albicans, with MIC values ≥ 10μM for MAP, ≥120μM for VP22 and ≥ 31μM for IDRY. AFM analyses revealed significant alterations in the surface topography of peptide-treated cells, characterized by reduced surface compared with the untreated controls, indicating peptide-induced modifications at cell walls. These findings demonstrate that MAP and IDRY exhibit antibacterial action, while VP22 showed activity only against C.albicans. Furthermore, the topographical changes observed by AFM highlight the potential of this technique for investigating, at the nanoscale, the interactions between bioactive peptides and the cell surface of bacteria and fungi, contributing to a better understanding of their possible mechanisms of action.