Dynamics of the antimicrobial peptide PGLa action on Escherichia coli monitored by atomic force microscopy

Atomic force microscopy (AFM) images of living cells in physiological solution were used to monitor the different stages involved in the interaction between Escherichia coli and the antimicrobial peptide PGLa. Damage on bacterial membranes was observed in the past using standard electron microscopy; stiffness measurements and images scanned in physiological solution demonstrate the advantage of AFM for such studies. From force versus separation curve measurements it is possible to determine the variation of the cellular stiffness. PGLa action on components of the cell structure like the outer membrane, the bacterial pili, the peptidoglycan wall and the inner membrane was determined by the comparison of AFM images of bacteria before and after PGLa addition. The interaction of Escherichia coli with PGLa in the culture medium has two stages. The first is characterized by the loss of surface stiffness and the formation of micelles probably originating from the disruption of the outer membrane and the loss of the bacteria's ability to adhere to the substrates. In the second stage there is further damage, which resulted in total cell rupture. AFM images of bacteria in air and surface roughness measurements were also used to estimate peptide damage.