Growing antimicrobial resistance necessitates the development of new, effective therapeutics for the continued treatment of infection and disease, for which antimicrobial peptides are promising candidates. Many antimicrobial peptides are membrane-active but their mode of action remains elusive. Our work provides insight into the action of antimicrobial peptides by measuring the effect of a model antimicrobial peptide on living planktonic Klebsiella pneumoniae cells. This has been achieved using atomic force microscopy to probe the mechanobiology of live bacteria to obtain the turgor pressure, cell wall elasticity, and bacterial capsule thickness and level of organisation.
The addition of the peptide had a significant effect on the turgor pressure and Young’s modulus of the cell wall. The turgor pressure increased upon peptide addition and the subsequent decrease suggests that cell lysis occurred and that cytoplasm leaked through a compromised membrane. The Young’s modulus increased after exposure to the peptide, indicating that interaction with the peptide increased the stiffness of the cell wall. A surprising result is that the bacterial capsule does not prevent cell lysis by the lytic peptide. Nor does the capsule appear to be affected by exposure to the peptide, as the thickness and organisation varied no more than expected within a population of bacteria. This result is ratified by mechanical measurements.