Stiffening of bacteria cells as a first manifestation of bactericidal attack.

Development of new antibacterial agents requires generation of new methods that will allow characterization of mechanisms of novel antibacterial actions as well as observation of pathogen susceptibility to antibiotics. A family of synthetic cationic lipids, such as ceragenins, which mimic the action of membrane-targeting natural cationic antibacterial peptides, shows promise as a new family of bactericidal agents. Here we report that treatment of Bacillus subtillis cells with human cathelicidin LL-37 and ceragenins CSA-13 and CSA-131 (1-100μg/mL) leads to physicochemical changes of bacterial cells surface. Especially nanomechanical alternations as quantified by bacterial stiffness and adhesion measurements shows that bacteria respond actively to treatment by tuning their surface properties and this response is tightly controlled by the concentration of bactericidal agents. Following the real-time changes of bacteria stiffness, we observed, that Bacillus subtilis cell envelope can initially stiffen, but in the long-time treatment it softens compare to untreated cells. We postulate, that nanomechanical responses may be considered as a new way to assess some aspects of antimicrobial activity that evolve in time, especially for molecules targeting bacterial membranes.