Antibacterial Activity of Chlorhexidine-Killed Bacteria: The Zombie Cell Effect

We report a biocidal zombie effect of chlorhexidine, a wide-scope biocidal agent commonly used in disinfectant and antiseptic formulations. The zombie effect refers to the ability of dead bacteria killed by a biocidal agent to act as efficient biocidal agents toward a new generation of viable bacteria. The killed bacteria serve as a reservoir for the antibacterial agent incorporated within them; and the new viable population of bacteria acts as a trap of the bioactive agent, shifting the equilibrium of this agent between the reservoir in the dead cells and their aqueous environment. This report is a major generalization of the zombie phenomenon reported previously for silver from the points of view of extending to organic antibacterial agents; extending the effect to both Gram-negative - Pseudomonas aeruginosa PAO1 - and Gram positive - Staphylococcus aureus - representative bacteria; showing that the zombie effect is maintained in the second and third generations; showing the effect to operate in an environment of growth media, which extends it to life-supporting environments; and proving that cross-killing is possible, that is, killed S. aureus cells fully inactivated viable P. aeruginosa.