Dynamic mechanisms of the bactericidal action of an Al2O3-TiO2-Ag granular material on an Escherichia coli strain

Abstract

The bactericidal activity of an Al2O3-TiO2-Ag granular material against an Escherichia coli strain was confirmed by a culturebased method. In particular, 100% of microorganisms were permanently inactivated in 30 to 45 min. The present work aimed to investigate the mechanisms of the bactericidal action of this material and their dynamics on Escherichia coli using different techniques. Observations by transmission electron microscopy (TEM) at different times of disinfection revealed morphological changes in the bacteria as soon as they were put in contact with the material. Notably highlighted were cell membrane damage; cytoplasm detachment; formation of vacuoles, possibly due to DNA condensation, in association with regions exhibiting different levels of electron density; and membrane lysis. PCR and flow cytometry analyses were used to confirm and quantify the observations of cell integrity. The direct exposure of cells to silver, combined with the oxidative stress induced by the reactive oxygen species (ROS) generated, was identified to be responsible for these morphological alterations. From the first 5 min of treatment with the Al2O3-TiO2-Ag material, 98% of E. coli isolates were lysed. From 30 min, cell viability decreased to reach total inactivation, although approximately 1% of permeable E. coli cells and 1% of intact cells (105 genomic units · ml-1) were evidenced. This study demonstrates that the bactericidal effect of the material results from a synergic action of desorbed and supported silver. Supported silver was shown to generate the ROS evidenced.