Structural and molecular interactions of quaternary ammonium compounds with bacteria

Abstract

The molecular interaction of quaternary ammonium compounds (QAC) with bacterial cells was investigated using various techniques to obtain the knowledge on what kinds of damage are done to the cell that results in cell death. Electron microscopy and flow cytometry were used to evaluate direct effects of didecyldimethyl ammonium chloride (DDAC) to Pseudomonas aeruginosa cells. The characteristic effects of DDAC observed on the bacterial structure were the formation and pinching off of blebs from the outer membrane and the formation of multilamellar structures within the nucleoid area. This was followed by coagulation of the entire nucleoid region and cytoplasm. The morphological changes observed in the intact gram-negative bacterium probably accounted for the bactericidal action of DDAC. An extensive theoretical discussion was provided to explain the observed effects of DDAC on the bacterial envelopes and cytoplasm. A flow cytometric approach was evaluated to determine its use for assessing antibacterial activity of a variety of QAC on P. aeruginosa ATCC 15442. LIVE/DEAD BacLight{dollar}\rm \sp{lcub}TM{rcub}{dollar} viability kit and the SYTOX{dollar}\rm \sp{lcub}TM{rcub}{dollar} kit were used as vital fluorescent stains. The DDAC had the greatest activity by converting the most number of bacteria in the population from green to red, followed by tetradecyl benzalkonium chlorides (BKC), hexadecyl BKC and dodecyl BKC. The flow cytometer provided an excellent means to assess the antibacterial activity of individual QAC. In order to decipher the interaction of DDAC with E. coli lipids, a steady-state and time-resolved emission spectra of 2-dimethylamino-6-lauroyinaphthalene (Laurdan), and steady-state anisotropy decay of 1,6-diphenyl-1,3,5,-hexatriene (DPH) were performed. Finally, {dollar}\sp{lcub}31{rcub}{dollar}P NMR spectroscopy allowed for convenient and qualitative measurements of the effects of DDAC on lipid polymorphism which seems relevant in the DDAC effects on membrane perturbation and possibly bactericidal activity. A schematic model for DDAC-membrane interactions that takes into account Laurdan spectral data, and turbidimetric data in conjunction with {dollar}\sp{lcub}31{rcub}{dollar}P NMR results was presented, which attempts to explain the different modes of action of DDAC on lipid supramolecular structures with respect to molar % of DDAC to lipids.