Interactions between cationic liposomes and bacteria: the physical-chemistry of the bactericidal action

The bactericidal effect of dioctadecyldimethylammonium bromide (DODAB), a l iposome forming synthetic amphiphile, is further evaluated for Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa, and Staphylococcus au reus in order to establish susceptibilities of different bacteria species t owards DODAB at a fixed viable bacteria concentration (2.5 x 10(7) viable b acteria/mL), For the four species, susceptibility towards DODAB increases f rom E. coli to S. aureus in the order above. Typically, cell viability decr eases to 5% over 1 h of interaction time at DODAB concentrations equal to 5 0 and 5 mu M for E. coli and 5, aureus, respectively. At charge neutralizat ion of the bacterial cell, bacteria flocculation by DODAB vesicles is shown to be a diffusion-controlled process. Bacteria flocculation does not yield underestimated counts of colony forming units possibly because dilution pr ocedures done before plating cause deflocculation, The effect of vesicle si ze on cell viability demonstrates that large vesicles, due to their higher affinity constant for the bacteria (45.20 M-1) relative to the small vesicl es (0.14 M-1), kill E. coil at smaller DODAB concentrations. For E. coil an d S. aureus, simultaneous determination of cell viability and electrophoret ic mobility as a function of DODAB concentration yields a very good correla tion between cell surface charge and cell viability. Negatively charged cel ls are 100% viable whereas positively charged cells do not survive. The res ults show a clear correlation between simple adsorption of entire vesicles generating a positive charge on the cell surfaces and cell death.