Antimicrobial mechanism of action of surfactant lipid preparations in enteric Gram-negative bacilli

Two surfactant lipid preparations (SLPs) were investigated to determine the ir mechanism of antimicrobial action. 8N8, a water-in-oil emulsion, and W60 C, a liposome, both have bactericidal activity against Gram-positive bacter ia and non-enteric Gram-negative bacteria. Additionally, W60C is bactericid al for enteric Gram-negative bacilli when suspended in deionized water. Zet a potential measurements suggested that the resistance of Gram-negative bac illi to 8N8 might be caused by ionic repulsion. Addition of 50 mu mol l(-1) ethylene diamine tetra acetic acid in 100 mmol l(-1) Tris buffer to either SLPs yielded efficient bactericidal activity against Gramnegative bacilli. This appeared to be due to disruption of the outer membrane and the chelat ion of divalent cations, as the addition of excess calcium inhibited the an timicrobial effect. Electron microscopy studies documented that 8N8 disrupt s the bacterial cell wall, lysing the bacteria, while W60C fuses and intern alizes within the cell, causing damage without immediate cell lysis. Unders tanding the mechanisms of action of these biocidal formulations will help t o produce improved formulations with broader spectra of activity.