Ampicillin is a beta-lactam antibiotic which inhibits transpeptidation during bacterial cell-wall peptidoglycan synthesis. Colistin (polymyxin E) is a cyclopeptide antibiotic that disrupts plasma membrane structures of sensitive Gram-negative bacteria. The first step of colistin anti-bacterial action is binding to outer membrane lipopolysaccharides (endotoxins, LPSs). The aim of this investigation was to compare the diffusion process of ampicillin and colistin in the absence and presence of Proteus vulgaris 025 native and O-deacylated forms of LPS by laser interferometry. Antibiotics and LPSs at concentrations of 4 mg/ml and 100 mu g/ml, respectively, were used. The interferometric system consisted of a Mach-Zehnder interferometer with a He-Ne laser, TV-CCD camera, computerised data acquisition system, and a membrane system with two cuvettes separated by a horizontally placed cellulose membrane. Diffusion of the substance through a membrane separating solutions of different concentrations leads to the formation of concentration boundary layers (CBLs). Computer-assisted analysis of the antibiotic CBLs showed that the amount of ampicillin and colistin transported through the membrane was not influenced by the presence of 025 LPSs, in contrast to colistin diffusion, which depended on the O-deacylated form of 025 LPS. The diffusion coefficient of colistin was 1.7 times greater in the presence than in the absence of endotoxin. The ionic interaction between colistin and 025 LPSs may have facilitated the transport through the membrane. (c) 2007 Elsevier B.V. All rights reserved.