Outer membrane permeability of Acinetobacter calcoaceticus and its implication in antibiotic resistance

In order to understand high and broad antibiotic resistance of Acinetobacter calcoaceticus, the outer membrane permeability was studied. The permeability coefficients of zwitterionic cephalosporins in the intact cell outer membrane were 0.14-1.12 x 10(-4) cm3/min/mg protein. These values were two to seven times lower than the permeability coefficients of the same beta-lactams in the outer membrane of Pseudomonas aeruginosa. The diffusion rates of carbapenems and zwitterionic cephalosporins into liposomes containing purified outer membrane appeared to be about 1-3%, that of the Escherichia coli B outer membrane. These results indicate that the outer membrane of A. calcoaceticus acts as a substantial barrier against the penetration of these antibiotics. About 80 times of the A. calcoaceticus outer membrane was needed in the liposome compared with E. coli B to show the same extent of saccharide permeability. Two minor outer membrane proteins (less than 5% of total outer membrane protein) were identified to be the porins. These results suggest that one of the causes for the high antibiotic resistance of A. calcoaceticus is attributable to the presence of a small number of small-sized porins.