Outer membrane of gram negative bacteria. XII. Molecular sieving function of cell wall

Abstract

The permeability function of the cell wall of gram negative bacteria such as Salmonella typhimurium was investigated by producing cells with an expanded periplasmic volume, and incubating them with radioactive nonutilizable oligo and polysaccharides or polyethylene glycols. To quantitate the extent of penetration of these hydrophilic compounds into the periplasm, the radioactivity of the cell pellet was determined after centrifugation. It was found that only di and trisaccharides could fully diffuse into the periplasm, whereas higher molecular weight saccharides were nonpenetrable. In addition, low molecular weight polyethylene glycols rapidly diffused across the cell wall. Kinetics experiments also showed that both sucrose and raffinose in the periplasm exchanged rapidly with sugars in the medium, even at 0°C. These results suggest that the cell wall acts as a molecular sieve, with an exclusion limit near 550 to 650 daltons for saccharides. It is suggested that the diffusion of these hydrophilic compounds most likely occurs through water filled pores present in the cell wall of gram negative bacteria.