The PhoPQ two-component system of Salmonella enterica serovar Typhimurium produces a remodeling of the lipid A domain of the lipopolysaccharide, including the PagP-catalyzed addition of palmitoyl residue, the PmrAB-regulated addition of the cationic sugar 4-aminoarabinose and phosphoethanolamine, and the LpxOcatalyzed addition of a 2-OH group onto one of the fatty acids. By using the diffusion rates of the dyes ethidium, Nile red, and eosin Y across the outer membrane, as well as the susceptibility of cells to large, lipophilic agents, we evaluated the function of this membrane as a permeability barrier. We found that the remodeling process in PhoP-constitutive strains produces an outer membrane that serves as a very effective permeability barrier in an environment that is poor in divalent cations or that contains cationic peptides, whereas its absence in phoP null mutants produces an outer membrane severely compromised in its barrier function under these conditions. Removing combinations of the lipid A-remodeling functions from a PhoP-constitutive strain showed that the known modification reactions explain a major part of the PhoPQ-regulated changes in permeability. We believe that the increased barrier property of the remodeled bilayer is important in making the pathogen more resistant to the stresses that it encounters in the host, including attack by the cationic antimicrobial peptides. On the other hand, drug-induced killing assays suggest that the outer membrane containing unmodified lipid A may serve as a better barrier in the presence of high concentrations (e.g., 5 mM) of Mg 2؉ .Cells of gram-negative bacteria are surrounded by the outer membrane (OM), which functions primarily as a permeability barrier (35). Large, hydrophilic compounds are excluded by the narrow porin channels, and lipophilic compounds cross the unusual, asymmetric bilayer of this membrane only slowly. Our early analysis by using steroids as probes (37) showed that the diffusion across the OM bilayer is nearly 2 orders of magnitude slower than the diffusion across a typical phospholipid bilayer, such as the phospholipid bilayer that exists in the inner, cytoplasmic membrane. A recent study (11) suggested that this estimate likely needs to be adjusted somewhat downward. Nevertheless, the OM bilayer clearly functions as a formidable permeation barrier, because perturbing this bilayer results in a striking sensitization of Escherichia coli and Salmonella enterica serovar Typhimurium to various lipophilic inhibitors (44).One major factor that contributes to this barrier property is presumably the asymmetric structure of the OM bilayer, whose outer leaflet is composed nearly entirely of lipopolysaccharides (LPS) (25). (In fact, very low permeability was found in symmetric LPS bilayers assembled in the laboratory [40].) According to the lattice model for diffusion in liquids, a statistical average of rapidly fluctuating variable distances creates transient holes within the bilayer into which solutes can migrate (41). Thus, an effective memb...

A product in the culture supernatant fluid of Clostridium perfringens NCTC 8239 stimulated the sporulation of a test strain, NCTC 8679, of the same organism. The responsible factor, termed sporulation factor (SF), was present in seven cultures of Cl. perfringens grown in either a defined or complex medium. The SF reversed glucose-mediated catabolite repression of sporulation by this organism. Preliminary characterization of the SF demonstrated a resistance to elevated temperatures and proteases and a molecular weight of less than 500 Da. The known association of Cl. perfringens enterotoxin with sporulation highlights the importance of interactions between strains of this organism as may occur in the human intestine during foodborne illness.