Polycations sensitize enteric bacteria to antibiotics

Binding of polymyxin B nonapeptide to gram-negative bacteria

The binding of the outer membrane-disorganizing peptide polymyxin B nonapeptide (PMBN) to gram-negative bacteria was studied by using tritium-labeled PMBN. Smooth Salmonella typhimurium had a binding capacity of ca. 6 nmol of PMBN per mg (dry weight) of bacteria, which corresponds to ca. 1 X 10(6) to 2 X 10(6) molecules of PMBN per single cell. The binding was of relatively high affinity (Kd, 1.3 microM). The isolated outer membrane of S. typhimurium bound ca. 100 nmol of PMBN per mg of outer membrane protein (Kd, 1.1 microM), whereas the cytoplasmic membrane bound 9 to 10 times less. Other bacteria which are susceptible to the action of PMBN (Escherichia coli strains, Pseudomonas aeruginosa, Haemophilus influenzae) also bound large amounts of PMBN. The S. typhimurium pmrA mutant, Neisseria gonorrhoeae, and Proteus mirabilis (all known as resistant to polymyxin and PMBN) bound 3.3, 4, and 12 times less than S. typhimurium, respectively. The binding of PMBN to S. typhimurium was effectively inhibited by low concentrations of polymyxin B, compound EM49 (octapeptin), polylysine, and protamine. Spermine, Ca2+, and Mg2+ also inhibited the PMBN binding although they were ca. 160, 700, and 2,400 times less active (based on molarity) than polymyxin B, respectively. No binding inhibition was found at the tested concentrations of streptomycin, tetralysine, spermidine, or cadaverine.

Manipulation of intracellular magnesium content in polymyxin B nonapeptide-sensitized Escherichia coli by ionophore A23187

Escherichia coli B cells were sensitized to ionophore A23187 by polymyxin B nonapeptide, and the induced magnesium and potassium ion fluxes were studied. Combined ionophore treatment permeabilized the cytoplasmic membrane of E. coli in an ion-specific manner and allowed the manipulation of intracellular Mg2+ content from the outside. A23187-induced Mg2+ efflux or influx was dependent on the free Mg2+ concentration gradient between the outside and inside of the cytoplasmic membrane and on the pH gradient. Most of the intracellular Mg2+ was bound, whereas only 1 to 2 mM was free in solution in the cellular sap.

In vitro and in vivo effect of immunoglobulin G on the integrity of bacterial membranes

The interaction between a modified 7S immunoglobulin (MISG) and bacterial membranes was studied by adopting in vitro as well as in vivo techniques. Preincubation of Escherichia coli and Pseudomonas aeruginosa with MISG resulted in a release of enzymatic markers from the periplasmic space, whereas no cytoplasmic or membrane-bound enzymes were liberated. Due to the interaction of MISG with the outer membrane of gram-negative rods, the bacteria became more susceptible to the antibacterial action of poorly penetrating penicillins because of a significantly increased rate of uptake. These in vitro effects were corroborated under in vivo conditions by adopting the granuloma pouch model. A single intravenous injection of MISG enhanced the therapeutic efficacy of mezlocillin against E. coli; similarly, the antibacterial activity of penicillin G, oxacillin, cephalothin and cefamandole against Staphylococcus aureus was augmented by MISG. These in vivo effects of MISG were not due to an increased rate of phagocytosis or complement activity. Thus, MISG sensitized bacteria to several beta-lactam antibiotics by disorganizing their outer membrane.

Dissipation of membrane potential of Escherichia coli cells induced by macromolecular polylysine

Macromolecular polylysine caused a rapid and drastic decrease in the membrane potential of Escherichia coli cells, monitored with the uptake of the lipophilic cation, tetraphenylphosphonium cation (TPP+). Respiration of cells was not affected to a significant extent by polylysine. Polylysine treatment induced the efflux of K+ from cells, suggesting that an increase in permeability of cytoplasmic membrane may be responsible for the decrease in the membrane potential. The action of polylysine is discussed in comparison with that of colicin E1 which is known to dissipate the membrane potential of E. coli cells.

Compounds which increase the permeability of the Pseudomonas aeruginosa outer membrane

Hydrolysis of the chromogenic beta-lactam nitrocefin by periplasmic beta-lactamase in intact Pseudomonas aeruginosa cells was used to assess the influence of various compounds on the permeability of the P. aeruginosa outer membrane. In addition to the five previously described outer membrane-active compounds EDTA, polymyxin B, gentamicin, poly-L-lysine, and Tris, seven other compounds were shown to increase outer membrane permeability to nitrocefin by 14- to 63-fold. These other compounds included poly-L-ornithine, neomycin, cetyltrimethylammonium bromide, nitrilotriacetate, L-ascorbate, and acetylsalicylate. In each case, Mg2+ ions antagonized, to different extents, the enhancement of outer membrane permeability. The same compounds increased the permeability of the outer membrane to the protein lysozyme and to the hydrophobic fluorescent probe 1-N-phenylnaphthylamine, although L-ascorbate and acetylsalicylate showed only very weak enhancement of uptake in these assays. In this report, we discuss the possibility that these compounds act at a common outer membrane site at which divalent cations noncovalently cross-bridge adjacent lipopolysaccharide molecules.

Susceptibility of gram-negative bacteria to polymyxin B nonapeptide

Subinhibitory concentrations of polymyxin B nonapeptide sensitized all 21 polymyxin-susceptible gram-negative bacterial strains studied to hydrophobic antibiotics such as fusidic acid, novobiocin, and erythromycin. The susceptibility increases were usually 30- to 300-fold. The strains included representatives of Escherichia coli with different O- and K-antigens, Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter cloacae, Enterobacter agglomerans, Salmonella typhimurium, Acinetobacter calcoaceticus, Pseudomonas aeruginosa, and Pseudomonas maltophilia. In contrast, polymyxin-resistant strains (Proteus mirabilis, Proteus vulgaris, Morganella morganii, Providencia stuartii, and Serratia marcescens) were resistant to the action of polymyxin B nonapeptide.

In vitro and in vivo effect of immunoglobulin G on the integrity of bacterial membranes

The interaction between a modified 7S immunoglobulin (MISG) and bacterial membranes was studied by adopting in vitro as well as in vivo techniques. Preincubation of Escherichia coli and Pseudomonas aeruginosa with MISG resulted in a release of enzymatic markers from the periplasmic space, whereas no cytoplasmic or membrane-bound enzymes were liberated. Due to the interaction of MISG with the outer membrane of gram-negative rods, the bacteria became more susceptible to the antibacterial action of poorly penetrating penicillins because of a significantly increased rate of uptake. These in vitro effects were corroborated under in vivo conditions by adopting the granuloma pouch model. A single intravenous injection of MISG enhanced the therapeutic efficacy of mezlocillin against E. coli; similarly, the antibacterial activity of penicillin G, oxacillin, cephalothin and cefamandole against Staphylococcus aureus was augmented by MISG. These in vivo effects of MISG were not due to an increased rate of phagocytosis or complement activity. Thus, MISG sensitized bacteria to several beta-lactam antibiotics by disorganizing their outer membrane.

Polycations as outer membrane-disorganizing agents

The outer membrane-disorganizing effect of a short (10-min) treatment with polycationic agents was studied with smooth Salmonella typhimurium used as a test organism. The polycationic agents were the protamine salmine, a lysine polymer with 20 lysine residues (lysine20), and the deacylated polymyxin B derivative polymyxin B nonapeptide. Two different types of outer membrane-disorganizing were found. Protamine and lysine20 released 20 to 30% of the lipopolysaccharide from the outer membrane and sensitized the bacteria to the anionic detergent sodium dodecyl sulfate but did not (under these conditions) make the bacteria permeable to the hydrophobic probes fusidic acid and actinomycin D. In contrast, polymyxin B nonapeptide did not release lipopolysaccharide or sensitize the bacteria to sodium dodecyl sulfate but made the outer membrane permeable to the hydrophobic probes. None of the agents was bactericidal under the conditions used or caused any leakage of periplasmic beta-lactamase. Polymyxin B was used as a reference and showed characteristic outer membrane-disorganizing action. In thin-section electron microscopy, polymyxin B nonapeptide caused the appearance of long, narrow, finger-like projections on the outer membrane. Protamine and lysine20 caused a distinctly wrinkled appearance of the outer membrane but no projections.