Interaction of Gram-Negative Bacteria with the Lysosomal Fraction of Polymorphonuclear Leukocytes II. Changes in the Cell Envelope of Escherichia coli

Comparative cytotoxicity of nanoparticles and ions to Escherichia coli in binary mixtures

The objective of this study was to understand toxicity of mixture of nanoparticles (NPs) (ZnO and TiO₂) and their ions to Escherichia coli. Results indicated the decrease in percentage growth of E. coli with the increase in concentration of NPs both in single and mixture setups. Even a small concentration of 1mg/L was observed to be significantly toxic to E. coli in binary mixture setup (exposure concentration: 1mg/L ZnO and 1mg/L TiO₂; 21.15% decrease in plate count concentration with respect to control). Exposure of E. coli to mixture of NPs at 1000mg/L (i.e., 1000mg/L ZnO and 1000mg/L TiO₂) resulted in 99.63% decrease in plate count concentration with respect to control. Toxic effects of ions to E. coli were found to be lesser than their corresponding NPs. The percentage growth reduction was found to be 36% for binary mixture of zinc and titanium ions at the highest concentration (i.e., 803.0mg/L Zn and 593.3mg/L Ti where ion concentrations are equal to the Zn ions present in 1000mg/L ZnO NP solution and Ti⁺⁴ ions present in 1000mg/L TiO₂ NP solution). Nature of mixture toxicity of the two NPs to E. coli was found to be antagonistic. The alkaline phosphatase (Alp) assay indicated that the maximum damage was observed when E. coli was exposed to 1000mg/L of mixture of NPs. This study tries to fill the knowledge gap on information of toxicity of mixture of NPs to bacteria which has not been reported earlier.

Role of cell wall structure of salmonella in the interaction with phagocytes

Wild-type and cell wall mutants of Salmonella were examined for sensitivity to ingestion and intracellular killing in vitro by mouse peritoneal macrophages and guinea pig polymorphonuclear leukocytes. A complete polysaccharide core of the cell wall is important for resisting ingestion and intracellular killing, and the presence of the O-specific side chains contributes further resistance. Uridine diphosphate-gal-4-epimeraseless mutants grown on galactose-supplemented medium, rendering them smooth phenotypes, showed resistance to ingestion and intracellular killing similar to the wild type.

Virulence of R-Factor-Bearing Salmonella typhimurium

Antibiotic-resistant recombinants obtained from mating antibiotic-resistant Escherichia coli with virulent, antibiotic-sensitive Salmonella typhimurium are generally avirulent. After 32 consecutive transfers, two of four avirulent recombinants regained their virulence without loss of episome-mediated resistance.

Antimicrobial peptides: pore formers or metabolic inhibitors in bacteria?

Antimicrobial peptides are an abundant and diverse group of molecules that are produced by many tissues and cell types in a variety of invertebrate, plant and animal species. Their amino acid composition, amphipathicity, cationic charge and size allow them to attach to and insert into membrane bilayers to form pores by 'barrel-stave', 'carpet' or 'toroidal-pore' mechanisms. Although these models are helpful for defining mechanisms of antimicrobial peptide activity, their relevance to how peptides damage and kill microorganisms still need to be clarified. Recently, there has been speculation that transmembrane pore formation is not the only mechanism of microbial killing. In fact several observations suggest that translocated peptides can alter cytoplasmic membrane septum formation, inhibit cell-wall synthesis, inhibit nucleic-acid synthesis, inhibit protein synthesis or inhibit enzymatic activity. In this review the different models of antimicrobial-peptide-induced pore formation and cell killing are presented.

Interaction of human defensins with Escherichia coli. Mechanism of bactericidal activity

Defensins are small, cysteine-rich antimicrobial peptides that are abundant in human, rabbit, and guinea pig neutrophils (PMN). Three defensins (human neutrophil peptide defensin [HNP]-1, HNP-2, and HNP-3) constitute between 30 and 50% of the total protein in azurophil granules of human PMN. We examined the mechanism of HNP-mediated bactericidal activity against Escherichia coli ML-35 (i-, y-, z+) and its pBR322-transformed derivative, E. coli ML-35p. Under conditions that supported bactericidal activity, HNP-1 sequentially permeabilized the outer membrane (OM) and inner membrane (IM) of E. coli. Coincident with these events, bacterial synthesis of DNA, RNA, and protein ceased and the colony count fell. Although these events were closely coupled under standard assay conditions, OM permeabilization was partially dissociated from IM permeabilization when experiments were performed with E. coli that had been plasmolyzed by mannitol. Under such conditions, the rate and extent of bacterial death more closely paralled loss of IM integrity than OM permeabilization. Electron microscopy of E. coli that had been killed by defensins revealed the presence of striking electron-dense deposits in the periplasmic space and affixed to the OM. Overall, these studies show that HNP-mediated bactericidal activity against E. coli ML-35 is associated with sequential permeabilization of the OM and IM, and that inner membrane permeabilization appears to be the lethal event.

Resistance of gram-negative bacteria to purified bactericidal leukocyte proteins: relation to binding and bacterial lipopolysaccharide structure

The sensitivity or resistance of gram-negative bacteria to antibacterial systems appears to be related to the length of the saccharide chain of the bacterial envelope lipopolysaccharides (LPS). To explore this relationship further, we made use of two bactericidal, membrane-active cationic proteins, recently purified to near homogeneity, one from human and one from rabbit polymorphonuclear leukocytes (PMN). We have studied the effects of these two closely similar proteins on strains of Salmonella typhimurium and Escherichia coli, each separate strain differing in the saccharide chain length of its outer membrane LPS. Binding of these proteins to the bacterial outer membrane is required for killing, and is accompanied by an almost immediate increase in outer membrane permeability to normally impermeant actinomycin D. Sensitivity to the bactericidal and permeability-increasing activities of the human and rabbit proteins increases with decreasing LPS-saccharide chain length (chemotype: [S < Ra < Rb(3) < Rc < Rd(1)]). S. typhimurium G-30 and E. coli J5, mutant strains lacking UDP-galactose-4-epimerase, synthesize incomplete LPS (chemotype Rc) when grown without galactose, and are then as sensitive to both PMN proteins as the S. typhimurium strains 395 R10 (Rd(1)) and R5 (Rb(3)). However, when these mutants are grown with galactose, they synthesize complete LPS (chemotype S) and exhibit nearly the same relative insensitivity as the smooth strains S. typhimurium 395 MS and E. coli 0111:B4. The differences among strains in sensitivity to the effects of the proteins on bacterial viability and permeability correspond to differences in bacterial binding of these PMN proteins. Thus, at protein concentrations that produce maximal antibacterial activity toward the rough bacteria, but little or no activity toward the smooth strains, rough bacteria bind from 3- to 10-fold more protein (S. typhimurium 395 R10; S. typhimurium G-30, and E. coli J5 [grown without galactose]) than do the smooth bacteria (S. typhimurium 395 MS; E. coli 0111:B4; S. typhimurium G-30 and E. coli J5 [grown with galactose]). These findings suggest that bacterial sensitivity or resistance to these purified bactericidal PMN proteins is determined by the binding properties of the outer membrane, which in turn depends upon the LPS-saccharide chain length.

Bactericidal activity of fractionated granule contents from human polymorphonuclear leukocytes

Proteins from human polymorphonuclear leukocyte granules were extracted with 0.2 M acetate, pH 4.0, and fractionated by Sephadex G-100 column chromatography. The fractions demonstrated selective bactericidal action against a deep rough cell wall mutant of Escherichia coli O111:B4 with rough lipopolysacharide and cell wall mutants of Salmonella typhimurium LT-2 with lipoplysacharide of Ra, Rc, Rd1, Rd2, and Re types. Smooth parent strains were most resistant to the bactericidal action. Fractions with greatest activity for the mutants were from valley regions (regions of low protein concentration) between three high protein peaks comprising myeloperoxidase, protease, and lysozyme, respectively. Susceptibility of the mutants to bactericidal action increased as sugar residues decreased in lipopolysaccharide. Gram-positive bacteria were susceptible to different fractions than were the gram-negative bacteria.

Bactericidal activity of specific and azurophil granules from human neutrophils: studies with outer-membrane mutants of Salmonella typhimurium LT-2

Extracts of specific granules and azurophil granules from human neutrophils were tested for their bactericidal activity against various lipopolysaccharide mutants of Salmonella typhimurium LT-2. Three purified granule populations, one specific and two azurophil, were obtained by isopycnic centrifugation of homogenized neutrophils. Each was extracted with 0.2 M acetate buffer (pH 4), and the extracts were dialyzed against phosphate-buffered saline (pH 7) to remove acetate. These extracts contained >/=84% of the lysozyme, lactoferrin, or myeloperoxidase initially present in the whole granules. The S. typhimurium mutants possessed Ra, Rc, Rd(1), Rd(2), or Re lipopolysaccharide. As the carbohydrate content of the lipopolysaccharide decreased, the bacteria became increasingly more susceptible to the bactericidal activity of all granule extracts. Bactericidal activity of the extracts was in the order: mixed (azurophil + specific) >/= azurophil >> specific. Specific granules were bacteriostatic for S through Rd(2) bacteria. They were bactericidal only for the Re mutant. Both azurophil granule populations were equally bactericidal. Extracts boiled for 30 min retained none of their bactericidal activity for any of the bacteria; however, they remained bacteriostatic for the deep rough (Rd(2), Re) mutants. Bactericidal activity was dependent upon pH, in that mixed and azurophil granule contents killed the smooth parent and Ra mutant best at pH 5, the Rc and Rd(1) mutants to the same degree at pH 5 to 8, and the deep rough mutants (Rd(2) and Re) best at pH 8. Specific granule contents were most bacteriostatic for S through Rd(2) bacteria at pH 5 and killed the Re mutant only at pH 8. Thus, as the S. typhimurium lipopolysaccharide content decreased, the bactericidal pH optimum increased. Killing by all extracts was dependent upon incubation temperature, with almost no bactericidal or bacteriostatic activity observed when bacteria and granule fractions were incubated on ice (2 degrees C) and plated immediately. Intermediate killing was observed at 22 degrees C. If bacteria were incubated with granule extracts at 2 degrees C, washed free of extract, suspended in medium without extract, and reincubated at 37 degrees C, killing was observed. This suggested that a component(s) of the extracts was sticking to the bacteria at 2 degrees C but killing only at 37 degrees C.

Susceptibility of lipopolysaccharide mutants to the bactericidal action of human neutrophil lysosomal fractions

Acetate extracts of purified human neutrophil granules (a mixed population containing specific and azurophil granules) were dialyzed against phosphate-buffered saline (pH 7.0) and tested for bactericidal activity against smooth parent and rough mutant, gram-negative bacteria. Rough (Re) mutants of Escherichia coli, Salmonella typhimurium, and Salmonella minnesota were exquisitely more sensitive to extracts of human polymorphonuclear leukocyte granules than were their smooth (S) parents. The mean lethal dose (LD(50)) for the parent strains was 25 to 50 mug of granule protein per ml. As much as 500 mug of extract per ml failed to kill 100% of the S parents. The LD(50) for the rough mutants was 1.5 to 2.0 mug of the same granule extract per ml; 100% killing occurred with 5 to 10 mug of lysosomal protein per ml. Conditions affecting the growth of the bacteria greatly affected their sensitivity to the granule extracts. Granule extract killed bacteria grown with aeration to log phase 10 to 15 times more efficiently than the same bacteria grown to stationary phase under static conditions. The bactericidal incubation mixture also influenced results, in that greater killing occurred with tryptone than with phosphate or N-2-hydroxyethyl piperazine-N'-2-ethanesulfonic acid-buffered saline. Bactericidal activity depended on lysosomal protein concentration, time, and temperature. Boiled lysosomal fractions failed to kill the S parents but retained 20 to 50% of their ability to kill the Re mutants. Parents (smooth) were killed more efficiently at pH 5 to 6, whereas their Re mutants were killed more efficiently at pH 7 to 8.