The cell envelope of gram-negative bacteria: new aspects of its function in transport and chemotaxis

Effect of Ca+ on the photobactericidal efficacy of methylene blue and toluidine blue against gram-negative bacteria and the dye affinity for lipopolysaccharides

BACKGROUND AND OBJECTIVES

Methylene blue (MB) and toluidine blue (TB) form metachromatic complexes with lipopolysaccharides (LPS). The greater photobactericidal efficacy of TB may be explained by its affinity for LPS. This study aims to elucidate the difference in photobactericidal efficacies between the dyes using Ca(2+) as a competitor for dye-binding sites on the bacterial outer membrane.

STUDY DESIGN/MATERIALS AND METHODS

Fixed dye concentration solutions with gram-negative bacteria and increasing concentrations of CaCl(2) were exposed to red laser light. Bacterial survival and spectrophotometry were used to describe the effect of Ca(2+) on dye interaction with bacteria and LPS.

RESULTS

MB-mediated photokilling was inhibited more significantly than that of TB. CaCl(2) inhibited dye photobleaching and suppressed the metachromatic reaction between the dyes and LPS, in particular TB.

CONCLUSIONS

CaCl(2) inhibits bacterial photokilling by binding with LPS, as well as other anionic polymers including outer membrane proteins. LPS is chiefly involved in TB-mediated photokilling, whereas outer membrane proteins probably are more involved in MB-mediated photokilling.

Porins and lipopolysaccharide induce apoptosis in human spermatozoa

Treatment of human spermatozoa with porins or lipopolysaccharide (LPS) increases spontaneous apoptosis in these cells. Porins and LPS were extracted from Salmonella enterica serovar Typhimurium and Pasteurella multocida and were mixed with human spermatozoa for detection of levels of apoptosis.

Haemophilus influenzae porin contributes to signaling of the inflammatory cascade in rat brain

In the present study we observed that the Haemophilus influenzae type b (Hib) porin, among the different surface bacterial components, is involved in the pathophysiology of bacterial meningitis. This study demonstrates that inoculation of Hib porin into the fourth cerebral ventricle causes the simultaneous expression of interleukin-1alpha (IL-1alpha), tumor necrosis factor alpha (TNF-alpha), and macrophage inflammatory protein 2 (MIP-2) at 6 h after inoculation. At 24 h, the expression of MIP-2 decreases while the expression of IL-1alpha and TNF-alpha increases. The mRNA expression of IL-1alpha, TNF-alpha, and MIP-2 is correlated with injury to the blood-brain barrier as demonstrated by the appearance of serum proteins and leukocytes in cerebrospinal fluid and by the increase in brain water content.

The wzz (cld) protein in Escherichia coli: amino acid sequence variation determines O-antigen chain length specificity

The O antigen is a polymer with a repeated unit. The chain length in most Escherichia coli strains has a modal value of 10 to 18 O units, but other strains have higher or lower modal values. wzz (cld/rol) mutants have a random chain length distribution, showing that the modal distribution is determined by the Wzz protein. Cloned wzz genes from E. coli strains with short (7 to 16), intermediate (10 to 18), and long (16 to 25) modal chain lengths were transferred to a model system, and their effects on O111 antigen were studied. The O111 chain length closely resembled that of the parent strains. We present data based on the construction of chimeric wzz genes and site-directed mutagenesis of the wzz gene to show that the modal value of O-antigen chain length of E. coli O1, O2, O7, and O157 strains can be changed by specific amino acid substitutions in wzz. It is concluded that the O-antigen chain length heterogeneity in E. coli strains is the result of amino acid sequence variation of the Wzz protein.

Chemical structure of the lipid A component of lipopolysaccharides of the genus Pectinatus

The chemical structure of the lipid A components of smooth-type lipopolysaccharides isolated from the type strains of strictly anaerobic beer-spoilage bacteria Pectinatus cerevisiiphilus and Pectinatus frisingensis were analyzed. The hydrophilic backbone of lipid A was shown, by controlled degradation of lipopolysaccharide combined with chemical assays and 31P-NMR spectroscopy, to consist of the common beta 1-6-linked disaccharide of pyranosidic 2-deoxy-glucosamine (GlcN), phosphorylated at the glycosidic position and at position 4'. In de-O-acylated lipopolysaccharide, the latter phosphate was shown to be quantitatively substituted with 4-amino-4-deoxyarabinose, whereas the glycosidically linked phosphate was present as a monoester. Laser-desorption mass spectrometry of free dephosphorylated lipid A revealed that the distal (non-reducing) GlcN was substituted at positions 2' and 3' with (R)-3-(undecanoyloxy)tridecanoic acid, whereas the reducing GlcN carried two unsubstituted (R)-3-hydroxytetradecanoic acids at positions 2 and 3. The lipid A of both Pectinatus species were thus of the asymmetric hexaacyl type. The linkage of lipid A to polysaccharide in the lipopolysaccharide was relatively resistant to acid-catalyzed hydrolysis, enabling the preparation of a dephosphorylated and deacylated saccharide backbone. Methylation analysis of the backbone revealed that position 6' of the distal GlcN of lipid A was the attachment site of the polysaccharide. Despite the quantitative substitution of the lipid A 4'-phosphate by 4-amino-4-deoxyarabinose, which theoretically should render the bacteria resistant to polymyxin, P. cerevisiiphilus was shown to be susceptible to this antibiotic. P. cerevisiiphilus was, however, also susceptibile to vancomycin and bacitracin, indicating that the outer membrane of this bacterium does not act as an effective permeability barrier.

Agents that increase the permeability of the outer membrane

The outer membrane of gram-negative bacteria provides the cell with an effective permeability barrier against external noxious agents, including antibiotics, but is itself a target for antibacterial agents such as polycations and chelators. Both groups of agents weaken the molecular interactions of the lipopolysaccharide constituent of the outer membrane. Various polycations are able, at least under certain conditions, to bind to the anionic sites of lipopolysaccharide. Many of these disorganize and cross the outer membrane and render it permeable to drugs which permeate the intact membrane very poorly. These polycations include polymyxins and their derivatives, protamine, polymers of basic amino acids, compound 48/80, insect cecropins, reptilian magainins, various cationic leukocyte peptides (defensins, bactenecins, bactericidal/permeability-increasing protein, and others), aminoglycosides, and many more. However, the cationic character is not the sole determinant required for the permeabilizing activity, and therefore some of the agents are much more effective permeabilizers than others. They are useful tools in studies in which the poor permeability of the outer membrane poses problems. Some of them undoubtedly have a role as natural antibiotic substances, and they or their derivatives might have some potential as pharmaceutical agents in antibacterial therapy as well. Also, chelators (such as EDTA, nitrilotriacetic acid, and sodium hexametaphosphate), which disintegrate the outer membrane by removing Mg2+ and Ca2+, are effective and valuable permeabilizers.

Resistance of Escherichia coli to osmotically introduced complement component C9

Investigation into the action of osmotically introduced C9 in Escherichia coli (in the absence of any other complement components) revealed that C9 could inhibit inner membrane respiration and cause a decrease in the viability of cells that were normally complement sensitive. This effect is analogous to the loss of inner membrane function and viability due to the assembly of the C5b-9 complex on these cells. Complement-resistant cells showed no such inhibition of respiration or loss of viability when subjected to the osmotic introduction of C9. The reason for this failure of C9 to affect complement-resistant cells was explored to determine whether this resistance to C9 was due to an inability of proteins in general to be osmotically introduced into the complement-resistant cells. The protein toxins melittin and colicin E1 were showed to be able to kill these complement-resistant cells (as well as complement-sensitive cells) when osmotically introduced into the periplasm. Therefore, cellular resistance to osmotically introduced C9 is not due to an inability of proteins to be introduced into the cells and may be related to a mechanism of cellular resistance to the C5b-9 complex.

Identification of bacterial periplasmic glycine betaine-binding protein after electrophoresis and affinity labeling

Antibodies were elicited in rabbits against periplasmic proteins obtained by cold osmotic shock from the Gram-negative eubacterium Rhizobium meliloti. When analyzed by crossed immunoelectrophoresis (CIE), the periplasmic proteins gave rise to 20 distinct immunoprecipitates corresponding to the same number of bands in polyacrylamide gel electrophoresis (PAGE) under non-denaturing conditions and in SDS-PAGE. The periplasmic glycine betaine-binding protein (GB-BP) was identified by autoradiography after affinity labeling with [14C]glycine betaine in PAGE and in CIE gels. The binding proved to be quite specific to glycine betaine, since the GB-BP was not labeled by choline (a metabolic precursor of glycine betaine in Escherichia coli and Rhizobium meliloti) and 15 distinct L-amino acids, including L-proline which, like glycine betaine is also an osmoprotectant. Affinity labeling of the GB-BP with [14C]glycine betaine after protein separation by PAGE or CIE is a simple and sensitive technique permitting the GB-BP to the unambiguously detected and identified in samples of complex protein mixtures containing down to 2 micrograms of GB-BP in PAGE and only 0.2 micrograms in CIE.

Compartmentalization of the periplasm at cell division sites in Escherichia coli as shown by fluorescence photobleaching experiments

Morphological evidence has previously indicated that the periplasmic space of Escherichia coli is compartmentalized at sites corresponding to future sites of cell division. The borders of these morphological compartments are formed by localized zones of adhesion (periseptal annuli). In the present study, the technique of fluorescence recovery after photobleaching was used to determine whether these structures act as barriers to the free movement of proteins within the periplasm. The recovery of fluorescence in the ftsA filaments was found to be uniformly low over at potential sites of cell division and at the cell poles, indicating that these regions are biochemically sequestered from the remainder of the periplasmic space. Our results provide direct evidence for local compartments within the periplasm, primarily located at the sites of past or future cell divisions. The implications of this finding for cell division and other periplasmic processes are discussed.

A bacterial c-type cytochrome can be translocated to the periplasm as an apo form; the biosynthesis of cytochrome cd1 (nitrite reductase) from Paracoccus denitrificans

An apo form of cytochrome cd1 (nitrite reductase) of Paracoccus denitrificans has been detected immunologically in the periplasm of a mutant that lacks all c-type cytochromes. A method for the preparation of apo-nitrite reductase (lacking both c- and d-type haem) from the holoenzyme of wild-type cells has been developed. The apoprotein synthesized by the mutant is indistinguishable from the chemically prepared apoprotein in respect of: (i) subunit molecular weight; (ii) formation of a homodimer; (iii) properties on anion exchange chromatography. The holoenzyme has similar properties in respect of (i) and (ii) but behaves differently during anion exchange. A suggested mode of assembly of cytochrome cd1 is translocation into the periplasm of a precursor polypeptide, maturation by a signal peptidase to give an apoprotein identical to that prepared chemically from the holoenzyme, followed by insertion of c-type and d-type haem in an as yet unknown order.

Characterization of the nucleoside-binding site inside the Tsx channel of Escherichia coli outer membrane. Reconstitution experiments with lipid bilayer membranes

Reconstitution of purified Tsx protein from Escherichia coli into lipid bilayer membranes showed that Tsx formed small ion-permeable channels with a single-channel conductance of 10 pS in 1 M KCl. The dependence of conductance versus salt concentration was linear, suggesting that Tsx has no binding site for ions. Conductance was inhibited by the addition of 20 mM adenosine. Titration of the Tsx-mediated membrane conductance with different solutes including free bases, nucleosides, and deoxynucleosides suggested that the channel contains a binding site for nucleosides but not for sugars or amino acids, and binding increased in the following order: free base, nucleoside, and deoxynucleoside. Among the five nucleosides the stability constant for the binding increased in the order of cytidine, guanosine, uridine, adenosine, and thymidine. Control experiments revealed that the binding of the nucleosides is independent of ion concentration in the aqueous phase, i.e. there was no competition between nucleosides and ions for the binding site inside the channel. The binding of the solutes to the channel interior can be explained by a one-site two-barrier model for the Tsx channel. The advantage of a binding site inside a specific porin for the permeation of solutes is discussed with respect to the properties of a general diffusion pore.

Identification of endogenous inducers of the mal regulon in Escherichia coli

The expression of the maltose regulon in Escherichia coli is induced when maltose or maltodextrins are present in the growth medium. Mutations in malK, which codes for a component of the transport system, result in the elevated expression of the remaining mal genes. Uninduced expression in the wild type, as well as elevated expression in malK mutants, is strongly repressed at high osmolarity. In the absence of malQ-encoded amylomaltase, expression remains high at high osmolarity. We found that uninduced expression in the wild type and elevated expression in malK mutants were paralleled by the appearance of two types of endogenous carbohydrates. One, produced primarily at high osmolarity, was identified as comprising maltodextrins that are derived from glycogen or glycogen-synthesizing enzymes. The other, produced primarily at low osmolarity, consisted of an oligosaccharide that was not derived from glycogen. We isolated a mutant that no longer synthesized this oligosaccharide. The gene carrying this mutation, termed malI, was mapped at min 36 on the E. coli linkage map. A Tn10 insertion in malI also resulted in the loss of constitutivity at low osmolarity and delayed the induction of the maltose regulon by exogenous inducers.