Fusion of small unilamellar vesicles with viable EDTA-treated Escherichia coli cells

Penetration of enveloped double-stranded RNA bacteriophages phi13 and phi6 into Pseudomonas syringae cells

Bacteriophages phi6 and phi13 are related enveloped double-stranded RNA viruses that infect gram-negative Pseudomonas syringae cells. phi6 uses a pilus as a receptor, and phi13 attaches to the host lipopolysaccharide. We compared the entry-related events of these two viruses, including receptor binding, envelope fusion, peptidoglycan penetration, and passage through the plasma membrane. The infection-related events are dependent on the multiplicity of infection in the case of phi13 but not with phi6. A temporal increase of host outer membrane permeability to lipophilic ions was observed from 1.5 to 4 min postinfection in both virus infections. This enhanced permeability period coincided with the fast dilution of octadecyl rhodamine B-labeled virus-associated lipid molecules. This result is in agreement with membrane fusion, and the presence of temporal virus-derived membrane patches on the outer membrane. Similar to phi6, phi13 contains a thermosensitive lytic enzyme involved in peptidoglycan penetration. The phage entry also caused a limited depolarization of the plasma membrane. Inhibition of host respiration considerably decreased the efficiency of irreversible virus binding and membrane fusion. An active role of cell energy metabolism in restoring the infection-induced defects in the cell envelope was also observed.

The influence of poly-(L-lysine) and porin on the domain structure of mixed vesicles composed of lipopolysaccharide and phospholipid: an infrared spectroscopic study

Fourier transform infrared (FTIR) spectroscopy has been used to study the thermotropic phase behavior of binary lipid mixtures composed of deuterated phospholipids (PLs) and lipopolysaccharides (LPSs). Furthermore, the influence of an extrinsic high-molecular, polycationic polypeptide (poly-(L-lysine), PLL(500)) and an intrinsic membrane protein (outer membrane protein F, OmpF) on these binary mixtures was investigated by FTIR spectroscopy. "Deep rough" mutant LPS (ReLPS), isolated from Salmonella minnesota R595, and perdeuterated 1,2-dimyristoylphosphatidylethanolamine (DMPEd54) were used as model lipids. Deuteration of one of the lipids permitted the detection of lipid protein interaction with each lipid component separately. For this purpose, the symmetric >CH2 and >CD2 stretching bands were utilized as specific monitors to scrutinize the state of order of the membranes. From the individual phase transition temperatures Tm and the shape of the phase transition profiles, it is established that ReLPS and DMPEd54 are molecularly immiscible. In addition to the two domains of the pure lipid components, a third, domain-like structure is detected that may coexist with these pure domains. This domain-like structure undergoes a gel to liquid-crystalline L1 (beta <--> alpha) phase transition at temperatures distinctly different from that of the respective pure lipid domains. The nature of this type of domain is discussed in terms of a "border region" model that adequately explains the experimentally observed complex phase transition profiles. It is further demonstrated that the extrinsic polycationic polypeptide PLL(500) and the intrinsic, pore-forming protein OmpF isolated from Escherichia coli interact preferentially and highly specifically with the negatively charged ReLPS. Both the synthetic polypeptide and the pore-forming protein increased the tendency of ReLPS and DMPEd54 to segregate into distinct, well-separated domains. Whereas the transition profiles of the ternary system ReLPS/DMPEd54/PLL(500) showed the features of a phase segregation phenomenon not affecting the transition temperatures of the pure lipid components, the ternary system composed of ReLPS/DMPEd54 and OmpF exhibited phase transition curves that were characterized by an unspecific (DMPEd54/OmpF) and a strong and unique (ReLPS/OmpF) type of lipid-protein interaction. Furthermore, semiquantitative estimations supported the supposition that OmpF might be able to induce bilayer asymmetry in preformed symmetrical ReLPS/DMPEd54 vesicles.

C9-mediated killing of bacterial cells by transferred C5b-8 complexes: transferred C5b-9 complexes are nonbactericidal

The formation of the C5b-9 complex on the outer membrane of complement-sensitive cells of Escherichia coli results in inhibition of inner membrane function and the death of the cell. Cells bearing a precursor of the C5b-9 site, the C5b-8 complex, suffer no loss in viability. Antibiotic-sensitive, complement-sensitive donor cells bearing precursor C5b-8 complexes were incubated with equal numbers of antibiotic-resistant, complement-sensitive acceptor cells that had not been exposed to a complement source. This cell mixture was incubated with 5 mM EDTA for 5 min and then with calcium chloride (20 mM) for various times. The excess calcium ion concentration was effectively reduced with additional EDTA, and the cell mixture was washed and resuspended in buffer. The viability of the acceptor cells was assayed by plating on antibiotic-containing media. C9 was added to the mixture, and the mixture was incubated for 10 min at 37 degrees C and then plated as described above. It was found that the acceptor cells were killed by the addition of purified C9 only after incubation with donor cells bearing C5b-8 sites during the transfer procedure. This indicates that precursor C5b-8 sites that support C9-mediated killing could be transferred between cells. No loss in viability was detected for acceptor cells subjected to the procedure described above in the presence of donor cells bearing complete C5b-9 complexes, formed prior to mixing with acceptor cells for the transfer procedure.

In vivo study of the state of order of the membranes of gram-negative bacteria by Fourier-transform infrared spectroscopy (FT-IR)

Temperature-induced order/disorder transition profiles were obtained from the membranes of intact Gram-negative bacterial cells by FT-IR analysis of the frequency shifts of the acyl chain methylene symmetric stretching band as a monitor. Cells grown at different temperatures yielded distinct transition profiles. At the individual growth temperatures, however, the nearly alike frequency values indicated a very similar 'state of order' of the bacterial membranes. The FT-IR data were complemented by GC analysis of whole cell fatty acid composition. The FT-IR data obtained in vivo gave direct evidence of the adaptation of the 'state of order' and 'fluidity' of bacterial membranes to varying growth temperatures.

Small unilamellar vesicles are able to fuse with Mycoplasma capricolum cells

We have investigated the fusion characteristics of intact Mycoplasma capricolum cells and small unilamellar vesicles (SUV). The rate and extent of fusion was monitored continuously by octadecylrhodamine B (R18) fluorescence dequenching assay, as well as by intracellular contents mixing, and by sucrose density gradient analysis. The fusion of SUV with M. capricolum cells was found to be dependent on poly(ethylene glycol) (PEG 8000), divalent cations in the medium, and on the cholesterol content of the lipid vesicles. Maximal levels of fusion were obtained with SUV containing 40 mol% cholesterol in the presence of 5% PEG. The rate and extent of fusion were affected by temperature, pH, osmotic pressure, and SUV/mycoplasma ratio. Under optimal fusion conditions, PEG did not increase the rate of exchange of either cholesterol or phospholipids between M. capricolum cells and SUV. Throughout the fusion process, M. capricolum cells remained intact as measured by the retention of [3H]thymidine-labeled components, and viable. M. capricolum cells were rendered nonfusogenic by treatment with glutaraldehyde (greater than 0.01%) or chlorpromazine (greater than 10 microM). Fusion was partially inhibited by treating the cells with the uncoupler CCCP (5 microM) or proteolytic enzymes, suggesting that a proton gradient across the cell membrane is required for the fusion, and that the cells possess proteinase-sensitive receptors that are responsible for a tighter contact with the lipid vesicles.