Enhanced Sensitivity of Salmonella to Antimicrobial Blue Light Caused by Inactivating rfaC Gene Involved in Lipopolysaccharide Biosynthesis.

Salmonella is a global foodborne pathogen that causes human diseases ranging from mild gastroenteritis to severe systemic infections. Recently, antimicrobial blue light (aBL) showed effective bactericidal activity against a variety of bacteria (e.g., Salmonella) with varying efficiency. However, the antimicrobial mechanism of aBL has not been fully elucidated. Our previous report showed that the outer membrane (OM) is a key target of aBL. The major component of the OM, lipopolysaccharide (LPS), may play a role in aBL bactericidal effect. Therefore, the influence of LPS truncation on the sensitivity of Salmonella Typhimurium SL1344 to aBL was investigated for the first time. First, the rfaC gene in the SL1344 strain likely involved in linking lipid A to the core region of LPS was inactivated and the influence on LPS structure was verified in the mutant strain SL1344ΔrfaC. SL1344ΔrfaC showed a significant increase in sensitivity to aBL, and the bactericidal efficiency exceeded 8 log CFU at an aBL dose of 383 J/cm2, while that of its parental SL1344 strain approached 4 log CFU. To discover the possible mechanism of higher sensitivity, the permeability of OM was determined. Compared to SL1344, SL1344ΔrfaC showed 2.7-fold higher permeability of the OM at 20 J/cm2, this may explain the higher vulnerability of the OM to aBL. Furthermore, the fatty acid profile was analyzed to reveal the detailed changes in the OM and inner membrane of the mutant. Results showed that the membrane lipids of SL1344ΔrfaC were markedly different to SL1344, indicating that change in fatty acid profile might mediate the enhancement of OM permeability and the increased sensitivity to aBL in SL1344ΔrfaC. Hence, we concluded that disruption of rfaC in Salmonella Typhimurium led to the formation of truncated LPS and thus enhanced the permeability of the OM, which contributed to the increased sensitivity to aBL.

Increased production of outer membrane vesicles by cultured freshwater bacteria in response to ultraviolet radiation.

Secretion of membrane vesicles is an important biological process of both eukaryotic and prokaryotic cells. This process has been characterized in pathogenic bacteria, but is less clear in non-pathogenic bacteria from aquatic ecosystems. Here, we investigated, for the first time, the process of formation of outer membranes vesicles (OMVs), nanoscale vesicles extruded from the outer membrane (OM) of gram-negative bacteria, in cultures of freshwater bacteria after exposure or not to ultraviolet radiation (UVR) as an environmental stressor. Non-axenic cultures of freshwater bacteria isolated from a Brazilian aquatic ecosystem (Funil reservoir) were exposed or not to UVR (UVA+UVB) over a 3h period, during which cell density, viability and ultrastructure were analyzed. First, we showed that UVR induce bacterial death. UVR triggered significant negative effect on cell density after 3h of UVR treatment. This decrease was directly associated with cell death as revealed by a cell viability fluorescent probe that enables the distinction of live/dead bacteria. Transmission electron microscopy (TEM) revealed changes indicative of cell death after 3h of UVR exposure, with significant increase of damaged cells compared to the control group. Second, we demonstrated that gram-negative bacteria release OMVs during normal growth and after UVR exposure. OMVs were clearly identified as round, membrane-bound vesicles budding off from the bacterial OM as isolated or clustered vesicles or free in the extracellular medium. Remarkably, quantitative TEM analyses showed that bacteria respond to UVR with increased formation of OMVs. Moreover, while OMVs numbers per intact or damaged cell did not differ in the untreated group, UVR led to a higher vesiculation by bacteria in process of death. This means that degenerating bacteria release OMVs before lysis and that this secretion might be an adaptive/protective response to rapid changes in environmental conditions such as UV radiation.

Reconstitution and alignment by a magnetic field of a beta-barrel membrane protein in bicelles.

A protocol is described for the reconstitution of a transmembrane beta-barrel protein domain, tOmpA, into lipid bicelles. tOmpA is the largest protein to be reconstituted in bicelles to date. Its insertion does not prevent bicelles from orienting with their plane either parallel or perpendicular to the magnetic field, depending on the absence or presence of paramagnetic ions. In the latter case, tOmpA is shown to align with the axis of the beta-barrel parallel to the magnetic field, i.e. perpendicular to the plane of the bilayer, an orientation conforming to that in natural membranes and favourable to structural studies by solid-state NMR. Reconstitution into bicelles may offer an interesting approach for structural studies of membrane proteins in a medium resembling a biological membrane, using either NMR or other biophysical techniques. Our data suggest that alignment in the magnetic field of membrane proteins included into bicelles may be facilitated if the protein is folded as a beta-barrel structure.

Electron holography of non-stained bacterial surface layer proteins.

We report transmission electron microscopy (TEM) investigations on bacterial surface layers (S-layers) which belong to the simplest biomembranes existing in nature. S-layers are regular 2D protein crystals composed of single protein or glycoprotein species. In their native form, S-layers are weak phase objects giving only poor contrast in conventional TEM. Therefore, they are usually examined negatively stained. However, staining with heavy metal compounds may cause the formation of structural artefacts. In this work, electron microscopy studies of non-stained S-layers of Bacillus sphaericus NCTC 9602 were performed. Compared to other proteins, these S-layers are found relatively stable against radiation damage. Electron holography was applied where information about phase and amplitude of the diffracted electron wave is simultaneously obtained. In spite of small phase shifts observed, the phase image reconstructed from the hologram of the non-stained S-layer is found to be sensitive to rather slight structure and thickness variations. The lateral resolution, obtained so far, is less than that of conventional electron microscopy of negatively stained S-layers. It corresponds to the main lattice planes of 12.4 nm observed in the reconstructed electron phase image. In addition, as a unique feature of electron holography the phase image provides thickness information. Thus, the existence of double layers of the protein crystals could be easily visualized by the height profile of the specimen.

Ligand binding and protein dynamics in cupredoxins.

Type 1 copper sites bind nitric oxide (NO) in a photolabile complex. We have studied the NO binding properties of the type 1 copper sites in two cupredoxins, azurin and halocyanin, by measuring the temperature dependence of the ligand binding equilibria and the kinetics of the association reaction after photodissociation over a wide range of temperature (80-280 K) and time (10(-6)-10(2) s). In both proteins, we find nonexponential kinetics below 200 K that do not depend on the NO concentration. Consequently, this process is interpreted as geminate recombination. In azurin, the rebinding can be modeled with the Arrhenius law using a single pre-exponential factor of 10(8.3) s-1 and a Gaussian distribution of enthalpy barriers centered at 22 kJ/mol with a width [full width at half-maximum (FWHM)] of 11 kJ/mol. In halocyanin, a more complex behavior is observed. About 97% of the rebinding population can also be characterized by a Gaussian distribution of enthalpy barriers at 12 kJ/mol with a width of 6.0 kJ/mol (FWHM). The pre-exponential of this population is 1.6 x 10(12) s-1 at 100 K. After the majority population has rebound, a power-law phase that can be modeled with a gamma-distribution of enthalpy barriers is observed. Between 120 and 180 K, an additional feature that can be interpreted as a relaxation of the barrier distribution toward higher barriers shows up in the kinetics. Above 200 K, a slower, exponential rebinding appears in both cupredoxins. Since the kinetics depend on the NO concentration, this process is identified as bimolecular rebinding.(ABSTRACT TRUNCATED AT 250 WORDS)