Impact of growth environment and physiological state on metal immobilization by Pseudomonas aeruginosa PAO1

Interaction of ethidium and tetraphenylphosphonium cations with Salmonella enterica cells

BACKGROUND AND OBJECTIVE

One of the main causes of bacterial resistance to antimicrobials is multidrug resistance induced by the increased efficiency of the efflux pumps. In this study we analyzed how the conditions of assay affect the efflux of indicator substrates ethidium (Et⁺) and tetraphenylphosphonium (TPP⁺) in Salmonella enterica ser. Typhimurium cells. Impact of the outer membrane permeability barrier, composition and temperature of the medium on accumulation of the indicator compounds also was analyzed.

MATERIALS AND METHODS

The fluorescence of Et⁺ and Nile Red was measured using 96-well plates and a plate reader. In parallel to traditional studies of fluorescence we applied a constructed selective electrode to follow the accumulation of Et⁺ in S. enterica cells. Simultaneously with monitoring of Et⁺ concentration in the cell incubation medium, electrochemical measurements of TPP⁺ accumulation were performed. Furthermore, Et⁺ and TPP⁺ were used within the same sample as agents competing for the interaction with the efflux pumps. An inhibitor phenylalanyl-arginyl-β-naphtylamide (PAβN) was applied to evaluate the input of RND-family pumps in the total efflux of these indicator compounds.

RESULTS

S. enterica cells with the intact outer membrane (OM) bound very low amounts of Et⁺ or TPP⁺. Cells with the permeabilized OM accumulate considerably higher amounts of the indicator compounds at pH 8.0, but only Et⁺ was considerably accumulated at pH 6.5. At conditions of electrochemical monitoring accumulation of Et⁺ by the permeabilized cells at 37°C was considerably faster than at 23°C, but at the higher temperature most of the cell-accumulated Et⁺ was extruded back to the medium. The fluorescence of Et⁺ in suspension of cells incubated in 400mmol/L Tris buffer was about twice higher compared to 100mmol/L one. The inhibitory action of TPP⁺ on Et⁺ efflux was evident only in 400mmol/L Tris although PAβN effectively increased Et⁺ fluorescence at both buffer concentrations.

CONCLUSIONS

Results of our experiments indicate that ionic strength of the incubation medium influence the selectivity, the medium temperature and the assay conditions impact the kinetics of efflux. The lower accumulated amount and the weaker fluorescence of Et⁺ registered in slightly acidic medium indicate that ΔΨ plays a role in the accumulation of this indicator cation. The bound amount of Et⁺ to the de-energized or permeabilized cells considerably varies depending on the conditions and methods of de-energization or permeabilization of cells. Tris/EDTA permeabilization of the cells does not inhibit the efflux.

Impact of an urban multi-metal contamination gradient: metal bioaccumulation and tolerance of river biofilms collected in different seasons

The aim of this study was to investigate the repeatability and seasonal variability of the biological response of river biofilms chronically exposed to a multi-metal pressure in an urban contamination gradient. Biofilms were grown on immersed plastic membranes at three sites on the Seine river upstream (site 1) and downstream (sites 2 and 3) from Paris (France). They were collected in four different seasons (autumn, spring, summer and winter). Biofilm tolerance to Cu, Ni, Pb and Zn was measured using a PICT (Pollution-Induced Community Tolerance) approach with a previously developed short-term toxicity test based on β-glucosidase (heterotrophic) activity. Metal concentrations in the river and also in the biofilm samples (total and non-exchangeable bioaccumulated metals) were also monitored. Biofilm-accumulated metal concentrations reflected the increase of the multi-metal exposure along the urban gradient. These concentrations were strongly correlated with dissolved and particulate organic carbon and with the total metal fraction in the river water, which recalls the significant influence of the environmental parameters on metal uptake processes in river biofilms. Overall, natural biofilms allow monitoring water quality by integrating the variations of a diffuse metal contamination overtime. Tolerance levels globally increased from site 1 to site 3 reflecting the metal pollution gradient measured in the river water collected at the three sites. Cu tolerance tended to increase during warm seasons but no clear seasonal tendency could be found for Ni, Pb and Zn. Furthermore, principal component analysis clearly discriminated samples collected upstream (site 1) from samples collected downstream (sites 2 and 3) along the first principal component which was correlated to the metal gradient. Samples collected in winter were also separated from the others along the second principal component correlated to parameters like water temperature and Total Suspended Solids concentration. This study shows that chronic in situ exposure to environmental metal concentrations has a significant impact on natural biofilms. Biofilm tolerance to metals and biofilm metal bioaccumulation both reflect metal exposure levels although they remain low when compared to Environmental Quality Standards from the European Water Framework Directive. Yet temperature appears as an important environmental variable shaping community structure and response to toxic exposure which shows that the sampling date is an important parameter to consider when using natural river biofilms to assess the impacts of urban pressure.

Gold(III) reduction by the rhizobacterium Azospirillum brasilense with the formation of gold nanoparticles

For the soil nitrogen-fixing bacterium Azospirillum brasilense, the ability to reduce [AuCl4](-) and to form gold nanoparticles (GNPs) has been demonstrated, with the appearance of a mauve tint of the culture. To validate the shapes and chemical nature of nanoparticles, transmission electron microscopy (TEM) and X-ray fluorescence analysis were used. For the widely studied agriculturally important wild-type strains A. brasilense Sp7 and Sp245, GNPs formed after 10 days of incubation of cell biomass with 0.25 mM [AuCl4](-) were shown (using TEM) to be mainly of spherical form (5 to 20 nm in diameter), with rare occasional triangles. In the course of cultivation with [AuCl4](-), after 5 days, a mauve tint was already visible for cells of strain Sp245.5, after 6 days for Sp245 and after 10 days for Sp7. Thus, for the mutant strain Sp245.5 (which has significant differences in the structure and composition of cell-surface polysaccharides as compared with Sp245), a more rapid formation of GNPs was observed. Moreover, their TEM images (also obtained after 10 days) showed different shapes: nano-sized spheres, triangles, hexagons and rods, as well as larger round-shaped flower-like nanoparticles about 100 nm in size. Since by the time of GNP formation in our experiments the cells were found to be already not viable, this confirms the dominating role of cell surface structure and chemical composition in shaping the GNPs formed in the course of [AuCl4](-) reduction to Au(0). This finding may be useful for understanding the natural biogeochemical mechanisms of gold reduction and formation of GNPs involving microorganisms. The data obtained may also help in developing protocols for environmentally friendly synthesis of nanoparticles and possible use of bacterial cells with modified surface structure and composition for their fabrication.

Copper susceptibility in Acinetobacter junii BB1A is related to the production of extracellular polymeric substances

Acinetobacter junii BB1A cells, grown in different media, were differentially inhibited in the presence of the copper. The minimum inhibitory concentration of Cu(2+) was influenced by the nutrient status of the media. The production of extracellular polymeric substances (EPS) was stimulated by the copper present in the growth medium. The nature of the EPS was anionic showing non-Newtonian pseudoplastic behaviour. The thermal behaviour of the EPS was studied by differential scanning calorimetry. The EPS was amorphous in nature with a crystalline index of 0.16. Scanning electron micrographs revealed its porous structure. Cells grown in the presence of quorum sensing inhibitor (QSI: 4-Nitropyridine-N-oxide) did not produce EPS and were found to be more sensitive to Cu(2+) than cells which produced EPS in the absence of QSI. EPS production in different media in the presence and absence of Cu(2+) was determined. The production of EPS was the highest in brain heart Infusion medium and the lowest in AB minimal medium. The sorption of Cu(2+) by EPS extracted from cells grown in non-copper-complexing AB medium was demonstrated by energy dispersive X-ray spectroscopy. A pertinent functional aspect of EPS in providing protection to A. junii in copper stress condition has been revealed.