Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Molecular Basis of Bacterial Outer Membrane Permeability Revisited

When titanium dioxide (TiO(2)) is irradiated with near-UV light, this semiconductor exhibits strong bactericidal activity. In this paper, we present the first evidence that the lipid peroxidation reaction is the underlying mechanism of death of Escherichia coli K-12 cells that are irradiated in the presence of the TiO(2) photocatalyst. Using production of malondialdehyde (MDA) as an index to assess cell membrane damage by lipid peroxidation, we observed that there was an exponential increase in the production of MDA, whose concentration reached 1.1 to 2.4 nmol. mg (dry weight) of cells(-1) after 30 min of illumination, and that the kinetics of this process paralleled cell death. Under these conditions, concomitant losses of 77 to 93% of the cell respiratory activity were also detected, as measured by both oxygen uptake and reduction of 2,3,5-triphenyltetrazolium chloride from succinate as the electron donor. The occurrence of lipid peroxidation and the simultaneous losses of both membrane-dependent respiratory activity and cell viability depended strictly on the presence of both light and TiO(2). We concluded that TiO(2) photocatalysis promoted peroxidation of the polyunsaturated phospholipid component of the lipid membrane initially and induced major disorder in the E. coli cell membrane. Subsequently, essential functions that rely on intact cell membrane architecture, such as respiratory activity, were lost, and cell death was inevitable.

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Bactericidal activity of photocatalytic TiO(2) reaction: toward an understanding of its killing mechanism.

Iso- and anteiso-fatty acids in bacteria: biosynthesis, function, and taxonomic significance.

The phospholipid fatty acid (PLFA) pattern was analyzed in a forest humus and in an arable soil experimentally polluted with Cd, Cu, Ni, Pb, or Zn at different concentrations. In both soil types, there were gradual changes in the PLFA patterns for the different levels of metal contamination. The changes in the forest soil were similar irrespective of which metal was used, while in the arable soil the changes due to Cu contamination differed from those due to the other metals. Several PLFAs reacted similarly to the metal amendments in the two soil types, while others showed different responses. In both soils, the metal pollution resulted in a decrease in the iso-branched PLFAs i15:0 and i17:0 and in the monounsaturated 16:1ω5 and 16:1ω7c fatty acids, while increases were found for i16:0, the branched br17:0 and br18:0, and the cyclopropane cy17:0 fatty acids. In the forest soil, the methyl branched PLFAs 10Me16:0, 10Me17:0, and 10Me18:0 increased in metal-polluted soils, indicating an increase in actinomycetes, while in the arable soil a decrease was found for 10Me16:0 and 10Me18:0 in response to most metals. The bacterial PLFAs 15:0 and 17:0 increased in all metal-contaminated samples in the arable soil, while they were unaffected in the forest soil. Fatty acid 18:2ω6, which is considered to be predominantly of fungal origin, increased in the arable soil, except in the Cu-amended samples, in which it decreased instead. Effects on the PLFA patterns were found at levels of metal contamination similar to or lower than those at which effects on ATP content, soil respiration, or total amount of PLFAs had occurred.

Phospholipid Fatty Acid composition, biomass, and activity of microbial communities from two soil types experimentally exposed to different heavy metals.

The effects of lime and wood-ash on the microbial community structure were evaluated by analyzing the phospholipid fatty acid (PLFA) composition of soils from two areas in the south of Sweden. A pine forest was amended with lime or ash at two concentrations, and a spruce forest was limed at one concentration. The treatments were carried out 5–6 years before sampling and raised the pH from approx. 4.0 to values between 4.8 and 7.0. At both sites there was a difference in the PLFA composition between the treated plots and the controls. The changes found were similar at both sites and correlated to the pH changes. No difference was found between limed plots and those treated with wood-ash. The methyl-branched fatty acids i15:0, i16:0 and 10Me16:0, the monounsaturated fatty acids 16: 1ω 7t and 18: 1ω 9, the cyclopropane fatty acid cy 19:0, and the saturated fatty acid 20:0 were more abundant in the control plots. In the plots with the highest pH there was a three-fold increase in the fatty acid 16: lω 5. An increase was also found for the fatty acids i14:0, 16:lω9, 16:lω 7c, cy17:0, 18:lω 7 and 10Me18:0. No effect on 18:2ω6 was found. The changes in PLFA pattern indicated that the increased pH caused a shift in the bacterial community to more Gram-negative and fewer Gram-positive bacteria, while the amount of fungi was unaffected. The increase in 10Me18:0 in limed soils indicated an increase in actinomycetes.

Sg Wilkinson

Papers

Gram-negative bacteria