The interaction of protozoa with their potential prey bacteria in the rhizosphere

The O-antigen mediates differential survival of Salmonella against communities of natural predators

Antigenically distinct members of bacterial species can be differentially distributed in the environment. Predators known to consume antigenically distinct prey with different efficiencies are also differentially distributed. Here we show that antigenically distinct, but otherwise isogenic and physiologically indistinct, strains of Salmonella enterica show differential survival in natural soil, sediment and intestinal environments, where they would face a community of predators. Decline in overall cell numbers is attenuated by factors that inhibit the action of predators, including heat and antiprotozoal and antihelminthic drugs. Moreover, the fitness of strains facing these predators - calculated by comparing survival with and without treatments attenuating predator activity - varies between environments. These results suggest that relative survival in natural environments is arbitrated by communities of natural predators whose feeding preferences, if not species composition, vary between environments. These data support the hypothesis that survival against natural predators may drive the differential distribution of bacteria among microenvironments.

Differential Salmonella survival against communities of intestinal amoebae

Predation from intestinal amoebae may provide selective pressure for the maintenance of high genetic diversity at the Salmonella enterica rfb locus, whereby serovars better escape predators in particular environments depending on the O-antigens they express. Here, the hypothesis that amoebae from a particular intestinal environment collectively prefer one serovar over another is tested. Collections of Acanthamoeba, Tetramitus, Naegleria and Hartmannella were isolated from the intestinal tracts of several vertebrate hosts, including bullfrog tadpoles, goldfish, turtles and bearded dragons, and their feeding preferences were determined. Congeneric amoebae from the same environment had significantly similar feeding preferences. Strikingly, even unrelated amoebae - such as Naegleria and Tetramitus from goldfish - also had significantly similar feeding preferences. Yet amoebae isolated from different environments showed no similarity in prey choice. Thus, feeding preferences of amoebae appear to reflect their environment, not their taxonomic relationships. A mechanism mediating this phenotypic convergence is discussed.

16S rRNA gene analyses of bacterial community structures in the soils of evergreen broad-leaved forests in south-west China

Bacterial community structure was studied in humus and mineral soils of evergreen broad-leaved forests in Ailaoshan and Xishuangbanna, representing subtropical and tropical ecosystems, respectively, in south-west China using sequence analysis and terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA genes. Clone sequences affiliated to Acidobacteria were retrieved as the predominant bacterial phylum in both forest soils, followed by those affiliated to members of the Proteobacteria, Planctomycete and Verrucomicrobia. Despite higher floristic richness at the Xishuangbanna forest than at the Ailaoshan forest, soil at Xishuangbanna harbored a distinctly high relative abundance of Acidobacteria-affiliated sequences (80% of the total clones), which led to a lower overall bacterial diversity than at Ailaoshan. Bacterial communities in humus and mineral soils of the two forests appeared to be well differentiated, based on 16S rRNA gene phylogeny, and correlations were found between the bacterial T-RFLP community patterns and the organic carbon and nutrient contents of the soil samples. The data reveal that Acidobacteria dominate soil bacterial communities in the evergreen broad-leaved forests studied here and suggest that bacterial diversity may be influenced by soil carbon and nutrient levels, but is not related to floristic richness along the climatic gradient from subtropical to tropical forests in south-west China.

A ClC chloride channel homolog and ornithine-containing membrane lipids of Rhizobium tropici CIAT899 are involved in symbiotic efficiency and acid tolerance

Rhizobium tropici CIAT899 is highly tolerant to several environmental stresses and is a good competitor for nodule occupancy of common bean plants in acid soils. Random transposon mutagenesis was performed to identify novel genes of this strain involved in symbiosis and stress tolerance. Here, we present a genetic analysis of the locus disrupted by the Tn5 insertion in mutant 899-PV9, which lead to the discovery of sycA, a homolog of the ClC family of chloride channels and Cl-/H+ exchange transporters. A nonpolar deletion in this gene caused serious deficiencies in nodule development, nodulation competitiveness, and N2 fixation on Phaseolus vulgaris plants, probably due to its reduced ability to invade plant cells and to form stable symbiosomes, as judged by electron transmission microscopy. A second gene (olsC), found downstream of sycA, is homologous to aspartyl/asparaginyl beta-hydroxylases and modifies two species of ornithine-containing lipids in vivo, presumably by hydroxylation at a still-unknown position. A mutant carrying a nonpolar deletion in olsC is symbiotically defective, whereas overexpressed OlsC in the complemented strain provokes an acid-sensitive phenotype. This is the first report of a ClC homolog being essential for the establishment of a fully developed N2-fixing root nodule symbiosis and of a putative beta-hydroxylase that modifies ornithine-containing membrane lipids of R. tropici CIAT899, which, in turn, are contributing to symbiotic performance and acid tolerance.

Protozoan predation, diversifying selection, and the evolution of antigenic diversity in Salmonella

Extensive population-level genetic variability at the Salmonella rfb locus, which encodes enzymes responsible for synthesis of the O-antigen polysaccharide, is thought to have arisen through frequency-dependent selection (FDS) by means of exposure of this pathogen to host immune systems. The FDS hypothesis works well for pathogens such as Haemophilus influenzae and Neisseria meningitis, which alter the composition of their O-antigens during the course of bloodborne infections. In contrast, Salmonella remains resident in epithelial cells or macrophages during infection and does not have phase variability in its O-antigen. More importantly, Salmonella shows host-serovar specificity, whereby strains bearing certain O-antigens cause disease primarily in specific hosts; this behavior is inconsistent with FDS providing selection for the origin or maintenance of extensive polymorphism at the rfb locus. Alternatively, selective pressure may originate from the host intestinal environment itself, wherein diversifying selection mediated by protozoan predation allows for the continued existence of Salmonella able to avoid consumption by host-specific protozoa. This selective pressure would result in high population-level diversity at the Salmonella rfb locus without phase variation. We show here that intestinal protozoa recognize antigenically diverse Salmonella with different efficiencies and demonstrate that differences solely in the O-antigen are sufficient to allow for prey discrimination. Combined with observations of the differential distributions of both serotypes of bacterial species and their protozoan predators among environments, our data provides a framework for the evolution of high genetic diversity at the rfb locus and host-specific pathogenicity in Salmonella.

The effect of chloroquine, quinacrine, and metronidazole on both soybean plants and soil microbiota

Chloroquine, quinacrine, and metronidazole are used extensively for therapeutic purposes. Substantial quantities of these compounds end up in the environment. The potential effect of these compounds on soybean and on the protozoa in soil was assessed. The growth of soybean plants was affected by increasing concentrations of the chloroquine, metronidazole, and quinacrine dihydrochloride. The plants were particularly sensitive to low concentrations of metronidazole. The number of bacteria and protozoa in soil was either unchanged or increased in the presence of chloroquine and quinacrine. However, in the presence of only 0.5 mg metronidazole g(-1) soil, the density of protozoa in the rhizosphere was reduced by a 10-fold.