Biosorption of uranium by Myxococcus xanthus

This paper deals with uranium biosorption by Myxococcus xanthus biomass in which dry biomass, accumulating up to 2.4 mM of uranium g⁻¹, is demonstrated to be a more efficient biosorbent than wet biomass. For uranium concentrations of 0.1–0.3 mM, between 95.79% and 95.99% of the uranium was taken up from the solution. Dry biomass biosorption was found to be relatively rapid, reaching equilibrium after 5–10 min. In addition, the pH influenced biosorption, pH 4.5 promoting maximum uptake. It was also established that the biosorbed uranium is located on the cellular wall and within the extracellular mucopolysaccharide of this microorganism. Furthermore, using sodium carbonate as a desorbent agent, 80.82% of the biosorbed uranium could be recovered. The results obtained indicate the possible utilization of M. xanthus biomass to solve some problems of the water contaminated by uranium.     

Short spacing between the Shine-Dalgarno sequence and P codon destabilizes codon-anticodon pairing in the P site to promote +1 programmed frameshifting

Myxococcus xanthus is a Gram‐negative bacterium capable of complex developmental processes involving vegetative swarming and fruiting body formation. Social (S‐) gliding motility, one of the two motility systems used by M. xanthus, requires at least two cell surface structures: type IV pili (TFP) and extracellular polysaccharides (EPS). Extended TFP that are composed of thousands of copies of PilA retract upon binding to EPS and thereby pull the cell forward. TFP also act as external sensor to regulate EPS production. In this study, we generated a random PilA mutant library and identified one derivative, SW1066, which completely failed to undergo developmental processes. Detailed characterization revealed that SW1066 produced very little EPS but wild‐type amounts of PilA. These mutated PilA subunits, however, are unable to assemble into functional TFP despite their ability to localize to the membrane. By preventing the mutated PilA of SW1066 to translocate from the cytoplasm to the membrane, fruiting body formation and EPS production were restored to the levels observed in mutant strains lacking PilA. This apparent connection between PilA membrane accumulation and reduction in surface EPS implies that specific cellular PilA localization are required to maintain the EPS level necessary to sustain normal S‐motility in M. xanthus.     

Lanthanum biosorption by a Pseudomonas sp.: equilibrium studies and chemical characterization

Lanthanum biosorption by a Pseudomonas sp. was characterized in terms of equilibrium metal loading, model fitting, kinetics, effect of solution pH, lanthanum–bacteria interaction mechanism and recovery of sorbed metal. Lanthanum sorption by the bacterium was rapid and optimum at pH 5.0 with equilibrium metal loading as high as 950 mg g⁻¹ biomass dry wt. Scatchard model and potentiometric titration suggested the presence of at least two types of metal-binding sites, corresponding to a strong and a weak binding affinity. The chemical nature of metal–microbe interaction has been elucidated employing FTIR spectroscopy, energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). FTIR spectroscopy and XRD analysis revealed strong involvement of cellular carboxyl and phosphate groups in lanthanum binding by the bacterial biomass. EDX and the elemental analysis of the sorption solution ascertained the binding of lanthanum with the bacterial biomass via displacement of cellular potassium and calcium. Transmission electron microscopy exhibited La accumulation throughout the bacterial cell with some granular deposits in cell periphery and in cytoplasm. XRD confirmed the presence of LaPO₄ crystals onto the bacterial biomass after La accumulation for a long period. A combined ion-exchange–complexation–microprecipitation mechanism could be involved in lanthanum accumulation by the biomass. Almost 98% of biomass-bound La could be recovered using CaCO₃ as the desorbing agent.     

Exopolysaccharides promote Myxococcus xanthus social motility by inhibiting cellular reversals

The biofilm‐forming bacterium Myxococcus xanthus moves on surfaces as structured swarms utilizing type IV pili‐dependent social (S) motility. In contrast to isolated cells that reverse their moving direction frequently, individual cells within swarms rarely reverse. The regulatory mechanisms that inhibit cellular reversal and promote the formation of swarms are not well understood. Here we show that exopolysaccharides (EPS), the major extracellular components of M. xanthus swarms, inhibit cellular reversal in a concentration‐dependent manner. Thus, individual wild‐type cells reverse less frequently in swarms due to high local EPS concentrations. In contrast, cells defective in EPS production hyper‐reverse their moving direction and show severe defects in S‐motility. Surprisingly, S‐motility and wild‐type reversal frequency are restored in double mutants that are defective in both EPS production and the Frz chemosensory system, indicating that EPS regulates cellular reversal in parallel to the Frz pathway. Here we clarify that besides functioning as the structural scaffold in biofilms, EPS is a self‐produced signal that coordinates the group motion of the social bacterium M. xanthus.     

Bioaccumulation of silver by a multispecies community of bacteria

A stable community of bacteria that had unusually high tolerance of soluble silver was isolated from soil by chemostat enrichment. The community consisted of three bacteria: Pseudomonas maltophilia, Staphylococcus aureus and a coryneform organism. The pseudomonas was primarly responsible for the silver resistance. The tolerance of high silver concentrations, up to 100 mM Ag⁺, was greatly reduced when the community was grown in the absence of silver. Pseudomonas maltophilia comprised approximately 50% by numbers of the community when grown in chemostats in the presence or absence of Ag⁺ but large fluctuations occurred in population sizes of the other two bacteria; the S. aureus population was small (less than 1%) in the presence of Ag⁺ but comparised a third of the total numbers when Ag⁺ was omitted from the medium. Silver-resistant respiration of the silveradapted community was significant even when it was confronted with high concentrations of Ag⁺. In contrast the respiration of the coryneform organism and particularly S. aureus was highly sensitive to silver. The inhibition constants for silver-sensitive respiration were 0.78 mM and 0.04 mM for silver acclimatized and nonacclimatized communities respectively.The community had great capacity for silver bioaccumulation. Maximum concentrations of over 300 mg silver per g dry weight of biomass were recorded at an accumulation rate of 21 mg Ag⁺ h^-1 (g biomass)^-1. The extent of silver removal from solution was a function of initial concentration of silver; at low external concentrations (ca. 1 mM) all the silver was rapidly removed from solution, at high concentrations (ca. 12 mM) 84% removal occurred in 15 h.     

Bacterial Community Dynamics During the Application of a Myxococcus xanthus-Inoculated Culture Medium Used for Consolidation of Ornamental Limestone

In this study, we investigated under laboratory conditions the bacterial communities inhabiting quarry and decayed ornamental carbonate stones before and after the application of a Myxococcus xanthus-inoculated culture medium used for consolidation of the stones. The dynamics of the community structure and the prevalence of the inoculated bacterium, M. xanthus, were monitored during the time course of the consolidation treatment (30 days). For this purpose, we selected a molecular strategy combining fingerprinting by denaturing gradient gel electrophoresis (DGGE) with the screening of eubacterial 16S rDNA clone libraries by DGGE and sequencing. Quantification of the inoculated strain was performed by quantitative real-time PCR (qPCR) using M. xanthus-specific primers designed in this work. Results derived from DGGE and sequencing analysis showed that, irrespective of the origin of the stone, the same carbonatogenic microorganisms were activated by the application of a M. xanthus culture. Those microorganisms were Pseudomonas sp., Bacillus sp., and Brevibacillus sp. The monitoring of M. xanthus in the culture media of treated stones during the time course experiment showed disparate results depending on the applied technique. By culture-dependent methods, the detection of this bacterium was only possible in the first day of the treatment, showing the limitation of these conventional techniques. By PCR-DGGE analysis, M. xanthus was detected during the first 3–6 days of the experiment. At this time, the population of this bacterium in the culture media varied between 10⁸–10⁶ cells ml⁻¹, as showed by qPCR analyses. Thereafter, DGGE analyses showed to be not suitable for the detection of M. xanthus in a mixed culture. Nevertheless, qPCR analysis using specific primers for M. xanthus showed to be a more sensitive technique for the detection of this bacterium, revealing a population of 10⁴ cells ml⁻¹ in the culture media of both treated stones at the end of the consolidation treatment. The molecular strategy used in this study is proposed as an effective monitoring system to evaluate the impact of the application of a bacterially induced carbonate mineralization as restoration/conservation treatment for ornamental stones.     

Turgor regulation in a novel Halomonas species

The mechanisms by which a novel eubacterium, identified as belonging to the genus Halomonas, adapted to increases in the extracellular osmotic potential were investigated. It was shown that the ability of the bacterium to grow after hyperosmotic shock was dependent on the presence of potassium ions. Growth of the bacterium in 2 M NaCl medium could be limited by low concentrations of K⁺ and this enabled the affinity for K⁺ to be determined (K _s=21.5 M). Rubidium salts could be substituted for those of potassium, but the lowest concentration of Rb⁺ that allowed growth in 2 M NaCl medium was 50-fold greater than the minimum concentration of K⁺. ¹³C-NMR spectroscopy and HPLC analysis were used to demonstrate the accumulation of organic solutes in the cytoplasm after exposure to high salinities. The major osmolyte was ectoine, but glutamate and ectoine hydroxide were also present. Addition of exogenous glycine betaine to 3.25 M NaCl medium resulted in the accumulation of high intracellular concentrations of glycine betaine in the bacterium. This reduced the level of ectoine accumulation but did not fully inhibit the synthesis of this compound in the cytoplasm.Abbreviation Specific growth rate (generations/h)     

Spectroscopic and Microscopic Characterization of Uranium Biomineralization in Myxococcus xanthus

In this work, synchrotron-based X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) studies were carried out to elucidate at molecular scale the interaction mechanisms of Myxococcus xanthus with uranium at different pH values. Extended X-ray absorption fine structure (EXAFS) spectroscopic measurements showed that there are significant differences in the structural parameters of the U complexes formed by this bacterium at pH 2 and 4.5. At very low acidic pH of 2, the cells accumulated U(VI) as organic phosphate-metal complexes. At pH 4.5, however, the cells of this bacterium precipitated U(VI) as meta-autunite-like phase. TEM analyses demonstrated that at pH 2 the uranium accumulates were located mainly at the cell surface, whereas at pH 4.5 a uranium precipitation occurred on the cell wall and within the extracellular polysaccharides (EPS) characteristic of this bacterium. Dead/live staining studies showed that 30% and 50% of the uranium treated cell populations were alive at pH 2 and 4.5, respectively. The precipitation of U(VI) as mineral meta-autunite-like phase is possibly due to the bacterial acidic phosphatase activity. The precipitation of uranium as mineral phases may lead to more stable U(VI) sequestration that may be suitable for remediation purposes. These observations, combined with the very high uranium accumulation capability of the studied bacterial cells indicate that M. xanthus may significantly influence the fate of uranium in soil environments where these bacterial species are mainly found.     

Type IV‐pili dependent motility is co‐regulated by PilSR and PilS2R2 two‐component systems via distinct pathways in Myxococcus xanthus

Myxococcus xanthus is an environmental bacterium with two forms of motility. One type, known as social motility, is dependent on extension and retraction of Type‐IV pili (T4P) and production of extracellular polysaccharides (EPS). Several signaling systems have been linked to regulation of T4P‐dependent motility. In particular, expression of the pilin subunit pilA requires the PilSR two‐component signaling system (TCS). A second TCS, PilS2R2, encoded within the same locus that encodes PilSR, has also been linked to M. xanthus T4P‐dependent motility. We demonstrate that PilSR and PilS2R2 regulate M. xanthus T4P‐dependent motility through distinct pathways. Consistent with known roles of PilSR, our results indicate that the primary function of PilSR is to regulate expression of pilA. In contrast, PilS2 and PilR2 have little to no affect on PilA protein levels. However, deletion of pilR2 resulted in a reduction of assembled pili, significant decreases in EPS production and loss of T4P‐dependent motility. Furthermore, the pilR2 mutation led to increased production of outer membrane vesicles (OMV). Collectively, we propose that PilS2R2 is required for proper assembly of T4P and regulation of OMV production, and hypothesize that production of these vesicles is related to M. xanthus motility.     

Experimental silver bioaccumulation in the polychaetePomatoceros triqueter (L.)

Summary The tubicolous polychaetePomatoceros triqueter was exposed for 6–7 weeks to 200 or 400 g · l^–1 silver introduced as the nitrate into sea water. Survival conditions and mortality were evaluated and silver bioaccumulation analysed by atomic absorption spectrometry. Characteristic morphological lesions were recognized. Histopathologic examination was performed on paraffin or semi-thin sections and at the ultrastructural level. Histochemical examination mainly concerned the metals, reducing groups and sulfur-containing proteins. Microanalytical study involved the use of a wavelength-dispersive X-ray spectrometry microprobe and ion microanalyzer, and the use of an energy-dispersive X-ray spectrometry microprobe at the ultrastructural level. Our results emphasize the role of the branchial crown for metal penetration. Its cuticle accumulates silver as a metal, in particulate form. The internal accumulation of mainly extracellular deposits concerns the basement membranes and connective tissue present in the axis of the branchial crown filaments, or surrounding the nephridial pouches and the gut sinus. The carrier role of the closed vascular system is suggested by ultrastructural observations. The silver route from transepithelial uptake to nephridial excretion involves at least two intracellular transits, plus the vascular mesothelium. Nephridia play a role in silver storage (lysosomes) and elimination (concretions). In all parts internal to the crown cuticle, silver is at least partly associated with protein SH-groups (metallothionein-like); deposits can be enriched with silver sulfide and metallic silver.     

Enhancement of extracellular enzymatic activities produced by immobilized growing cells of Myxococcus xanthus

Summary The gram-negative soil bacterium Myxococcus xanthus was immobilized by entrapping into carrageenan gel beads. Unexpectedly, the growth rate was hardly increased, and the released free cell concentration remained low. However, extracellular proteolytic and bacteriolytic activities produced in the medium or inside the beads was greatly increased and (or) stabilized as compared to the control. These properties might be quite useful in view of using Myxococcus xanthus as a cloning vehicle for secretion of foreign proteins.     

Genetic Population Structure of the Soil Bacterium Myxococcus xanthus at the Centimeter Scale

Myxococcus xanthus is a gram-negative soil bacterium best known for its remarkable life history of social swarming, social predation, and multicellular fruiting body formation. Very little is known about genetic diversity within this species or how social strategies might vary among neighboring strains at small spatial scales. To investigate the small-scale population structure of M. xanthus, 78 clones were isolated from a patch of soil (16 by 16 cm) in Tübingen, Germany. Among these isolates, 21 genotypes could be distinguished from a concatemer of three gene fragments: csgA (developmental C signal), fibA (extracellular matrix-associated zinc metalloprotease), and pilA (the pilin subunit of type IV pili). Accumulation curves showed that most of the diversity present at this scale was sampled. The pilA gene contains both conserved and highly variable regions, and two frequency-distribution tests provide evidence for balancing selection on this gene. The functional domains in the csgA gene were found to be conserved. Three instances of lateral gene transfer could be inferred from a comparison of individual gene phylogenies, but no evidence was found for linkage equilibrium, supporting the view that M. xanthus evolution is largely clonal. This study shows that M. xanthus is surrounded by a variety of distinct conspecifics in its natural soil habitat at a spatial scale at which encounters among genotypes are likely.     

Branched-chain fatty acids: the case for a novel form of cell-cell signalling during Myxococcus xanthus development

The esg locus is required for the formation of muiti-cellular fruiting bodies and spores by the developmental bacterium Myxococcus xanthus Studies have suggested that esg mutants are defective in the production of an essential signal (E-signal) used in cell-cell communication and that E-signalling is required for the expression of many developmental genes. Recently we have determined that the esg locus encodes components of a branched-chain keto acid dehydrogenase. a multienzyme complex involved in branched-chain amino acid metabolism in many bacteria and higher organisms. During vegetative growth in M. xanthus. this enzyme complex appears to participate in the production of the branched-chain fatty acids found in this organism. M. xanthus fatty acids (including the branched-chain fatty acids) have been observed to have a variety of effects on developing cells. These effects include; (i) the lysis of M. xanthus cells (autocide activity), (ii) acceleration of the rate of sporulation and (iii) rescue of sporulation by certain development-defective mutants. These and other results suggest a model in which the branched-chain fatty acids. Synthesized during growth, are released from cellular phospholipid by a developmentally regulated phospholipase during fruiting-body formation. This model proposes that one or more of the branched-chain fatty acids that are released constitutes the E-signal which must be transmitted between cells to complete M. xanthus development.     

Eukaryotic-like protein serine/threonine kinases in Myxococcus xanthus,a developmental bacterium exhibiting social behavior

Myxococcus xanthus, a gram-negative bacterium exhibits a spectacular life cycle and social behavior. Its developmental cycle and multicellular morphogenesis resemble those of eukaryotic slime molds such as Dictyostelium discoideum. On the basis of this resemblance, we explored the existence of eukaryotic-like protein serine/threonine kinases which are known to play important roles in signal transduction during development of D. discoideum. It was indeed found that M. xanthus contains a large family of protein serine/threonine kinases related to the eukaryotic enzymes. This is the first unambiguous demonstration of eukaryotic-like protein serine/threonine kinases in the prokaryotes. © 1993 Wiley-Liss, Inc.     

Plasmid-mediated UV-protection in Myxococcus xanthus

Summary Plasmid R46 was successfully transferred from Escherichia coli K-12 into Myxococcus xanthus strain MD-1 but not into M. xanthus strain XK. Plasmid R68.45 was transferred from E. coli K-12 into both strains of M. xanthus. The effects of these plasmids on survival of M. xanthus after ultraviolet (UV)-254 nm irradiation, the ability of M. xanthus to reactivate irradiated myxophages, and Weigle reactivation of UV-irradiated myxophages by M. xanthus were studied. Plasmid R46 had no effect on UV survival of M. xanthus, but increased the host's ability to reactivate irradiated myxophages. Plasmid R68.45 protected M. xanthus strains MD-1 and XK against the lethal effects of UV irradiation and also increased the host's ability to reactivate irradiated myxophages.     

Autocides and a paracide,antibiotic TA,produced byMyxococcus xanthus

Myxococcus xanthus produces two categories of low molecular weight antibacterial materials, autocides and paracides, that have diametrically opposite host ranges. Low concentrations of autocides lyseM. xanthus, the producing organism, whereas paracides exert their effects on other bacteria. Antibiotic TA (a paracide) kills all growing bacteria tested that have a peptidoglycan cell wall exceptM. xanthus. It is a macrocyclic polyketide with a molecular weight of 623. The two major autocides produced byM. xanthus are phosphatidylethanolamine and a mixture of fatty acids. The modes of action, host ranges and biosynthesis of antibiotic TA and the autocides are presented, and then an attempt is made to explain their role in the complex life cycle ofM. xanthus. In addition, the remarkable adhesion properties of antibiotic TA and a new semisynthetic derivative of it, focusin, are presented.     

Production of a hypoglycemic, extracellular polysaccharide from the submerged culture of the mushroom, Phellinus linteus

Mycelial growth and extracellular polysaccharide production of Phellinus linteus were optimal at pH 5 and 25 °C. Maximum biomass production (14.2 g l^–1) was after 15 d of cultivation, whereas, extracellular polysaccharide was maximal (3.5 g l^–1) after 21 d. The hypoglycemic effect of the polysaccharide, investigated in streptozotocin-induced diabetic rats, decreased plasma glucose, total cholesterol and triacylglycerol concentrations by 49%, 32%, and 28%, respectively, and aspartate aminotransferase activity by 20%. The results indicate the potential of this polysaccharide to prevent hyperglycemia in diabetic patients.     

Endotoxin-like activities inMyxococcus xanthus

Vegetative cells as well myxospores ofMyxococcus xanthus have shown anticomplementary activity and the capacity to be used as active agents in the skin preparation of the Shwartzman reaction and in its intravenous induction. These endotoxin-like properties were not extractable by the hot phenol-water methods. Our results suggest the presence of a lipid A analog in both vegetative cells and myxospores, and emphasize the difficulty of lipopolysaccharide detection; this is perhaps a consequence of a developing associated change in polysaccharide moiety of the myxobacterial lipopolysaccharides; this may be the basis of the special immunomodulation pattern shown byM. xanthus myxospores.