Revealing the Hidden Spin-Polarized Bands in a Superconducting Tl Bilayer Crystal.

The interplay of spin-orbit coupling and crystal symmetry can generate spin-polarized bands in materials only a few atomic layers thick, potentially leading to unprecedented physical properties. In the case of bilayer materials with global inversion symmetry, locally broken inversion symmetry can generate degenerate spin-polarized bands, in which the spins in each layer are oppositely polarized. Here, we demonstrate that the hidden spins in a Tl bilayer crystal are revealed by growing it on Ag(111) of sizable lattice mismatch, together with the appearance of a remarkable phenomenon unique to centrosymmetric hidden-spin bilayer crystals: a novel band splitting in both spin and space. The key to success in observing this novel splitting is that the interaction at the interface has just the right strength: it does not destroy the original wave functions of the Tl bilayer but is strong enough to induce an energy separation.

Gut bacteria identified in colorectal cancer patients promote tumourigenesis via butyrate secretion.

Emerging evidence is revealing that alterations in gut microbiota are associated with colorectal cancer (CRC). However, very little is currently known about whether and how gut microbiota alterations are causally associated with CRC development. Here we show that 12 faecal bacterial taxa are enriched in CRC patients in two independent cohort studies. Among them, 2 Porphyromonas species are capable of inducing cellular senescence, an oncogenic stress response, through the secretion of the bacterial metabolite, butyrate. Notably, the invasion of these bacteria is observed in the CRC tissues, coinciding with the elevation of butyrate levels and signs of senescence-associated inflammatory phenotypes. Moreover, although the administration of these bacteria into ApcΔ14/+ mice accelerate the onset of colorectal tumours, this is not the case when bacterial butyrate-synthesis genes are disrupted. These results suggest a causal relationship between Porphyromonas species overgrowth and colorectal tumourigenesis which may be due to butyrate-induced senescence.

Efficacy of antimicrobial photodynamic therapy with Rose Bengal and blue light against cariogenic bacteria.

OBJECTIVE: We evaluated the effect of antimicrobial photodynamic therapy (a-PDT) with Rose Bengal and blue light LED on bacteria that initiate and promote dental caries. DESIGN: Colony forming units of Streptococcus mutans, Streptococcus sobrinus, Streptococcus sanguinis, and Lactobacillus salivarius under planktonic and biofilm conditions were counted after a-PDT treatment using Rose Bengal and blue light LED. In addition, cariogenic bacteria from saliva and dental plaques from ten volunteers were used for evaluation of a-PDT treatment. RESULTS: We found that a-PDT using Rose Bengal at > 10 µg/mL had antimicrobial effects on oral Gram-positive S. mutans, S. sobrinus, S. sanguinis, and L. salivarius under both planktonic and biofilm conditions. The effect was also observed for cariogenic bacteria that formed biofilms containing water-insoluble glucans, through which the bacteria are firmly attached to the tooth surface. Moreover, a-PDT led to a marked reduction in cariogenic bacteria in saliva and dental plaques. CONCLUSION: a-PDT could be a useful approach for controlling dental caries in dental surgery.

Analysis of the Butyrate-Producing Pathway in Porphyromonas gingivalis.

Butyrate is one of the most harmful metabolic end products found in the oral cavity. Thus, it would be important to characterize the enzymes responsible for production of this metabolite to elucidate the pathogenicity of periodontogenic bacteria. Here, a spectrophotometric assay for butyryl-CoA:acetate CoA transferase activity and gas chromatography-mass spectrometry measurement of butyrate and other short chain fatty acids such as acetate, propionate, isobutyrate, and isovalerate are described.

Scanning tunneling microscopy on cleaved Mn3Sn(0001) surface.

We have studied in-situ cleaved (0001) surfaces of the magnetic Weyl semimetal Mn3Sn by low-temperature scanning tunneling microscopy and spectroscopy (STM/S). It was found that freshly cleaved Mn3Sn surfaces are covered with unknown clusters, and the application of voltage pulses in the tunneling condition was needed to achieve atomically flat surfaces. STM topographs taken on the flat terrace show a bulk-terminated 1 × 1 honeycomb lattice with the Sn site brightest. First-principles calculations reveal that the brightest contrast at the Sn site originates from the surrounding surface Mn d orbitals. Tunneling spectroscopy performed on the as-cleaved and voltage-pulsed surfaces show a prominent semimetal valley near the Fermi energy.

Characterization of the phosphotransacetylase-acetate kinase pathway for ATP production in Porphyromonas gingivalis.

Acetyl phosphate (AcP) is generally produced from acetyl coenzyme A by phosphotransacetylase (Pta), and subsequent reaction with ADP, catalyzed by acetate kinase (Ack), produces ATP. The mechanism of ATP production in Porphyromonas gingivalis is poorly understood. The aim of this study was to explore the molecular basis of the Pta-Ack pathway in this microorganism. Pta and Ack from P. gingivalis ATCC 33277 were enzymatically and structurally characterized. Structural and mutational analyses suggest that Pta is a dimer with two substrate-binding sites in each subunit. Ack is also dimeric, with a catalytic cleft in each subunit, and structural analysis indicates a dramatic domain motion that opens and closes the cleft during catalysis. ATP formation by Ack proceeds via a sequential mechanism. Reverse transcription-PCR analysis demonstrated that the pta (PGN_1179) and ack (PGN_1178) genes, tandemly located in the genome, are cotranscribed as an operon. Inactivation of pta or ack in P. gingivalis by homologous recombination was successful only when the inactivated gene was expressed in trans. Therefore, both pta and ack genes are essential for this microorganism. Insights into the Pta-Ack pathway reported herein would be helpful to understand the energy acquisition in P. gingivalis.

Bulk ferromagnetic tips for spin-polarized scanning tunneling microscopy.

We characterized the performance of electrochemically etched bulk Fe and Ni tips as a probe of spin-polarized scanning tunneling microscopy (SP-STM). Through the observation of the striped contrast on the conical spin-spiral structure formed in Mn double layers on a W(110) substrate, the capability of both the tips to detect the magnetic signal was clarified. We also confirmed that the magnetized direction of the Fe and Ni tips can be flipped between the two out-of-plane directions by external magnetic fields. Our results demonstrate that the ex-situ prepared tips are reliable in SP-STM for the samples that are not susceptible to a stray magnetic field.

Biological and Biochemical Roles of Two Distinct Cyclic Dimeric Adenosine 3',5'-Monophosphate- Associated Phosphodiesterases in Streptococcus mutans.

Cyclic dimeric adenosine 3',5'-monophosphate (c-di-AMP), a recently identified secondary messenger in bacteria, plays a role in several bacterial processes, including biofilm formation. It is enzymatically produced by diadenylate cyclase and cleaved by c-di-AMP phosphodiesterase. c-di-AMP is believed to be essential for the viability of bacterial cells that produce it. In the current study, the biochemical and biological roles of GdpP (SMU_2140c) and DhhP (SMU_1297), two distinct Streptococcus mutans phosphodiesterases involved in the pathway producing AMP from c-di-AMP, were investigated. Liquid chromatography-tandem mass spectrometry revealed that c-di-AMP was degraded to phosphoadenylyl adenosine (pApA) by truncated recombinant GdpP, and pApA was cleaved by recombinant DhhP to yield AMP. In-frame deletion mutants lacking the dhhP gene (ΔdhhP) and both the gdpP and dhhP genes (ΔgdpPΔdhhP) displayed significantly more biofilm formation than the wild-type and a mutant strain lacking the gdpP gene (ΔgdpP; p < 0.01). Furthermore, biofilm formation was restored to the level of the wild type strain upon complementation with dhhP. Optical and electron microscopy observations revealed that ΔdhhP and ΔgdpPΔdhhP mutants self-aggregated into large cell clumps, correlated with increased biofilm formation, but cell clumps were not observed in cultures of wild-type, ΔgdpP, or strains complemented with gdpP and dhhP. Thus, deletion of dhhP presumably leads to the formation of bacterial cell aggregates and a subsequent increase in biofilm production.

Structural insights into the catalytic mechanism of cysteine (hydroxyl) lyase from the hydrogen sulfide-producing oral pathogen, Fusobacterium nucleatum.

Hydrogen sulfide (H2S) plays important roles in the pathogenesis of periodontitis. Oral pathogens typically produce H2S from l-cysteine in addition to pyruvate and [Formula: see text] However, fn1055 from Fusobacterium nucleatum subsp. nucleatum ATCC 25586 encodes a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes the production of H2S and l-serine from l-cysteine and H2O, an unusual cysteine (hydroxyl) lyase reaction (ß-replacement reaction). To reveal the reaction mechanism, the crystal structure of substrate-free Fn1055 was determined. Based on this structure, a model of the l-cysteine-PLP Schiff base suggested that the thiol group forms hydrogen bonds with Asp232 and Ser74, and the substrate α-carboxylate interacts with Thr73 and Gln147 Asp232 is a unique residue to Fn1055 and its substitution to asparagine (D232N) resulted in almost complete loss of ß-replacement activity. The D232N structure obtained in the presence of l-cysteine contained the α-aminoacrylate-PLP Schiff base in the active site, indicating that Asp232 is essential for the addition of water to the α-aminoacrylate to produce the l-serine-PLP Schiff base. Rapid-scan stopped-flow kinetic analyses showed an accumulation of the α-aminoacrylate intermediate during the reaction cycle, suggesting that water addition mediated by Asp232 is the rate-limiting step. In contrast, mutants containing substitutions of other active-site residues (Ser74, Thr73, and Gln147) exhibited reduced ß-replacement activity by more than 100-fold. Finally, based on the structural and biochemical analyses, we propose a mechanism of the cysteine (hydroxyl) lyase reaction by Fn1055. The present study leads to elucidation of the H2S-producing mechanism in F. nucleatum.

Guided Molecular Assembly on a Locally Reactive 2D Material.

Atomically precise engineering of the position of molecular adsorbates on surfaces of 2D materials is key to their development in applications ranging from catalysis to single-molecule spintronics. Here, stable room-temperature templating of individual molecules with localized electronic states on the surface of a locally reactive 2D material, silicene grown on ZrB2 , is demonstrated. Using a combination of scanning tunneling microscopy and density functional theory, it is shown that the binding of iron phthalocyanine (FePc) molecules is mediated via the strong chemisorption of the central Fe atom to the sp3 -like dangling bond of Si atoms in the linear silicene domain boundaries. Since the planar Pc ligand couples to the Fe atom mostly through the in-plane d orbitals, localized electronic states resembling those of the free molecule can be resolved. Furthermore, rotation of the molecule is restrained because of charge rearrangement induced by the bonding. These results highlight how nanoscale changes can induce reactivity in 2D materials, which can provide unique surface interactions for enabling novel forms of guided molecular assembly.

Experimental verification of the rotational type of chiral spin spiral structures by spin-polarized scanning tunneling microscopy.

We report on experimental verification of the rotational type of chiral spin spirals in Mn thin films on a W(110) substrate using spin-polarized scanning tunneling microscopy (SP-STM) with a double-axis superconducting vector magnet. From SP-STM images using Fe-coated W tips magnetized to the out-of-plane and [001] directions, we found that both Mn mono- and double-layers exhibit cycloidal rotation whose spins rotate in the planes normal to the propagating directions. Our results agree with the theoretical prediction based on the symmetry of the system, supporting that the magnetic structures are driven by the interfacial Dzyaloshinskii-Moriya interaction.

Atomic-scale visualization of surface-assisted orbital order.

Orbital-related physics attracts growing interest in condensed matter research, but direct real-space access of the orbital degree of freedom is challenging. We report a first, real-space, imaging of a surface-assisted orbital ordered structure on a cobalt-terminated surface of the well-studied heavy fermion compound CeCoIn5. Within small tip-sample distances, the cobalt atoms on a cleaved (001) surface take on dumbbell shapes alternatingly aligned in the [100] and [010] directions in scanning tunneling microscopy topographies. First-principles calculations reveal that this structure is a consequence of the staggered d xz -d yz orbital order triggered by enhanced on-site Coulomb interaction at the surface. This so far overlooked surface-assisted orbital ordering may prevail in transition metal oxides, heavy fermion superconductors, and other materials.

Novel fimbrilin PGN_1808 in Porphyromonas gingivalis.

Porphyromonas gingivalis, a periodontopathic gram-negative anaerobic bacterium, generally expresses two types of fimbriae, FimA and Mfa1. However, a novel potential fimbrilin, PGN_1808, in P. gingivalis strain ATCC 33277 was recently identified by an in silico structural homology search. In this study, we experimentally examined whether the protein formed a fimbrial structure. Anion-exchange chromatography showed that the elution peak of the protein was not identical to those of the major fimbrilins of FimA and Mfa1, indicating that PGN_1808 is not a component of these fimbriae. Electrophoretic analyses showed that PGN_1808 formed a polymer, although it was detergent and heat labile compared to FimA and Mfa1. Transmission electron microscopy showed filamentous structures (2‒3 nm × 200‒400 nm) on the cell surfaces of a PGN_1808-overexpressing P. gingivalis mutant (deficient in both FimA and Mfa1 fimbriae) and in the PGN_1808 fraction. PGN_1808 was detected in 81 of 84 wild-type strains of P. gingivalis by western blotting, suggesting that the protein is generally present in P. gingivalis.

Comparative analysis of motility and other properties of Treponema denticola strains.

The periodontitis-associated pathogen Treponema denticola is a spirochetal bacterium that swims by rotating its cell body like a corkscrew using periplasmic flagella. We compared physiologic and pathogenic properties, including motility, in four strains of T. denticola. Phase-contrast microscopy showed differential motility between the strains; ATCC 35404 showed the highest motility, followed by ATCC 33521, and the remaining two strains (ATCC 35405 and ATCC 33520) showed the lowest motility. Transmission electron microscopy showed that the low motility strains exhibited extracellular flagellar protrusions resulting from elongated flagella. Treponemal flagellar filaments are composed of three flagellins of FlaB1, FlaB2 and FlaB3. FlaB1 expression was comparable between the strains, whereas FlaB2 expression was lowest in ATCC 35404. FlaB3 expression varied among strains, with ATCC 35405, ATCC 33520, ATCC 33521, and ATCC 35404 showing the highest to lowest expression levels, respectively. Additionally, the low motility strains showed faster electrophoretic mobility of FlaB3, suggesting that posttranslational modifications of these proteins may have varied, because the amino acid sequences of FlaB3 were identical between the strains. These results suggest that inappropriate expression of FlaB2 and FlaB3 caused the unusual elongation of flagella that resulted in decreased motility. Furthermore, the low motility strains grew to higher bacterial density, and showed greater chymotrypsin-like protease activity, and more bacterial cells associated with gingival epithelial cells in comparison with the high motility strains. There may be a relationship between motility and these properties, but the genetic factors underlying this association remain unclear.

Three CoA Transferases Involved in the Production of Short Chain Fatty Acids in Porphyromonas gingivalis.

Butyryl-CoA:acetate CoA transferase, which produces butyrate and acetyl-CoA from butyryl-CoA and acetate, is responsible for the final step of butyrate production in bacteria. This study demonstrates that in the periodontopathogenic bacterium Porphyromonas gingivalis this reaction is not catalyzed by PGN_1171, previously annotated as butyryl-CoA:acetate CoA transferase, but by three distinct CoA transferases, PGN_0725, PGN_1341, and PGN_1888. Gas chromatography/mass spectrometry (GC-MS) and spectrophotometric analyses were performed using crude enzyme extracts from deletion mutant strains and purified recombinant proteins. The experiments revealed that, in the presence of acetate, PGN_0725 preferentially utilized butyryl-CoA rather than propionyl-CoA. By contrast, this preference was reversed in PGN_1888. The only butyryl-CoA:acetate CoA transferase activity was observed in PGN_1341. Double reciprocal plots revealed that all the reactions catalyzed by these enzymes follow a ternary-complex mechanism, in contrast to previously characterized CoA transferases. GC-MS analysis to determine the concentrations of short chain fatty acids (SCFAs) in culture supernatants of P. gingivalis wild type and mutant strains revealed that PGN_0725 and PGN_1888 play a major role in the production of butyrate and propionate, respectively. Interestingly, a triple deletion mutant lacking PGN_0725, PGN_1341, and PGN_1888 produced low levels of SCFAs, suggesting that the microorganism contains CoA transferase(s) in addition to these three enzymes. Growth rates of the mutant strains were mostly slower than that of the wild type, indicating that many carbon compounds produced in the SCFA synthesis appear to be important for the biological activity of this microorganism.

Acyl-CoA reductase PGN_0723 utilizes succinyl-CoA to generate succinate semialdehyde in a butyrate-producing pathway of Porphyromonas gingivalis.

The molecular basis of butyrate production in Porphyromonas gingivalis has not been fully elucidated, even though butyrate, a short chain fatty acid (SCFA), can exert both beneficial and harmful effects on a mammalian host. A database search showed that the amino acid sequence of PGN_0723 protein was 50.6% identical with CoA-dependent succinate semialdehyde dehydrogenase (SSADH) in Clostridium kluyveri. By contrast, the protein has limited identity (19.1%) with CoA-independent SSADH in Escherichia coli. Compared with the wild type, growth speed, and final turbidity were lower in the PGN_0723 deletion strain that was constructed by replacing the PGN_0723 gene with an erythromycin resistance cassette. Gas chromatography mass spectrometry revealed the supernatant concentrations of the SCFAs butyrate, isobutyrate, and isovalerate, but not propionate, in the PGN_0723 deletion strain were also lower than those in the wild type. The wild-type phenotype was restored in a complemented strain. We cloned the PGN_0723 gene, purified the recombinant protein, and computationally constructed its three-dimensional model. A colorimetric assay and liquid chromatography-tandem mass spectrometry analysis demonstrated that the recombinant PGN_0723 produces succinate semialdehyde, which is an intermediate in the P. gingivalis butyrate synthesis pathway, not from succinate but from succinyl-CoA in the presence of NAD(P)H via a ping-pong bi-bi mechanism. Asn110Ala and Cys239Ala mutations resulted in a significant loss of the CoA-dependent PGN_0723 enzymatic activity. The study provides new insights into butyrate production, which constitutes a virulence factor in P. gingivalis.

Production of 4-hydroxybutyrate from succinate semialdehyde in butyrate biosynthesis in Porphyromonas gingivalis.

BACKGROUND: Despite evidence demonstrating the importance of butyrate-producing bacteria in host health and disease, the characterization of enzymes responsible for butyrate production has not been fully elucidated in the periodontopathogen, Porphyromonas gingivalis. METHODS: LC-MS/MS and colorimetric analyses were employed to enzymatically characterize recombinant PGN_0724 in P. gingivalis as a succinate semialdehyde reductase. The concentration of short chain fatty acids in the culture supernatant of the wild-type bacteria and a mutant strain lacking the PGN_0724 gene were quantified using GC-MS. RESULTS: Incubation of recombinant PGN_0724 with succinate semialdehyde and NADH resulted in the production of 4-hydroxybutyrate as well as consumption of succinate semialdehyde. Double reciprocal plots showed that the reaction catalyzed by the PGN_0724 protein was associated with a ternary complex mechanism. The growth speed and final turbidity of the mutant strain were much lower than those of the wild-type cells. The capacity of the mutant strain to produce butyrate, isobutyrate, and isovalerate was 30%, 15%, and 45%, respectively, of that of the wild-type strain, while the mutant strain produced approximately 3.9-fold more propionate than the wild type. CONCLUSIONS: The pathway responsible for butyrate production is important for the growth of P. gingivalis and appears to be associated with production of the other short chain fatty acids. GENERAL SIGNIFICANCE: The aim of this study was to delineate the mechanisms involved in the production of 4-hydroxybutyrate, which is an intermediate in the biosynthetic pathway for production of butyrate, which is a virulence factor in P. gingivalis.

Mfa4, an Accessory Protein of Mfa1 Fimbriae, Modulates Fimbrial Biogenesis, Cell Auto-Aggregation, and Biofilm Formation in Porphyromonas gingivalis.

Porphyromonas gingivalis, a gram-negative obligate anaerobic bacterium, is considered to be a key pathogen in periodontal disease. The bacterium expresses Mfa1 fimbriae, which are composed of polymers of Mfa1. The minor accessory components Mfa3, Mfa4, and Mfa5 are incorporated into these fimbriae. In this study, we characterized Mfa4 using genetically modified strains. Deficiency in the mfa4 gene decreased, but did not eliminate, expression of Mfa1 fimbriae. However, Mfa3 and Mfa5 were not incorporated because of defects in posttranslational processing and leakage into the culture supernatant, respectively. Furthermore, the mfa4-deficient mutant had an increased tendency to auto-aggregate and form biofilms, reminiscent of a mutant completely lacking Mfa1. Notably, complementation of mfa4 restored expression of structurally intact and functional Mfa1 fimbriae. Taken together, these results indicate that the accessory proteins Mfa3, Mfa4, and Mfa5 are necessary for assembly of Mfa1 fimbriae and regulation of auto-aggregation and biofilm formation of P. gingivalis. In addition, we found that Mfa3 and Mfa4 are processed to maturity by the same RgpA/B protease that processes Mfa1 subunits prior to polymerization.

Characterization of Treponema denticola mutants defective in the major antigenic proteins, Msp and TmpC.

Treponema denticola, a gram-negative and anaerobic spirochete, is associated with advancing severity of chronic periodontitis. In this study, we confirmed that two major antigenic proteins were Msp and TmpC, and examined their physiological and pathological roles using gene-deletion mutants. Msp formed a large complex that localized to the outer membrane, while TmpC existed as a monomer and largely localized to the inner membrane. However, TmpC was also detected in the outer membrane fraction, but its cell-surface exposure was not detected. Msp defects increased cell-surface hydrophobicity and secretion of TNF-α from macrophage-like cells, whereas TmpC defects decreased autoagglutination and chymotrypsin-like protease activities. Both mutants adhered to gingival epithelial cells similarly to the wild-type and showed slightly decreased motility. In addition, in Msp-defective mutants, the TDE1072 protein, which is a major membrane protein, was abolished; therefore, phenotypic changes in the mutant can be, at least in part, attributed to the loss of the TDE1072 protein. Thus, the major antigenic proteins, Msp and TmpC, have significant and diverse impacts on the characteristics of T. denticola, especially cell surface properties.

Scanning tunneling microscopy/spectroscopy of picene thin films formed on Ag(111).

Using ultrahigh-vacuum low-temperature scanning tunneling microscopy and spectroscopy combined with first principles density functional theory calculations, we have investigated structural and electronic properties of pristine and potassium (K)-deposited picene thin films formed in situ on a Ag(111) substrate. At low coverages, the molecules are uniformly distributed with the long axis aligned along the [112̄] direction of the substrate. At higher coverages, ordered structures composed of monolayer molecules are observed, one of which is a monolayer with tilted and flat-lying molecules resembling a (11̄0) plane of the bulk crystalline picene. Between the molecules and the substrate, the van der Waals interaction is dominant with negligible hybridization between their electronic states; a conclusion that contrasts with the chemisorption exhibited by pentacene molecules on the same substrate. We also observed a monolayer picene thin film in which all molecules were standing to form an intermolecular π stacking. Two-dimensional delocalized electronic states are found on the K-deposited π stacking structure.