63 research outputs found
Differential growth of wrinkled biofilms
Biofilms are antibiotic-resistant bacterial aggregates that grow on moist
surfaces and can trigger hospital-acquired infections. They provide a classical
example in biology where the dynamics of cellular communities may be observed
and studied. Gene expression regulates cell division and differentiation, which
affect the biofilm architecture. Mechanical and chemical processes shape the
resulting structure. We gain insight into the interplay between cellular and
mechanical processes during biofilm development on air-agar interfaces by means
of a hybrid model. Cellular behavior is governed by stochastic rules informed
by a cascade of concentration fields for nutrients, waste and autoinducers.
Cellular differentiation and death alter the structure and the mechanical
properties of the biofilm, which is deformed according to Foppl-Von Karman
equations informed by cellular processes and the interaction with the
substratum. Stiffness gradients due to growth and swelling produce wrinkle
branching. We are able to reproduce wrinkled structures often formed by
biofilms on air-agar interfaces, as well as spatial distributions of
differentiated cells commonly observed with B. subtilis.Comment: 19 pages, 13 figure
Size effects in the magnetic behaviour of TbAl_2 milled alloys
The study of the magnetic properties depending upon mechanical milling of the
ferromagnetic polycrystalline TbAl_2 material is reported. The Rietveld
analysis of the X-ray diffraction data reveals a decrease of the grain size
down to 14 nm and -0.15 % of variation of the lattice parameter, after 300
hours of milling time. Irreversibility in the zero field cooled - field cooled
(ZFC-FC) DC-susceptibility and clear peaks in the AC susceptibility between 5
and 300 K show that the long-range ferromagnetic structure is inhibited in
favour of a disordered spin arrangement below 45 K. This glassy behaviour is
also deduced from the variation of the irreversibility transition with the
field (H^{2/3}) and frequency. The magnetization process of the bulk TbAl_2 is
governed by domain wall thermal activation processes. By contrast, in the
milled samples, cluster-glass properties arise as a result of cooperative
interactions due to the substitutional disorder. The interactions are also
influenced by the nanograin structure of the milled alloys, showing a variation
of coercivity with the grain size, below the crossover between the multi- and
single-domain behaviours.Comment: 23 pages, 11 figures, to appear in J. Phys.: Condens. Ma
Influence of primary particle density in the morphology of agglomerates
Agglomeration processes occur in many different realms of science such as
colloid and aerosol formation or formation of bacterial colonies. We study the
influence of primary particle density in agglomerate structure using
diffusion-controlled Monte Carlo simulations with realistic space scales
through different regimes (DLA and DLCA). The equivalence of Monte Carlo time
steps to real time scales is given by Hirsch's hydrodynamical theory of
Brownian motion. Agglomerate behavior at different time stages of the
simulations suggests that three indices (fractal exponent, coordination number
and eccentricity index) characterize agglomerate geometry. Using these indices,
we have found that the initial density of primary particles greatly influences
the final structure of the agglomerate as observed in recent experimental
works.Comment: 11 pages, 13 figures, PRE, to appea
First-order nature of the ferromagnetism in CeIn2 investigated using muon spin rotation and by systematic substitution of La for Ce
he nature of the first-order ferromagnetic transition in binary CeIn2 alloy is investigated by muon spin rotation (μSR) measurements and chemical substitution of Ce by La in the La1−xCexIn2 (0.9 x 1.0) series of alloys. Below 22 K, the analysis of μSR spectra shows two spin precession frequencies associated with the local field at the muon site created by the surrounding ferromagnetic ordered magnetic moments. These frequencies abruptly disappear above TC, indicating the first-order character of this transition, as previously reported. For temperatures between 22 and 24 K, the shape of the μSR spectra indicates the existence of an additional magnetic phase with features of an incommensurate magnetic structure. The presence of this magnetic phase is supported by dc(ac)-magnetic susceptibility and specific-heat results obtained on chemical diluted samples,which also show a magnetic contribution above the ferromagnetic transition. The combined analysis of these results clarifies the first-order character of the ferromagnetic transition in CeIn2, based on the existence of an intermediate magnetic phase between the paramagnetic and ferromagnetic state
The Pochonia chlamydosporia Serine Protease Gene vcp1 Is Subject to Regulation by Carbon, Nitrogen and pH: Implications for Nematode Biocontrol
The alkaline serine protease VCP1 of the fungus Pochonia chlamydosporia belongs to a family of subtilisin-like enzymes that are involved in infection of nematode and insect hosts. It is involved early in the infection process, removing the outer proteinaceous vitelline membrane of nematode eggs. Little is known about the regulation of this gene, even though an understanding of how nutrients and other factors affect its expression is critical for ensuring its efficacy as a biocontrol agent. This paper provides new information on the regulation of vcp1 expression. Sequence analysis of the upstream regulatory region of this gene in 30 isolates revealed that it was highly conserved and contained sequence motifs characteristic of genes that are subject to carbon, nitrogen and pH-regulation. Expression studies, monitoring enzyme activity and mRNA, confirmed that these factors affect VCP1 production. As expected, glucose reduced VCP1 expression and for a few hours so did ammonium chloride. Surprisingly, however, by 24 h VCP1 levels were increased in the presence of ammonium chloride for most isolates. Ambient pH also regulated VCP1 expression, with most isolates producing more VCP1 under alkaline conditions. There were some differences in the response of one isolate with a distinctive upstream sequence including a variant regulatory-motif profile. Cryo-scanning electron microscopy studies indicated that the presence of nematode eggs stimulates VCP1 production by P. chlamydosporia, but only where the two are in close contact. Overall, the results indicate that readily-metabolisable carbon sources and unfavourable pH in the rhizosphere/egg-mass environment may compromise nematode parasitism by P. chlamydosporia. However, contrary to previous indications using other nematophagous and entomopathogenic fungi, ammonium nitrate (e.g. from fertilizers) may enhance biocontrol potential in some circumstances
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