Marina Davydova

Nonlinear and structural aspects of transitions from damage to fracture in composites and structures

Abstract Statistical theory of defect evolution allows us to obtain nonlinear kinetic equations for tensor parameter of microcrack density. The laws of spatial damage localization on various structural levels are defined by the nonlinearity of the microcrack accumulation in the condition of intensive interaction of the defects. This leads to the multiscale generation of failure centers. The relation between typical nonlinearity of damage kinetics and spatial failure localization is the theoretical background for the explanation of experimental results and numerical simulation of fracture in heterogeneous materials. Topological features of fracture development were investigated numerically using percolation model of failure cluster growth.

Adaptation of Mycoplasmas to Antimicrobial Agents: Acholeplasma laidlawii Extracellular Vesicles Mediate the Export of Ciprofloxacin and a Mutant Gene Related to the Antibiotic Target

This study demonstrated that extracellular membrane vesicles are involved with the development of resistance to fluoroquinolones by mycoplasmas (class Mollicutes). This study assessed the differences in susceptibility to ciprofloxacin among strains of Acholeplasma laidlawii PG8. The mechanisms of mycoplasma resistance to antibiotics may be associated with a mutation in a gene related to the target of quinolones, which could modulate the vesiculation level. A. laidlawii extracellular vesicles mediated the export of the nucleotide sequences of the antibiotic target gene as well as the traffic of ciprofloxacin. These results may facilitate the development of effective approaches to control mycoplasma infections, as well as the contamination of cell cultures and vaccine preparations.

Genomic and proteomic profiles of Acholeplasma laidlawii strains differing in sensitivity to ciprofloxacin.

As a result of comparative analysis of complete genomes as well as cell and vesicular proteomes of A. laidlawii strains differing in sensitivity to ciprofloxacin, it was first shown that the mycoplasma resistance to the antibiotic is associated with the reorganization of genomic and proteomic profiles, which concerns many genes and proteins involved in fundamental cellular processes and realization of bacterial virulence.

Statistical Laws of Dynamic Fragmentation of ZrO2 Ceramics

Dynamic fragmentation of ceramic samples with different porosity were carried out using modified Hopkinson bar setup, which allow us to keep samples safe (in order to define fragment size distribution) and to measure fractoluminescence impulses occurred on the fracture surfaces (in order to establish the distribution of intervals between impulses). The analysis of experimental data reveals that the fragment size distribution and distribution of interval between fractoluminescence impulses obeys a power law, which exponent depends on ceramics porosity.

Extracellular vesicles of mycoplasmas and development of resistance to quinolones in bacteria.

34 Mycoplasmas infect humans, animals, and plants, contaminate cell cultures and vaccine preparations and, hence, represent a serious problem [1]. Myco plasmas quickly become resistant to antibiotics. How ever, despite its low efficiency, antibiotic therapy is the primary tool used in the treatment of mycoplasma infections and decontamination of cell cultures. The most widely used agents are fluoroquinolones—syn thetic antibacterial drugs such as enrofloxacin, ciprof loxacin, and sparfloxacin [2]. The mechanisms under lying the rapid development of mycoplasmal resis tance to fluoroquinolones remain obscure. According to some researchers, the mechanisms of formation of resistance to quinolones known for other bacteria, which are associated with the mutations in the target protein genes and limitation of accumulation of anti microbial drugs in microbial cells, are not the main mechanisms in mycoplasmas [3]. The elucidation of the mechanisms of the rapid development of resis tance to antibiotics and solving the problem of control of mycoplasmal infection and contamination are asso ciated with the studies of the adaptation of mycoplas mas to stress [3–5]. Successful implementation of genome projects for a number of mycoplasmas deter mined the possibility of using postgenomic technolo gies for studying respective processes. A unique species of mycoplasmas in terms of adaptive properties is Acholeplasma laidlawii, the causative agent of phyto mycoplasmoses and the main contaminant of cell cul tures and vaccines. Using transcriptomic and pro teomic analysis and nanoscopy, we for the first time identified the stress reactive proteins and genes of Acholeplasma laidlawii and showed that the adaptation to mycoplasma to stressors is associated with the secretion of extracellular vesicles (EVs) [4, 5]. Bacteria EVs are spherical nanostructures 20–200 nm in diam eter surrounded with a membrane, which mediate the traffic of a many compounds involved in signaling, intercellular interactions, and pathogenesis of infec tions [6]. Recently, it was shown that bacterial EVs may be involved in the development of resistance to antimi crobial agents [7]; however, such studies have not been conducted for mycoplasmas. The purpose of this study was to elucidate the role of extracellular membrane vesicles in the formation of the resistance of mycoplas mas (Acholeplasma laidlawii) to fluoroquinolones (ciprofloxacin).

Genome Sequences of Acholeplasma laidlawii Strains with Increased Resistance to Tetracycline and Melittin

ABSTRACT Acholeplasma laidlawii is a well-suited model for studying the molecular basis for adapting mollicutes to environmental conditions. Here, we present the whole-genome sequences of two strains of A. laidlawii with increased resistance to tetracycline and melittin.

The effect of porosity on fragmentation statistics of dynamically loaded ZrO2 ceramics

The effect of load intensity and porous material structure on the fragmentation statistics of ZrO2(MgO)-based ceramics is studied. Cylindrical samples were fragmented under dynamic compression. Experimental data processing showed that the shape of stress-strain curves, the fragment size distribution and distribution of time intervals between the fractoluminiscense impulses depend on the sample porosity and load intensity. The X-ray Computed Tomography (CT) study of porous material structures allowed us to link the fragmentation statistics with pronounced porosity clustering (about 97% of the total pore volume) formed due to sintering.

Fragmentation of Mansurov granite under quasi-static compression

The fragmentation statistics was studied in the quasi-static compression experiments conducted on prismatic specimens of Mansurov granite. The statistical analysis showed that the cumulative fragment mass distribution for granite specimens is well described by a power law function, but the fragment number-sieve size distribution deviates from the power law in the range of are equal to about 1 mm. In order to explain this fact, we investigated the structure of fractured material (Computed Tomography and microstructures study in thin sections). A sharp increase the number of grains in the range of size from to 0.5 mm to 1 mm and a change in the fragment shape (from splinter to oval) allow us to suppose that there is an addition fracture mechanism associated with disintegration of feldspar and amphiboles grains.