Phenogenetic profile and agronomic contribution of Azospirillum argentinense Az39T, a reference strain for the South American inoculant industry

Azospirillum sp. is a plant growth-promoting rhizobacteria largely recognized for its potential to increase the yield of different important crops. In this work, we present a thorough genomic and phenotypic analysis of A. argentinense Az39T to provide new insights into the beneficial mechanisms of this microorganism. Phenotypic analyses revealed the following in vitro abilities: growth at 20-38 °C (optimum, 28 °C), pH 6.0-8.0 (optimum, pH 6.8), and in the presence of 1% (w/v) NaCl; production of variable amounts of PHB as intracellular granules; nitrogen fixation under microaerophilic conditions; IAA synthesis in the presence of L-tryptophan. Through biochemical (API 20NE) and carbon utilization profiling (Biolog) assays, we proved that A. argentinense Az39T is able to use 15 substrates and metabolize 19 different carbon substrates. Lipid composition indicated a predominance of medium and long-chain saturated fatty acids. A total of 6 replicons classified as one main chromosome, three chromids, and two plasmids, according to their tRNA and core essential genes contents, were identified. Az39T genome includes genes associated with multiple plant growth-promoting (PGP) traits such as nitrogen fixation and production of auxins, cytokinin, abscisic acid, ethylene, and polyamines. In addition, Az39T genome harbor genetic elements associated with physiological features that facilitate its survival in the soil and competence for rhizospheric colonization; this includes motility, secretion system, and quorum sensing genetic determinants. A metadata analysis of Az39T agronomic performance in the pampas region, Argentina, demonstrated significant grain yield increases in wheat and maize, proving its potential to provide better growth conditions for dryland cereals. In conclusion, our data provide a detailed insight into the metabolic profile of A. argentinense Az39T, the strain most widely used to formulate non-legume inoculants in Argentina, and allow a better understanding of the mechanisms behind its field performance.

Unraveling Azospirillum's colonization ability through microbiological and molecular evidence

It is known that members of the bacterial genus Azospirillum can promote the growth of a great variety of plants, an ability harnessed by the industry to create bioproducts aimed to enhance the yield of economically relevant crops. Its versatile metabolism allows this bacterium to adapt to numerous environments, from optimal to extreme or highly polluted. The fact of having been isolated from soil and rhizosphere samples collected worldwide and many other habitats proves its remarkable ubiquity. Azospirillum rhizospheric and endophytic lifestyles are governed by several mechanisms, leading to efficient niche colonization. These mechanisms include cell aggregation and biofilm formation, motility, chemotaxis, phytohormone and other signaling molecules production, and cell-to-cell communication, in turn, involved in regulating Azospirillum interactions with the surrounding microbial community. Despite being infrequently mentioned in metagenomics studies after its introduction as an inoculant, an increasing number of studies detected Azospirillum through molecular tools (mostly 16S rRNA sequencing) as part of diverse, even unexpected, microbiomes. This review focuses on Azospirillum traceability and the performance of the available methods, both classical and molecular. An overview of Azospirillum occurrence in diverse microbiomes and the less-known features explaining its notorious ability to colonize niches and prevail in multiple environments is provided.

Genome-based reclassification of Azospirillum brasilense Az39 as the type strain of Azospirillum argentinense sp. nov

Strain Az39T of Azospirillum is a diazotrophic plant growth-promoting bacterium isolated in 1982 from the roots of wheat plants growing in Marcos Juárez, Córdoba, Argentina. It produces indole-3-acetic acid in the presence of l-tryptophan as a precursor, grows at 20–38 °C (optimal 38 °C), and the cells are curved or spiral-shaped, with diameters ranging from 0.5–0.9 to 1.8–2.2 µm. They contain C16 : 0, C18 : 0 and C18 : 1  ω7c/ω6c as the main fatty acids. Phylogenetic analysis of its 16S rRNA gene sequence confirmed that this strain belongs to the genus Azospirillum , showing a close relationship with Azospirillum baldaniorum Sp245T, Azospirillum brasilense Sp7T and Azospirillum formosense CC-Nfb-7T. Housekeeping gene analysis revealed that Az39T, together with five strains of the genus (Az19, REC3, BR 11975, MTCC4035 and MTCC4036), form a cluster apart from A. baldaniorum Sp245T, A. brasilense Sp7T and A. formosense CC-Nfb-7T. Average nucleotide identity (ANI) and digital DNA–DNA hybridization (dDDH) between Az39T and the aforementioned type strains revealed values below 96 %, the circumscription limit for the species delineation (ANI: 95.3, 94.1 and 94.0 %; dDDH: 62.9, 56.3 and 55.6 %). Furthermore, a phylogeny evaluation of the core proteome, including 809 common shared proteins, showed an independent grouping of Az39T, Az19, REC3, BR 11975, MTCC4035 and MTCC4036. The G+C content in the genomic DNA of these six strains varied from 68.3 to 68.5 %. Based on the combined phylogenetic, genomic and phenotypic characterization presented here, we consider that strain Az39T, along with strains Az19, REC3, BR 11975, MTCC4035 and MTCC4036, are members of a new Azospirillum species, for which the name Azospirillum argentinense sp. nov. is proposed. The type strain is Az39T (=LBPCV39T=BR 148428T=CCCT 22.01T).

The Azospirillum brasilense type VI secretion system promotes cell aggregation, biocontrol protection against phytopathogens and attachment to the microalgae Chlorella sorokiniana

The plant-growth-promoting bacterium Azospirillum brasilense is able to associate with the microalgae Chlorella sorokiniana. Attachment of A. brasilense increases the metabolic performances of the microalgae. Recent genome analyses have revealed that the A. brasilense Az39 genome contains two complete sets of genes encoding type VI secretion systems (T6SS), including the T6SS1 that is induced by the indole-3-acetic acid (IAA) phytohormone. The T6SS is a multiprotein machine, widespread in Gram-negative bacteria, that delivers protein effectors in both prokaryotic and eukaryotic cells. Here we show that the A. brasilense T6SS is required for Chlorella-Azospirillum synthetic mutualism. Our data demonstrate that the T6SS is an important determinant to promote production of lipids, carbohydrates and photosynthetic pigments by the microalgae. We further show that this is likely due to the role of the T6SS during the attachment stage and for the production of IAA phytohormones. Finally, we demonstrate that the A. brasilense T6SS provides antagonistic activities against a number of plant pathogens such as Agrobacterium, Pectobacterium, Dickeya and Ralstonia species in vitro, suggesting that, in addition to promoting growth, A. brasilense might confer T6SS-dependent bio-control protection to microalgae and plants against bacterial pathogens.

What Do We Know About the Publications Related with Azospirillum? A Metadata Analysis

Azospirillum is one of the most successful plant growth-promoting bacteria (PGPB) genera and it is considered a study model for plant-bacteria interactions. Because of that, a wide broad of topics has been boarded and discussed in a significant number of publications in the last four decades. Using the Scopus® database, we conducted a bibliographic search in order to analyze the number and type of publications, the authors responsible of these contributions, and the origin of the researchers, as well as the keywords and journals selected by the authors, among other related characteristics, with the aim to understand some less addressed details about the work done with Azospirillum worldwide since its discovery in 1925. Despite that the largest numbers of publications about this bacterium were obtained between the 1970 and 1980s, there is still a linear increase tendency in the number of published works. Understanding the mechanisms involved in the ability of these bacteria to promote growth in a wide broad of plant species under both laboratory and field conditions has been a preferential target for these published articles. This tendency could be considered a cause or consequence of the current increase in the number of commercial products formulated with Azospirillum around the world and a catalyzer for the increase of published articles along time.

Day and blue light modify growth, cell physiology and indole-3-acetic acid production of Azospirillum brasilense Az39 under planktonic growth conditions

AIM

In this work, we evaluated the effects of light on growth, cell physiology and stress response of Azospirillum brasilense Az39, a non-photosynthetic rhizobacteria, under planktonic growth conditions.

METHODS AND RESULTS

Exponential cultures of Az39 were exposed to blue (BL), red (RL) and daylight (DL) or maintained in darkness for 24, 48 and 72 h. The biomass production and indole 3-acetic acid (IAA) biosynthesis increased by exposition to DL. Conversely, BL decreased IAA concentration through a direct effect on the molecule. The DL increased superoxide dismutase activity, hydrogen peroxide and thiobarbituric acid reactive substances levels, but the last one was also increased by BL. Both DL and BL increased cell aggregation but only BL increased biofilm formation.

CONCLUSIONS

We demonstrated that both BL and DL are stress effectors for A. brasilense Az39 under planktonic growth conditions. The DL increased biomass production, IAA biosynthesis and bacterial response to stress, whereas BL induced cell aggregation and biofilms formation, but decreased the IAA concentration by photooxidation.

SIGNIFICANCE AND IMPACT OF THE STUDY

Blue light and DL changes growth capacity, cell physiology and plant growth promotion ability of A. brasilense Az39 and these changes could be considered to improve the production and functionality of biofertilizers.

[Development of sequence characterized amplified region markers for identification of Azospirillum brasilense Az39]

Azospirillum brasilense Az39 has been used since more than 30 years by several companies in South America for biofertilizers production. This strain may promote plants growth and development, as well as the ability of inoculated plants to tolerate environmental stresses, which determines an increase in the productivity under field conditions. At present, there are no protocols in Argentina to confirm the identity of Az39 in commercial products; however, such biofertilizers are formulated almost exclusively with this strain. Therefore, the objective of this paper was to develop a molecular methodology that allows the accurate identification of A. brasilense Az39. Using the complete genome sequence and several bioinformatics tools, fragments of DNA present only in the Az39 genome were recognized. A set of PCR primers to amplify these sequences were designed, and the specific products were observed only in the strain of our interest. The sensitivity of the methodology was evaluated, where the strain could be detected up to a titer of 10⁵ CFU/ml (4.5 ng/μl ADN) or 10² CFU/ml (0.88 ng/μl DNA) or in a minimal concentration of 0.098 ng/μl DNA, depending on the DNA extraction methodology used. Primers were tested against direct samples of commercial inoculants and cultures, in both cases there were specifics products, both in direct samples and in confirmatory tests from isolated colonies from those products. The procedure presented in this paper allows the accurate identification of A. brasilense Az39 in pure cultures, mixtures of microorganisms, and commercial biofertilizers.

Azospirillum brasilense Az39, a model rhizobacterium with AHL quorum-quenching capacity

AIMS

The aim of this research was to analyse the quorum-sensing (QS) and quorum-quenching (QQ) mechanisms based on N-acyl-l-homoserine lactones (AHLs) in Azospirillum brasilense Az39, a strain with remarkable capacity to benefit a wide range of crops under agronomic conditions.

METHODS AND RESULTS

We performed an in silico and in vitro analysis of the quorum mechanisms in A. brasilense Az39. The results obtained in vitro using the reporter strains Chromobacterium violaceum and Agrobacterium tumefaciens and liquid chromatography coupled with mass-mass spectrometry analysis showed that although Az39 does not produce AHL molecules, it is capable of degrading them by at least two hypothetical enzymes identified by bioinformatics approach, associated with the bacterial cell. In Az39 cultures supplemented with 500 nmol l^-1 of the C3 unsubstituted AHLs (C4, C6, C8, C10, C12, C14), AHL levels were lower than in noninoculated LB media controls. Similar results were observed upon the addition of AHLs with hydroxy (OH-) and keto (oxo-) substitutions in C3. These results not only demonstrate the ability of Az39 to degrade AHLs. They also show the wide spectrum of molecules that can be degraded by this bacterium.

CONCLUSIONS

Although A. brasilense Az39 is a silent bacterium unable to produce AHL signals, it is able to interrupt the communications between other bacteria and/or plants by a QQ activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report confirming by unequivocal methodology the ability of A. brasilense, one of the most agriculturally used benefic bacteria around the world, to degrade AHLs by a QQ mechanism.

Extended lymphadenectomy in elderly and/or highly co-morbid gastric cancer patients: A retrospective multicenter study

BACKGROUND

Gastrectomy with extended lymphadenectomy is considered the gold standard treatment for advanced gastric cancer, with no age- or comorbidity-related limitations. We evaluated the safety and efficacy of curative gastrectomy with extended nodal dissection, verifying survival in elderly and highly co-morbid patients.

METHODS

In a retrospective multicenter study, we examined 1322 non-metastatic gastric-cancer patients that underwent curative gastrectomy with D2 versus D1 lymphadenectomy from January 2000 to December 2009. Postoperative complications, overall survival (OS), and disease-specific survival (DSS) according to age and the Charlson Comorbidity Score were analyzed in relation to the extent of lymphadenectomy.

RESULTS

Postoperative morbidity was 30.4%. Complications were more frequent in highly co-morbid elderly patients, and, although general morbidity rates after D2 and D1 lymphadenectomy were similar (29.9% and 33.2%, respectively), they increased following D2 in highly co-morbid elderly patients (39.6%). D2-lymphadenectomy significantly improved 5-year OS and DSS (48.0% vs. 37.6% in D1, p < 0.001 and 72.6% vs. 58.1% in D1, p < 0.001, respectively) in all patients. In elderly patients, this benefit was present only in 5-year DSS. D2 nodal dissection induced better 5-year OS and DSS rates in elderly patients with positive nodes (29.7% vs. 21.2% in D1, p = 0.008 and 47.5% vs. 30.6% in D1, p = 0.001, respectively), although it was present only in DSS when highly co-morbid elderly patients were considered.

CONCLUSION

Extended lymphadenectomy confirmed better survival rates in gastric cancer patients. Due to high postoperative complication rate and no significant improvement of the OS, D1 lymphadenectomy should be considered in elderly and/or highly co-morbid gastric cancer patients.

Multiple 3D inversion recovery imaging for volume T1 mapping of the heart

In this article, a three-dimensional inversion recovery sequence was optimized with the aim of generating in vivo volume T(1) maps of the heart using a 1.5-T MR system. Acquisitions were performed before and after gadolinium diethylenetriamine penta-acetic acid (Gd-DTPA) administration in one patient with hypertrophic cardiomyopathy and in two healthy volunteers. Data were acquired with a multishot fast field echo readout using both ECG and respiratory triggers. A dedicated phantom, composed of four solutions with different T(1) values, was positioned on the subjects' thoracic region to perform patient-specific calibration. Pixel based T(1) maps were calculated with a custom Matlab(®) code. Phantom measurements showed a good accuracy of the technique and in vivo T(1) estimation of liver, skeletal muscle, myocardium, and blood resulted in good agreement with values reported in the literature. Multiple three-dimensional inversion recovery technique is a feasible and accurate method to perform T(1) volume mapping.

Validation of the new AJCC TNM staging system for gastric cancer in a large cohort of patients (n = 2,155): focus on the T category

BACKGROUND

The prognostic value of T subclassification in patients with gastric carcinoma has been just implemented in the new AJCC TNM staging system, which has reclassified T2a and T2b into T2 and T3 tumors, respectively. The aim of the present study was to validate the prognostic significance of the new T categorization within the frame of the latest TNM staging system.

METHODS

We retrospectively reviewed the records of 686 T2/T3 patients among 2155 subjects who underwent radical resection for gastric carcinoma at six Italian centers from 1988 through 2006.

RESULTS

Upon multivariate analysis, the new T categories, extent of lymph node dissection (D) and patient's age were retained by the survival model as independent prognostic factors. In particular, the death risk for patients with T3 tumors was higher than that of patients with T2 tumors (HR: 1.42, P = 0.005). Among the 686 patients previously classified as having T2 tumors, patients with T2 and T3 disease were 270 (39.4%) and 416 (60.6%), respectively. After a median follow-up of 55 months, the 5-year overall survival rates were 67.3% and 52.3% for patients with T2 and T3 tumors, respectively (P < 0.001). The survival advantage for the T2 as compared to T3 category was maintained even when N0 and N+ patients were separately considered (P = 0.0154 and P < 0.001, respectively).

CONCLUSIONS

Our data confirm the prognostic difference between the newly proposed T2 and T3 categories, which should be implemented in the routine clinical practice to improve risk stratification of patients with gastric cancer.

Changing clinical and pathological features of gastric cancer over time

BACKGROUND

The aim of the present multicentre observational study was to evaluate potential changes in clinical and pathological features of patients with gastric cancer (GC) treated in a 15-year interval.

METHODS

A centralized prospective database including clinical, surgical, pathological and follow-up data from 2822 patients who had resection of a primary GC was analysed. The analysis focused on three periods: 1991-1995 (period 1), 1996-2000 (period 2) and 2001-2005 (period 3). Surgical procedure, pathological classification and follow-up were standardized among centres.

RESULTS

The number of resections decreased from 1024 in period 1 to 955 and 843 in periods 2 and 3 respectively. More advanced stages and a smaller number of intestinal-type tumours of the distal third were observed over time. Five-year survival rates after R0 resection (2320 patients) did not change over time (overall: 56·6 and 51·2 per cent in periods 1 and 3; disease-free: 66·8 and 61·1 per cent respectively). Decreases in survival in more recent years were related particularly to more advanced stage, distal tumours and tumours in women. Multivariable analysis showed a lower probability of overall and disease-free survival in the most recent interval: hazard ratio 1·22 (95 per cent confidence interval 1·06 to 1·40) and 1·29 (1·06 to 1·58) respectively compared with period 1. Recurrent tumours were more frequently peritoneal rather than locoregional.

CONCLUSION

Overall and disease-free survival rates after R0 resection of GC were unchanged over time.

Dynamical heterogeneities in irreversible gels: analogy with spin glasses

We describe the sol-gel transition by introducing an order parameter, defined as the average of local variables, and its fluctuations. It can be shown that these quantities are related to percolation quantities, but in principle they can be measured without resorting to connectivity properties. In this framework it appears that the dynamical transition associated with gelation is a real thermodynamic transition, as happens in spin glasses. The strong analogies between the sol-gel transition and the spin glass transition are also discussed.

Self-tuning phase separation in a model with competing interactions inspired by biological cell polarization

We present a theoretical study of a system with competing short-range ferromagnetic attraction and a long-range antiferromagnetic repulsion, in the presence of a uniform external magnetic field. The interplay between these interactions, at sufficiently low temperature, leads to the self-tuning of the magnetization to a value which triggers phase coexistence, even in the presence of the external field. The investigation of this phenomenon is performed using a Ginzburg-Landau functional in the limit of an infinite number of order parameter components (large N model). The scalar version of the model is expected to describe the phase separation taking place on a cell surface when this is immersed in a uniform concentration of chemical stimulant. A phase diagram is obtained as a function of the external field and the intensity of the long-range repulsion. The time evolution of the order parameter and of the structure factor in a relaxation process is studied in different regions of the phase diagram.

Metachronous hepatic metastases from gastric carcinoma: a multicentric survey

BACKGROUND

The treatment of hepatic metastases from gastric cancer is controversial, due to biologic aggressiveness of the disease.

OBJECTIVE

To survey the clinical approach to the subset of patients presenting with metachronous hepatic metastases as sole site of recurrence after curative resection of gastric cancer, focusing on the results achieved by different therapies and to investigate the prognostic factors of major clinical relevance.

METHODS

Retrospective multi-center chart review evaluating 73 patients, previously submitted to D >or= 2 gastrectomy for gastric cancer, who developed exclusive hepatic recurrence. Prognostic factors related to the patient, to the gastric malignancy and its treatment, and to the metastatic disease and its therapy were evaluated.

RESULTS

Forty-five patients received supportive care, 17 were submitted to chemotherapy, and 11 to hepatic resection. Survival was independently influenced by the variables T (p=0.019), N (p=0.05) and G (p=0.018) of the gastric primary and by the therapeutic approach to the metastases (p<0.005). In particular, T4 gastric cancer, presence of lymph-node metastases and G3 tumor displayed a negative prognostic value. Therapeutic approach to the metastases was the principal prognostic variable: 1, 2, and 3 years survival rates were 22.2%, 4.4% and 2.2%, respectively, for patients without specific treatment; 44.9%, 12.8% and 6.4% after chemotherapy (p=0.08) and 80.8%, 30.3% and 20.2% after surgical resection (p<0.001).

CONCLUSIONS

Our data suggest some clinical criteria that may facilitate selection of therapy for patients with hepatic recurrence after primary gastric cancer resection. The best survival rates are associated with surgical treatment, which should be chosen whenever possible.

Dynamical heterogeneity in a model for permanent gels: different behavior of dynamical susceptibilities

We present a systematic study of dynamical heterogeneity in a model for permanent gels upon approaching the gelation threshold. We find that the fluctuations of the self-intermediate scattering function are increasing functions of time, reaching a plateau whose value, at large length scales, coincides with the mean cluster size and diverges at the percolation threshold. Another measure of dynamical heterogeneities-i.e., the fluctuations of the self-overlap-displays instead a peak and decays to zero at long times. The peak, however, also scales as the mean cluster size. Arguments are given for this difference in the long-time behavior. We also find that the non-Gaussian parameter reaches a plateau in the long-time limit. The value of the plateau of the non-Gaussian parameter, which is connected to the fluctuations of diffusivity of clusters, increases with the volume fraction and remains finite at the percolation threshold.

Shear- and vibration-induced order-disorder transitions in granular media

By molecular dynamics simulations we investigate the order-disorder transitions induced in granular media by an applied drive combining vibrations and shear. As the steady state is attained, the pack is found in disordered configurations for comparatively high intensities of the drive; conversely, ordering and packing fractions exceeding the random close packing are found when vibrations and shear are weak. As forcing amplitudes get smaller, we find diverging time scales in the dynamics, as the system enters a jamming region. Under this perspective, our picture supports the intuition that externally applied forcing has, in driven granular media, a role similar to temperature in thermal systems.

The prognostic value of N-ratio in patients with gastric cancer: validation in a large, multicenter series

AIMS

The proportion between metastatic and examined lymph nodes (N-ratio) has been proposed as an independent prognostic factor in patients with gastric cancer. In the present work we validated the reliability of N-ratio in a large, multicenter series.

PATIENTS AND METHODS

We retrospectively reviewed the data of 1853 patients who underwent radical resection for gastric carcinoma. Survival of patients with >15 (Group-1, n=1421) and those with < or =15 (Group-2, n=432) lymph nodes examined was separately analyzed in order to evaluate the influence of lymph node dissection on disease staging. N-ratio categories (N-ratio 0, 0%; N-ratio 1, 1-9%; N-ratio 2, 10-25%; N-ratio 3, >25%) were determined by the best cut-off approach.

RESULTS

At multivariate analysis, N-ratio (but not TNM N-category) was retained as an independent prognostic factor both in Group-1 and Group-2 (HR for N-ratio 1, N-ratio 2 and N-ratio 3=1.67, 2.96 and 6.59, and 1.56, 2.68 and 4.28, respectively). After a median follow-up of 45.5 months, the 5-year overall survival rates of TNM N0, N1 and N2 patients were significantly different in Group-1 vs Group-2. This was not the case when adopting the N-ratio classification, suggesting that a low number of excised lymph nodes can lead to patients being understaged using the N-category, but not N-ratio. Moreover, N-ratio identified subsets of patients with significantly different survival rates within TNM N1 and N2 categories in both groups.

CONCLUSIONS

N-ratio is a simple and reproducible prognostic tool that can stratify patients with gastric cancer, including those cases with limited lymph node dissection. These data support the rationale to propose the implementation of N-ratio into the current TNM staging system.

Phenomenology and theory of horizontally oscillated granular mixtures

We overview the physics of a granular mixture subject to horizontal oscillations, recently investigated via experiments and molecular dynamics simulations. First we discuss the rich phenomenology exhibited by this system, which encompasses both segregation and dynamical instabilities. Then we show that the phenomenology can be explained via an effective interaction approach, by which the driven, non-thermal, granular mixture in mapped into a monodispersed thermal system of particles interacting via an effective potential. After determining the effective interaction we discuss its microscopic origin and investigate how it induces the observed phenomenology. Finally, as much as in thermal fluids, from the effective interaction we derive a Cahn-Hilliard dynamics equation, which appears to capture the essential characteristics of the dynamics of the granular mixture.

Static and dynamic heterogeneities in a model for irreversible gelation

We study the structure and the dynamics in the formation of irreversible gels by means of molecular dynamics simulation of a model system where the gelation transition is due to the random percolation of permanent bonds between neighboring particles. We analyze the heterogeneities of the dynamics in terms of the fluctuations of the self-intermediate scattering functions: in the sol phase close to the percolation threshold, we find that this dynamic susceptibility increases with the time until it reaches a plateau. At the gelation threshold this plateau scales as a function of the wave vector k as k(eta-2), with eta being related to the decay of the percolation pair connectedness function. At the lowest wave vector, approaching the gelation threshold it diverges with the same exponent gamma as the mean cluster size. These findings suggest an alternative way of measuring critical exponents in a system undergoing chemical gelation.

Kinetics of bond formation in cross-linked gelatin gels

In chemical cross-linking of gelatin solutions, two different time scales affect the kinetics of the gel formation in the experiments. We complement the experimental study with Monte Carlo numerical simulations of a lattice model. This approach shows that the two characteristic time scales are related to the formation of single bond cross-linker-chain and of bridges between chains. In particular, their ratio turns out to control the kinetics of the gel formation. We discuss the effect of the concentration of chains. Finally our results suggest that by varying the probability of forming bridges as an independent parameter, one can finely tune the kinetics of the gelation via the ratio of the two characteristic times.

Columnar and lamellar phases in attractive colloidal systems

In colloidal suspensions, at low volume fraction and temperature, dynamical arrest occurs via the growth of elongated structures that aggregate to form a connected network at gelation. Here we show that, in the region of parameter space where gelation occurs, the stable thermodynamical phase is a crystalline columnar one. Near and above the gelation threshold, the disordered spanning network slowly evolves and finally orders to form the crystalline structure. At higher volume fractions the stable phase is a lamellar one, which seems to have a still longer ordering time.

Complex viscosity behavior and cluster formation in attractive colloidal systems

The increase in viscosity that is observed in attractive colloidal systems by varying the temperature or the volume fraction can be related to the formation of structures due to particle aggregation. In particular we have studied the nontrivial dependence of the viscosity from the temperature and the volume fraction in the copolymer-micellar system L64. The comparison of the experimental data with the results of numerical simulations in a simple model for gelation phenomena suggests that this intriguing behavior can be explained in terms of cluster formation and that this picture can be quite generally extended to other attractive colloidal systems.

Size segregation in granular media induced by phase transition

In order to study analytically the nature of the size segregation in granular mixtures, we introduce a mean field theory in the framework of a statistical mechanics approach, based on Edwards' original ideas. For simplicity we apply the theory to a lattice model for a hard sphere binary mixture under gravity, and we find a new purely thermodynamic mechanism that gives rise to the size segregation phenomenon. By varying the number of small grains and the mass ratio, we find a crossover from the Brazil nut to the reverse Brazil nut effect, which becomes a true phase transition when the number of small grains is larger then a critical value. We suggest that this transition is induced by the effective attraction between large grains due to the presence of small ones (depletion force). Finally the theoretical results are confirmed by numerical simulations of the 3d system under taps.

Jamming transition in granular media: a mean-field approximation and numerical simulations

In order to study analytically the nature of the jamming transition in granular material, we have considered a cavity method mean-field theory, in the framework of a statistical mechanics approach, based on Edwards' original idea. For simplicity, we have applied the theory to a lattice model, and a transition with exactly the same nature of the glass transition in mean-field models for usual glass formers is found. The model is also simulated in three dimensions under tap dynamics, and a jamming transition with glassy features is observed. In particular, two-step decays appear in the relaxation functions and dynamic heterogeneities resembling ones usually observed in glassy systems. These results confirm early speculations about the connection between the jamming transition in granular media and the glass transition in usual glass formers, giving moreover a precise interpretation of its nature.

Traumatic lesions of the diaphragm. Our experience in 33 cases and review of the literature

We reviewed 33 consecutive patients with diaphragmatic injuries. Twenty-nine were admitted in emergency conditions after blunt (22 patients) or penetrating injury, presenting shock, dyspnoea, coma or acute abdomen in 21 cases; major associated lesions were found in 23 patients. Four patients presented acute complications of visceral herniation 2, 4, 84 and 216 months after the trauma. The diagnosis was preoperative in 23 cases, intraoperative in 9; in one case it was missed at laparotomy, becoming evident the day after. The sensibility of preoperative chest x-ray and CT was 86% and 100% in presence of visceral herniation, 14% and 0% in absence of visceral hernia. The diaphragmatic repair was always obtained by direct suture, following 20 haemostatic procedures (liver, spleen, mesenterium) and two bowel resections. The mortality rate was 24.4%; the morbidity rate was 48%. Traumatic lesions of the diaphragm are generally expression of particularly severe trauma whose outcome is mainly influenced by the associated lesions. They are also correlated to specific morbidity and mortality, so the surgical exploration is mandatory whenever this injury is suspected, considering that the preoperative diagnosis relies on visceral dislocation. Associated lesions influence the surgical strategy but a direct suture is usually effective in preventing specific complications.

Percolation model for enzyme gel degradation

We study a model for the gel degradation by an enzyme, where the gel is schematized as a cubic lattice, and the enzyme as a random walker, that cuts the bonds over which it passes. The model undergoes a (reverse) percolation transition, which for low density of enzymes falls in a universality class different from random percolation. In particular, we have measured a gel fraction critical exponent beta=1.0+/-0.1, in excellent agreement with experiments made on the real system.

Percolation and Burgers' dynamics in a model of capillary formation

Capillary networks are essential in vertebrates to supply tissues with nutrients. Experiments of in vitro capillary formation show that cells randomly spread on a gel matrix autonomously organize to form vascular networks. Cells form disconnected networks at low densities and connected ones above a critical density. Above the critical density the network is characterized by a typical mesh size approximately 200 microm, which is approximately constant on a wide range of density values. In this paper we present a full characterization of a recently proposed model which reproduces the main features of the biological system, focusing on its dynamical properties, on the fractal properties of patterns, and on the percolative phase transition. We discuss the relevance of the model in relation with some experiments in living beings and proposed diagnostic methods based on the measurement of the fractal dimension of vascular networks.

[Results of elective open surgery for juxtarenal aortic aneurysms]

Review of the most significant surveys (data base: Pub Med on September 2003) of elective open surgery for Juxtarenal aneurysms and personal results of 106 cases (9.3% of AAA consecutively operated in the last 11 yrs.) are reported. Mortality and morbidity are discussed related to: technique of aortic cross-clamping; protective measures on splanchnic and renal perfusion; risks from previous CAD and chronic renal failure. Over all, the main predictive factor is the accuracy of the selected technique, without any difference among different approaches, and the same results of infrarenal aneurysms can be obtained.

Monodisperse model suitable to study the glass transition

We study the properties of a monodisperse lattice glass model with a simple geometrical interpretation, which reproduces many features of glass forming liquids, such as the cage effect, vanishing diffusivity, and the presence of two time scales in relaxation functions. The model has a crystalline ground state at high density, but has no tendency to crystallize when quenched, even at extremely low cooling rates, which makes it suitable for the study of the glass transition. We study the model in mean field on random regular graphs, finding a scenario analogous to p-spin models.