Stereology of nerve cross sections

Among strains of Haemophilus influenzae, the ability to catabolize tryptophan (as detected by indole production) varies and is correlated with pathogenicity. Tryptophan catabolism is widespread (70 to 75%) among harmless respiratory isolates but is nearly universal (94 to 100%) among strains causing serious disease, including meningitis. As a first step in investigating the relationship between tryptophan catabolism and virulence, we have identified genes in pathogenic H. influenzae which are homologous to the tryptophanase (tna) operon of Escherichia coli. The tna genes are located on a 3.1-kb fragment between nlpD and mutS in the H. influenzae type b (Eagan) genome, are flanked by 43-bp direct repeats of an uptake signal sequence downstream from nlpD, and appear to have been inserted as a mobile unit within this sequence. The organization of this insertion is reminiscent of pathogenicity islands. The tna cluster is found at the same map location in all indole-positive strains of H. influenzae surveyed and is absent from reference type d and e genomes. In contrast to H. influenzae, most other Haemophilus species lack tna genes. Phylogenetic comparisons suggest that the tna cluster was acquired by intergeneric lateral transfer, either by H. influenzae or a recent ancestor, and that E. coli may have acquired its tnaA gene from a related source. Genomes of virulent H. influenzae resemble those of pathogenic enterics in having an island of laterally transferred DNA next to mutS.     

Modeling grain growth dependence on the liquid content in liquid-phase-sintered materials

A model for grain growth during liquid-phase sintering (LPS) is presented. A Rayleigh grain size distribution is assumed based on both experimental and theoretical results. This asymmetric distribution provides a continuous driving force for coarsening. The model uses the solid grain contiguity to calculate the relative solid-state and liquid-phase contributions to coarsening. The level of grain agglomeration affects both the mean diffusion distance and interface area over which diffusion occurs. A cumulative grain growth rate is calculated assuming independent solid and liquid contributions to coarsening. Consequently, only the liquid volume fraction and solid-liquid dihedral angle are required to predict the change in grain coarsening rate with solid-liquid ratio. A prior empirical correlation between the grain growth rate constant and the liquid volume fraction is compared to the resulting analytic form, showing excellent agreement. The new model is projected to be generically applicable to microstructure coarsening in multiple phase materials, including porous structures.