Search for: All records

Abstract In this paper, we examine the cleavage fracture anisotropy in tantalum carbides, namely, TaC, α‐Ta₂C, and ζ‐Ta₄C_3 − x, using density functional theory (DFT) calculations. Our investigation identifies the presence of multiple low‐energy cleavage planes indicating multiple potential pathways for crack propagation in these ceramics, even the low symmetry compounds. The anisotropy characteristics of cleavage fractures exhibited by α‐Ta₂C and ζ‐Ta₄C_3 − xclosely align with the intrinsic fracture anisotropy observed in TaC. Notably, there exist at least three pyramidal planes in ζ‐Ta₄C_3 − xwhose cleavage energies are less than those of the carbon‐depleted basal planes, previously reported to have the lowest cleavage energy. The observed preference in experiments for cleavage along carbon‐depleted basal planes, exclusive of other identified low‐energy planes, points to factors beyond cleavage energy influencing cleavage plane preference. 

Bacteria use surface appendages called type IV pili to perform diverse activities including DNA uptake, twitching motility, and attachment to surfaces. The dynamic extension and retraction of pili are often required for these activities, but the stimuli that regulate these dynamics remain poorly characterized. To address this question, we study the bacterial pathogen Vibrio cholerae , which uses mannose-sensitive hemagglutinin (MSHA) pili to attach to surfaces in aquatic environments as the first step in biofilm formation. Here, we use a combination of genetic and cell biological approaches to describe a regulatory pathway that allows V. cholerae to rapidly abort biofilm formation. Specifically, we show that V. cholerae cells retract MSHA pili and detach from a surface in a diffusion-limited, enclosed environment. This response is dependent on the phosphodiesterase CdpA, which decreases intracellular levels of cyclic-di-GMP to induce MSHA pilus retraction. CdpA contains a putative nitric oxide (NO)–sensing NosP domain, and we demonstrate that NO is necessary and sufficient to stimulate CdpA-dependent detachment. Thus, we hypothesize that the endogenous production of NO (or an NO-like molecule) in V. cholerae stimulates the retraction of MSHA pili. These results extend our understanding of how environmental cues can be integrated into the complex regulatory pathways that control pilus dynamic activity and attachment in bacterial species.