Jaeyeon Kim

Effect of initial particle size on microstructure of liquid-phase sintered α-silicon carbide

Abstract Three α-SiC powders of different particle sizes (∼0.42, ∼0.50 and ∼0.71 μm), containing 7.2 wt% Y 3 Al 5 O 12 (YAG) and 4.8 wt% SiO 2 as sintering aids, were hot-pressed at 1850°C and subsequently annealed at 1950°C to initiate grain growth. All the hot-pressed specimens consisted of equiaxed grains and showed unimodal grain size distribution. The smaller the starting powder the finer the microstructure was developed. After annealing, the fine (∼0.7 μm) and the medium (∼1.1 μm) grain-sized specimens showed self-reinforced microstructures whereas the large (∼1.5 μm) grain-sized specimen maintained an unimodal grain size distribution. These results suggest that the abnormal grain growth of α-SiC grains during annealing is critically dependent on the average grain size of sintered materials, which in turn depends on initial particle size. The fracture toughnesses (5.6 and 6.1 MPa m 1 2 ) of the annealed specimens with self-reinforced microstructures were higher than for the specimen with an unimodal grain size distribution (5.0 MPa m 1 2 ).

Effect of annealing on mechanical properties of self-reinforced alpha-silicon carbide

Alpha-SIC powder containing 7.2 wt % Y3Al5O12 (YAG, yttrium aluminum garnet) and 4.8 wt % SiO2 as sintering aids were hot-pressed (SC0) at 1820°C for 1 h and subsequently annealed at 1920°C for 2 h (SC2), 4 h (SC4) and 8 h (SC8). When the annealing time was increased, the microstructure changed from equiaxed to elongated grains and resulted in self-reinforced microstructure consisted of large elongated grains and small equiaxed grains. Development of self-reinforced microstructure, consisted of mostly 6H phase, resulted in significant improvements in toughness. However, the improved toughness was offset by a significant reduction in strength as in the materials consisted of 4H originated from β-SiC. The fracture toughness and strength of the 8-h annealed materials were 5.5MPa · m1/2 and 490 MPa, respectively.

Analysis of Gastric Microbiota by Pyrosequencing: Minor Role of Bacteria Other Than Helicobacter pylori in the Gastric Carcinogenesis.

Little is known about the role of gastric microbiota except for Helicobacter pylori (HP) in human health and disease. We compared the differences of human gastric microbiota according to gastric cancer or control and HP infection status and assessed the role of bacteria other than HP.

R-curve behaviour and microstructure of liquid-phase sintered α-SiC

R-curves for two liquid-phase sintered α-SiC (SC-A and SC-B), of different microstructures, were characterized using indentation-strength method. Silicon carbide SC-B, with its coarser microstructure and 8 MPa · m1/2 toughness, showed higher resistance to crack growth and more damage tolerance than silicon carbide SC-A, with its finer microstructure and 4.5 MPa · m1/2 toughness. These results suggest that a coarse microstructure is beneficial to toughening and damage tolerance while a fine microstructure is beneficial to strengthening.

An Appropriate Cutoff Value for Determining the Colonization of Helicobacter pylori by the Pyrosequencing Method: Comparison with Conventional Methods

Sequencing of 16S ribosomal RNA (rRNA) gene has improved the characterization of microbial communities. It enabled the detection of low abundance gastric Helicobacter pylori sequences even in subjects that were found to be H. pylori negative with conventional methods. The objective of this study was to obtain a cutoff value for H. pylori colonization in gastric mucosa samples by pyrosequencing method.

Intestinal Pseudo-obstruction: Initial Manifestation of Systemic Lupus Erythematosus

A 20-year-old man was hospitalized for 2 months after the onset of vomiting, cramping abdominal pain and dysuria. He had been diagnosed as IgA nephropathy 4 months ago. His mother was recently diagnosed as systemic lupus erythematosus (SLE). Physical examination revealed hypoactive bowel sounds and distended abdomen with diffuse tenderness. There was no rash on the face and body. The laboratory investigations showed hemoglobin 11.2 g/dL, leukocyte 10,500/µL, platelet 160,000/µL and albumin 3.2 g/dL. Antinuclear antibody (titer of 1:320 with speckled pattern), anti-Sm and ribonucleotide protein antibodies were positive. The 24 hour urine protein was 1,100 mg/day. Abdominal X-ray demonstrated multiple air-fluid levels of small bowel (Fig. 1). Abdominal CT showed fluid-filled dilatation of small bowel, bilateral hydronephrosis and thickened bladder wall. There was no abnormality on esophagogastroduodenoscopy and colonoscopy. Anorectal manometry showed decreased anal sphincter pressure and intact anaorectal inhibitory reflex. Figure 1 Simple abdominal erect view. Simple abdominal erect film shows extensive dilatation of the small bowel and multiple air-fluid levels. He was treated with empirical prednisolone 30 mg/day for 3 days but abdominal pain was worsened and plain X-ray showed aggravation of ileus. To find out the underlying cause of intestinal pseudo-obstruction (IPO), subtotal colectomy was performed. Grossly there was no mass, ulcer or mechanical obstruction. Histopathology demonstrated diffuse muscular degeneration with little vasculitis suggestive of SLE (Fig. 2). Steroid and azathiopurine were started after surgery and he became well. Figure 2 Microscopic findings of colon. (A) There is diffuse muscular degeneration, fibrosis and multifocal eosinophil infiltration. (B) The vascular lumen is intact and few inflammatory cells are infiltrated around the vessel

The Effect of Helicobacter pylori on Epidermal Growth Factor Receptor-Induced Signal Transduction and the Preventive Effect of Celecoxib in Gastric Cancer Cells.

Background/Aims Helicobacter pylori infection induces cyclooxygenase-2 (COX-2) and epidermal growth factor receptor (EGFR) overexpression, and these factors may engage in cross-talk. The aim of the present study was to evaluate the effect of H. pylori on EGFR signaling pathways and to determine whether celecoxib has an inhibitory effect on this pathway. Methods The AGS cell line was cocultured with H. pylori G27 and the isogenic cagE- mutant. The expression of COX-2, EGFR, heparin binding-epidermal growth factor (HB-EGF), and transforming growth factor-β (TGF-β) was measured by real time-polymerase chain reaction (RT-PCR). Next, Western blot analyses of COX-2, EGFR, total Akt, phosphorylated Akt (pAkt), and phosphorylated glycogen synthase kinase-3β (pGSK3β) were performed after incubating H. pylori-treated AGS cells for 24 hours with various concentrations of celecoxib (0, 10, 20, and 30 µmol/L). Results H. pylori infection upregulated the mRNA levels of COX-2, EGFR, HB-EGF, and TGF-β, as detected by RT-PCR. However, AGS cells treated with cagE- mutants, which have a defective type IV secretion system, did not exhibit EGFR upregulation. Celecoxib had inhibitory effects on the H. pylori-induced overexpression of COX-2 (p=0.015), EGFR (p=0.025), pAkt (p=0.025), and pGSK3β (p=0.029) by Western blot analysis. Conclusions H. pylori with an intact type IV secretion system activated the COX-2 and EGFR-Akt pathways in the AGS cell line. As celecoxib exhibited inhibitory effects on the EGFR signaling pathway, the cross-talk of COX-2 and EGFR likely mediates H. pylori-induced gastric cancer.

Comparison of Gastric Microbiota Between Gastric Juice and Mucosa by Next Generation Sequencing Method

Background: Not much is known about the role of gastric microbiota except for Helicobacter pylori in human health and disease. In this study, we aimed to detect human gastric microbiota in both gastric mucosa and gastric juice by barcoded 454-pyrosequencing of the 16S rRNA gene and to compare the results from mucosa and juice. Methods: Gastric biopsies and stomach juices were collected from 4 subjects who underwent standard endoscopy at Seoul National University Bundang Hospital. Gastric microbiota of antral mucosa, corpus mucosa samples, and gastric fluids were analyzed by barcoded 454-pyrosequencing of the 16S rRNA gene. The analysis focused on bacteria, such as H. pylori and nitrosating or nitrate-reducing bacteria. Results: Gastric fluid samples showed higher diversity compared to that of gastric mucosa samples. The mean of operational taxonomic units was higher in gastric fluid than in gastric mucosa. The samples of gastric fluid and gastric mucosa showed different composition of phyla. The composition of H. pylori and Proteobacteria was higher in mucosa samples compared to gastric fluid samples (H. pylori, 66.5% vs. 3.3%, P = 0.033; Proteobacteria, 75.4% vs. 26.3%, P = 0.041), while Actinobacteria, Bacteroidetes, and Firmicutes were proportioned relatively less in mucosa samples than gastric fluid. However there was no significant difference. (Actinobacteria, 3.5% vs. 20.2%, P = 0.312; Bacteroidetes, 6.0% vs. 14.8%, P = 0.329; Firmicutes, 12.8% vs. 33.4%, P = 0.246). Conclusions: Even though these samples were small, gastric mucosa could be more effective than gastric fluid in the detection of meaningful gastric microbiota by pyrosequencing.

Analysis of Gastric Body Microbiota by Pyrosequencing: Possible Role of Bacteria Other Than Helicobacter pylori in the Gastric Carcinogenesis

Background Gastric microbiota along with Helicobacter pylori (HP) plays a key role in gastric disease. The aim of our study is to investigate the difference of human gastric microbiota between antrum and body according to disease (control vs. gastric cancer) and HP status. Methods Each antrum and body biopsy was collected from 12 subjects at Seoul National University Bundang Hospital. Gastric microbiota was analyzed by bar-coded 454 pyrosequencing of the 16S rRNA gene. Twelve subjects consisted of HP-negative control (n = 2), HP-negative cancer (n = 2), HP-positive control (n = 3), and HP-positive cancer (n = 5). The analysis was focused on non-HP urease-producing bacteria (UB) and non-HP nitrosating or nitroreducing bacteria (NB) between antrum and body. Results Gastric body samples showed higher diversity compared to gastric antrum mucosa samples but there was no significant difference. The mean of operational taxonomic units was higher in HP(−) cancer than HP(+) cancer (antrum, 273.5 vs. 228.2, P = 0.439; body, 585.5 vs. 183.2, P = 0.053). The number of non-HP UB and non-HP NB was higher in HP(−) cancer groups than the others. These differences were more pronounced in the body (P = 0.051 and P = 0.081, respectively). Analysis of overlap of non-HP UB and non-HP NB revealed the higher composition of Streptococcus pseudopneumoniae, S. parasanguinis, and S. oralis in HP(−) cancer groups than the others, only in the body (P = 0.030) but not in the antrum (P = 0.123). Conclusions Higher diversity and higher composition of S. pseudopneumoniae, S. parasanguinis, and S. oralis in HP(−) cancer group than the other groups in the body suggest that analysis of microbiota from body mucosa could be beneficial to identify a role of non-HP bacteria in the gastric carcinogenesis.

Dynamic Changes in Helicobacter pylori Status Following Gastric Cancer Surgery.

Background/Aims Helicobacter pylori eradication is recommended in patients with early gastric cancer. However, the possibility of spontaneous regression raises a question for clinicians about the need for “retesting” postoperative H. pylori status. Methods Patients who underwent curative gastrectomy at Seoul National University Bundang Hospital and had a positive H. pylori status without eradication therapy at the time of gastric cancer diagnosis were prospectively enrolled in this study. H. pylori status and atrophic gastritis (AG) and intestinal metaplasia (IM) histologic status were assessed pre- and postoperatively. Results One hundred forty patients (mean age, 59.0 years; 60.7% male) underwent subtotal gastrectomy with B-I (65.0%), B-II (27.1%), Roux-en-Y (4.3%), jejunal interposition (0.7%), or proximal gastrectomy (4.3%). Preoperative presence of AG (62.9%) and IM (72.9%) was confirmed. The mean period between surgery and the last endoscopic follow-up was 38.0±25.6 months. Of the 140 patients, 80 (57.1%) were found to be persistently positive for H. pylori, and 60 (42.9%) showed spontaneous negative conversion at least once during follow-up. Of these 60 patients, eight (13.3%) showed more complex postoperative dynamic changes between negative and positive results. The spontaneous negative conversion group showed a trend of having more postoperative IM compared to the persistent H. pylori group. Conclusions A high percentage of spontaneous regression and complex dynamic changes in H. pylori status were observed after partial gastrectomy, especially in individuals with postoperative histological IM. It is better to consider postoperative eradication therapy after retesting for H. pylori.