Dong Kwon - Academia.edu (original) (raw)

Papers by Dong Kwon

Journal of Applied Polymer Science, 2017

A novel and simple surface modification of cellulose nanocrystals (CNC) was performed by chloroac... more A novel and simple surface modification of cellulose nanocrystals (CNC) was performed by chloroacetylation and subsequent reaction with tertiary amines to form quaternary ammonium modified CNCs. The acetylation of CNC and quaternary ammonium modified CNCs was confirmed using IR spectroscopy and solid state NMR spectroscopy. The 13 C NMR spectrum of quaternary ammonium modified CNC showed several additional resonances ranging from 14.5 ppm to 58.0 ppm compared to 13 C NMR spectrum of pure CNC, suggesting that alkyl chains have been added to the pure CNC. The disc diffusion method was used to evaluate the antimicrobial properties of quaternary ammonium modified CNCs. It was found that modified CNCs with alkyl chain longer than ten carbons are effective antimicrobial agents against Staphylococcus aureus and E. coli bacteria. These CNCs can be chemically modified to tailor the properties to improve dispersion in the polymer matrix. This will expand the application of CNC as a reinforcing material.

Tetrahedron Letters, 2012

Poly(amidoamine) (PAMAM) (G3) dendrimer was modified into quaternary ammonium salts using tertiar... more Poly(amidoamine) (PAMAM) (G3) dendrimer was modified into quaternary ammonium salts using tertiary amines with different chain lengths: dimethyldodecyl amine, dimethylhexyl amine, and dimethylbutyl amine using an efficient synthetic route. The antimicrobial activity of these dendrimer ammonium salts against Staphylococcus and E-coli bacteria was examined using the disc diffusion method. It was found that quaternary ammonium salt prepared with the dimethyldodecyl amine exhibits antimicrobial efficacy against Staphalococus and E.coli bacteria.

Helicobacter, 2009

Background and Aims: Helicobacter pylori infection is associated with a variety of diseases inclu... more Background and Aims: Helicobacter pylori infection is associated with a variety of diseases including gastric cancer. Flavodoxin is an electron transfer protein containing a flavin mononucleotide prosthetic group and substituted an iron-containing electron transfer protein under iron-limiting conditions. H. pylori flavodoxin has been reported but its pathogenic role is unclear. The aim of this study is to understand a pathogenic role of H. pylori flavodoxin under ironlimiting condition. Methods: The flavodoxin-encoding gene (fldA) was cloned from one of clinical H. pylori isolates (DU17) and its transcript was quantified by primer extension, Northern hybridization, and real-time polymerase chain reaction in different concentrations of an iron chelator. The fldA transcript was also quantified in H. pylori ATCC 700392, lacking a ferric uptake regulatory (fur) protein. Result: Nucleotide sequence of the fldA from H. pylori DU17 revealed a 492-bp (164 amino acids) open reading frame with a deduced amino acid sequence having 97% identity to that from the complete genomic sequence of H. pylori 26695. The deduced promoter [-35,-10, and +1] of the fldA was 56-bp upstream from the first codon of FldA. The fldA transcript (~0.55-kb) was induced up to 14-fold in both wild-type and fur-knocked-out strains under iron-limiting conditions, suggesting that the fldA induction is independent to the Fur protein. Conclusion: The fldA gene may play an important role in iron starvation conditions.

Helicobacter, 2003

Antibiotic resistance among Helicobacter pylori has been increasing worldwide and has begun to af... more Antibiotic resistance among Helicobacter pylori has been increasing worldwide and has begun to affect the overall efficacy of current antibiotic regimens adversely. We examined 220 pairs of H. pylori isolates obtained from both the antrum and corpus of separate patients; 109 (50%) harbored antibioticresistant H. pylori : amoxicillin (0.5%), clarithromycin (5.9%), furazolidone (1.4%), metronidazole (45.5%), nitrofurantoin (1.4%), and tetracycline (6.8%). Heteroresistance among the two biopsy sites from each patient was present in 41 of the 109 patients (38%) with antibiotic resistant H. pylori (e.g. 34% with resistant strains would be misclassified as susceptible if a biopsy of the antrum alone used for antimicrobial susceptibility testing). DNA fingerprinting genotype analysis was carried out on the 41 pairs of isolates with heteroresistance. While different patients had different fingerprinting patterns, each pair of isolates showed identical or similar fingerprinting patterns. These results suggest that antibiotic-resistant H. pylori typically develop from pre-existing susceptible strain rather than coinfection with a different strain. The minor differences in genotype (degeneration of genotype) seen reflect one of the processes for development of genetic diversity in H. pylori. No biopsy single site can be considered representative for antimicrobial susceptibility testing.

FEMS Microbiology Letters, 2013

Klebsiella pneumoniae carbapenemase (KPC)-encoding genes containing promoter-deletions (bla KPC-2... more Klebsiella pneumoniae carbapenemase (KPC)-encoding genes containing promoter-deletions (bla KPC-2a , bla KPC-2b , and bla KPC-2c) have disseminated in Enterobacteriaceae. The minimal inhibitory concentrations (MICs) to b-lactams in clinical KPC-producing Enterobacteriaceae range from susceptible to highlevel resistant, resulting in diagnostic problems. To better understand the variability in b-lactam MICs among KPC-producing Enterobacteriaceae, three isoforms of bla KPC-2 gene were used to transform Escherichia coli W4573 and its deletion mutant of an efflux pump (AcrAB) to examine the effects on b-lactam susceptibility. MICs to b-lactams in E. coli W4573 and its acrAB mutant strain increased 1-to 500-fold (MIC from 0.125 to 64 lg mL À1 of aztreonam) in the bla KPC-2a , bla KPC-2b , and bla KPC-2c transformants compared with the cloning vector alone. However, transformants of the acrAB mutant strain remained susceptible to all b-lactams tested except for aztreonam and carbenicillin. Levels of the three promoters' length and carbapenemase activities in the transformants harboring the bla KPC-2a , bla KPC-2b , and bla KPC-2c were correlated to the levels of b-lactam MICs in both E. coli W4573 and its mutant of an efflux pump (AcrAB). Overall, these results suggest that promoter-deletions of bla KPC-2 gene and AcrAB may be associated with the variability in b-lactam MICs in KPC-producing Enterobacteriaceae.

FEMS Microbiology Letters, 2000

Metronidazole is a critical ingredient for combination therapies of Helicobacter pylori infection... more Metronidazole is a critical ingredient for combination therapies of Helicobacter pylori infection, the major cause of peptic ulcer and gastric cancer. It has been recently reported that metronidazole resistance from H. pylori ATCC43504 is caused by the insertion of a mini-IS605 sequence and deletion of sequences in an oxygen insensitive NAD(P)H nitroreductase encoding gene (rdxA). We also found that an additional gene (frxA) encoding NAD(P)H flavin oxidoreductase in the same strain was truncated by frame-shift mutations. To assess whether the frxA truncation is also involved in metronidazole resistance, metronidazole sensitive H. pylori strains ATCC43629 and SS1 were transformed by the truncated frxA gene cloned from strain ATCC43504. All transformed cells grew on agar plates containing 16 Wg ml 31 of metronidazole. The involvement of the frxA gene in metronidazole resistance was also confirmed by insertion inactivation of frxA and/or rdxA genes from strain ATCC43629 and one metronidazole sensitive clinical isolate H. pylori 2600. In addition, the frxA gene cloned from the H. pylori 2600 showed metronidazole nitroreductase activity in Escherichia coli and rendered ordinary metronidazole resistant E. coli to metronidazole sensitive cell. These results indicate that the frxA gene may also be involved in metronidazole resistance among clinical H. pylori isolates.

Clinical Infectious Diseases, 1999

resistance in Helicobacter pylori is due to null mutations in a gene (rdxA) that encodes an oxyge... more resistance in Helicobacter pylori is due to null mutations in a gene (rdxA) that encodes an oxygen-insensitive NADPH nitroreductase.

Clinical Infectious Diseases, 1998

With use of multiple-and single-colony expansion procedures, the results of susceptibility testin... more With use of multiple-and single-colony expansion procedures, the results of susceptibility testing of Helicobacter pylori isolates from patients with duodenal ulcer were assessed by Etest. The H. pylori genotype was assessed by repetitive extragenic palindrome-based polymerase chain reaction (REP-PCR). There was a high degree of genotypic heterogeneity between different patients, but a single REP-PCR pattern was found for 92% of patients. In contrast, a high degree of phenotypic heterogeneity was shown among the isolated colonies. Antibiogram susceptibility patterns differed only with respect to metronidazole but not with respect to clarithromycin or amoxicillin. The 42% rate of resistance to metronidazole determined with use of the conventional multiple-strains expansion method was increased to 92% when the single-colony expansion method was used. Similarly, dual clarithromycin/metronidazole resistance was increased from 8% to 42% with single-colony expansion. Despite evidence of a single genotype in most patients, single-colony expansion shows that routine susceptibility testing may greatly underestimate the frequency of metronidazole resistance. Helicobacter pylori is a gram-negative and microaerophilic ribotyping [12], PCR-based restriction fragment length polymorphism analysis [13, 14], and repetitive extragenic palin-bacterium whose normal niche is human gastric mucosa [1]. Although H. pylori is susceptible to many antimicrobials in drome-based PCR (REP-PCR) [7]. Molecular analysis of H. pylori isolates before and after therapy has proven to be vitro, clinical experience has demonstrated that H. pylori infection is not easy to cure in vivo. Successful treatment of very useful for distinguishing reinfection with a different strain from recrudescence [15, 16], and REP-PCR is one of the most H. pylori infection usually requires combinations of several antibiotics, often with acid suppression [2, 3]. The primary sensitive and efficient means for distinguishing individual H. pylori strains [7]. impediments to successful treatment are noncompliance with the drug regimens and antibiotic-resistant H. pylori [4, 5]. The traditional protocol for antibiotic susceptibility testing is analysis by a multiple-strain (or colony) expansion method, These findings have led to the search for successful treatment regimens. Resistance has been reported to most antimicrobials i.e., the use of several colonies from a primary culture per biopsy per patient. H. pylori isolates with identical RAPD pat-used in effective treatment protocols for H. pylori infection, including clarithromycin, metronidazole, tetracycline, and terns have been reported to show different patterns of susceptibility to metronidazole [11, 17, 18]. The goal of this study was amoxicillin [3, 5, 6]. to investigate whether genotyping by REP-PCR gave a clue about the presence of pathogens with mixed antibiotic suscepti-See editorial response by Solnick on pages 90-2. bility in the same biopsy specimen. The REP-PCR genotypes of isolates from duodenal ulcer patients in different regions of Tremendous genetic heterogeneity exists among H. pylori the United States were compared with their antibiotic susceptiisolates from different patients [7], and the finding of more bility profiles with regard to amoxicillin, clarithromycin, and than one genotype from a single patient has been reported [8]. metronidazole. A variety of methods have been used for genotyping H. pylori, including pulsed-field gel electrophoresis [9], random amplified Methods polymorphic DNA (RAPD) PCR of genomic DNA [10, 11], Patients and Specimens Gastric mucosal biopsy samples were obtained from 12 patients throughout the United States who had duodenal ulcers.

FEMS Microbiology Letters, 2009

Pseudomonas aeruginosa is a major causative agent of hospital-acquired infections and infections ... more Pseudomonas aeruginosa is a major causative agent of hospital-acquired infections and infections in cystic fibrosis patients. Treatment of P. aeruginosa is complicated by the presence of intrinsic and acquired multidrug-resistant isolates. Polymyxin B has often been used as the last option to treat the multidrug-resistant isolates. However, polymyxin B-resistant clinical isolates have been increasingly reported worldwide. To understand molecular details of polymyxin resistance we characterized polymyxin B-resistant clinical isolate of P. aeruginosa. The clinical isolate grew with 4 mg mL À1 of polymyxin B while a reference P. aeruginosa PAO1 grew with 0.25 mg mL À1. Polymyxin B susceptibility was restored (minimal inhibitory concentration from 8 to 0.5 mg mL À1) by an intact clone of pmrAB, but not by an intact clone of phoPQ or the cloning vector. DNA sequence analysis of pmrB from the resistant isolate revealed a single nucleotide substitution (T to C) substituted methionine to threonine at position 292 of PmrB. Involvement of this amino acid substitution in polymyxin B resistance was confirmed by complementation of a pmrAB null-mutant strain with the pmrAB containing the amino acid substitution. These results suggest that amino acid substitution in PmrB is one mechanism of polymyxin B resistance in clinical isolates of P. aeruginosa.