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W C Yam

Publications and source records attributed to W C Yam.

At least 37 records · Page 2Linked to original sources
PubMed2001

Oral colonization of aerobic and facultatively anaerobic gram-negative rods and cocci in irradiated, dentate, xerostomic individuals.

This study aimed at investigating the oral colonization of aerobic and facultatively anaerobic gram-negative rods and cocci in head- and neck-irradiated, dentate, xerostomic individuals. Subjects were recruited from a nasopharyngeal carcinoma clinic and were segregated into group A: <60 years (n=25, 48+/-6 years, 5+/-5 years post-irradiation) and group B: >or=60 years (n=8, 67+/-4 years, 2+/-2 years post-irradiation) and were compared with age- and sex-matched normal individuals, group C: <60 years (n=20, 44+/-12 years) and group D: >or=60 years (n=10, 70+/-3 years). Selective culture of the oral rinse samples was carried out to isolate, quantify and speciate (using API 20E kit) aerobic and facultatively anaerobic gram-negative rods and cocci recovery. All test subjects were put under comprehensive oral and preventive care for 3 months, and 12 group A and 5 group B subjects were recalled for reassessment of aerobic and facultatively anaerobic gram-negative rods and cocci colonization. All identical isolates, pre- and post-hygienic care, were phenotypically (Vitek, Hazelwood, MA and antibiogram profile) and genotypically (pulsed-field gel electrophoresis) evaluated. The aerobic and facultatively anaerobic gram-negative rods and cocci isolated from the first round oral rinse samples included: Acinetobacter, Neisseria, Chryseomonas, Flavimonas, Pseudomonas, Citrobacter, Enterobacter, Escherichia, Klebsiella, Flavobacterium and Weeksella species. The aerobic and facultatively anaerobic gram-negative rods and cocci isolation rate was high for irradiated individuals, and they were 64/25% and 100/80% for groups A/C and B/D, respectively. Recovery of aerobic and facultatively anaerobic gram-negative rods and cocci and Klebsiella pneumoniae subsp. pneumoniae in oral rinse samples were found to be significantly more prevalent in the irradiated subjects (groups A and B). Enterobacteriaceae were more frequently isolated from oral rinse samples of aged irradiated subjects (group B vs D, P<0.05), where the quantity of Citrobacter freundii (colony-forming units/ml oral rinse) was also significantly elevated. The isolation rate of aerobic and facultatively anaerobic gram-negative rods and cocci after hygienic care remained unchanged; 3 of 12 and 3 of 5 of the recalled subjects from groups A and B, respectively, harbored same aerobic and facultatively anaerobic gram-negative rods and cocci species. However, only two pairs of K. pneumoniae subsp. pneumoniae, sequentially isolated from same patients in group B, were found to be identical by pulsed-field gel electrophoresis. This may be due to reinfection of the microbes from the same source or permanent colonization. In conclusion, irradiation-induced xerostomia seems to favor frequent, repeated, transient intraoral colonization of aerobic and facultatively anaerobic gram-negative rods and cocci.

Acinetobacter↗
PubMed2001

Target site modifications and efflux phenotype in clinical isolates of Streptococcus pneumoniae from Hong Kong with reduced susceptibility to fluoroquinolones.

Ciprofloxacin-susceptible (n = 7) and -resistant (MIC >or=4 mg/L) (n = 15) clinical isolates of Streptococcus pneumoniae from diverse sources in Hong Kong were studied for target site modifications and efflux phenotype. Reserpine-inhibited efflux of ciprofloxacin and/or levofloxacin was common in both susceptible and non-susceptible isolates. The ParC substitutions K137N and/or S79F or Y were associated with increased ciprofloxacin MICS. The GyrA substitution S81F was only found in isolates with full resistance to ciprofloxacin (MIC >or=16 mg/L) and levofloxacin (MIC >or=8 mg/L). Among clinical isolates of S. pneumoniae, accumulation of target site mutations in strains with an efflux mechanism was associated with increasing MICs of fluoroquinolones.

Amino Acid Substitution↗
PubMed2001

Increasing resistance of Streptococcus pneumoniae to fluoroquinolones: results of a Hong Kong multicentre study in 2000.

The MICs of 13 antimicrobial agents including seven fluoroquinolones (ciprofloxacin, levofloxacin, sparfloxacin, grepafloxacin, gatifloxacin, moxifloxacin and clinafloxacin) for Streptococcus pneumoniae isolates obtained from all regions of Hong Kong in the year 2000 were determined by the Etest. Overall, 39.4% of 180 isolates were susceptible to penicillin, 11.7% were intermediate and 48.9% were resistant. The overall prevalence of fluoroquinolone non-susceptibility (levofloxacin MIC > or = 4 mg/L) was 13.3% but increased to 27.3% among the penicillin-resistant isolates. For the fluoroquinolone non-susceptible isolates, within-class cross-resistance was common. For the fluoroquinolone non-susceptible isolates, the median MICs of clinafloxacin, gatifloxacin, moxifloxacin, sparfloxacin and grepafloxacin were, respectively, six-, 24-, 32- 84- and 128-fold higher than those for the susceptible isolates. All fluoroquinolone non-susceptible strains were derived from adults. The prevalence of fluoroquinolone resistance was higher in isolates from older patients (17.1% among those > or = 65 years of age versus 9.1% among those 18-64 years of age, P < 0.001) and from adults with chronic obstructive pulmonary disease (24.6% versus 9.3%, P = 0.01). All fluoroquinolone non-susceptible strains were non-susceptible to penicillin (MIC range 2-4 mg/L), cefotaxime (MIC range 1-4 mg/L) and erythromycin (MIC range 4- > or = 256 mg/L). The fluoroquinolone non-susceptible isolates were genetically related to the Spain(23F)-1 clone when analysed by pulse-field gel electrophoresis and multilocus sequence typing. In conclusion, a rapid increase in the prevalence of fluoroquinolone resistance among S. pneumoniae was found in Hong Kong. Typing analysis suggests that this is due to the pan-regional dissemination of a fluoroquinolone-resistant variant (designated Hong Kong(23F)-1) of the globally distributed Spain(23)F-1 clone.

Adolescent↗
PubMed2001

Detection of RTX toxin gene in Vibrio cholerae by PCR.

A PCR that amplifies a recently discovered Vibrio cholerae RTX (repeat in toxin) toxin gene was developed. Among 166 clinical and environmental isolates of V. cholerae causing epidemics and sporadic cases of cholera in various parts of the world, all were found to be toxigenic by both PCR and HEp-2 cell cytotoxicity assay. Standard strains of the classical biotype containing a deletion within the gene cluster exhibited negative results by both assays. This is the first rapid genotyping method for differentiation of V. cholerae O1 classical biotype strains from El Tor biotype strains as well as strains of other non-O1 serogroups including serogroup O139. The PCR assay that was developed also specifically detects RTX toxin genes in V. cholerae, as clinical isolates of Vibrio parahaemolyticus, diarrheagenic Escherichia coli, Aeromonas species, and Plesiomonas species were all negative by the RTX toxin-specific PCR as well as the HEp-2 cytotoxicity assay. These findings highlight the characteristics of the RTX toxins in V. cholerae. Their role in the pathogenicity of the bacterium requires further investigation.

Acyltransferases↗
PubMed2000

Bacteremia due to Staphylococcus aureus with reduced susceptibility to vancomycin.

Four cases of bacteremia caused by Staphylococcus aureus with heteroresistance to vancomycin (hetero-VRSA) were described. In at least two of these four mortalities, the cause of death was temporally related to the hetero-VRSA bacteremia. The vancomycin and teicoplanin MICs of the resistant subpopulations of these four hetero-VRSA were 8 and 24 microg/ml, respectively. All isolates were producers of beta-lactamase, produced penicillin-binding protein PBP2a, and possessed the mecA gene accounting for methicillin resistance. Thickening of the peptidoglycan cell wall was observed by electron microscopy. When ampicillin was combined with vancomycin, in vitro synergism was detected using the checkerboard titration method (epsilonFIC = 0.13). The use of vancomycin plus ampicillin-sulbactam could be a viable option in treating severe hetero-VRSA infection in view of the higher affinity of ampicillin toward PBP2a.

Aged↗
PubMed2000

Oral colonization, phenotypic, and genotypic profiles of Candida species in irradiated, dentate, xerostomic nasopharyngeal carcinoma survivors.

The aim of this study was to investigate oral yeast colonization and oral yeast strain diversity in irradiated (head and neck), dentate, xerostomic individuals. Subjects were recruited from a nasopharyngeal carcinoma clinic and were segregated into group A (age, <60 years [n = 25; average age +/- standard deviation (SD), 48 +/- 6 years; average postirradiation time +/- SD, 5 +/- 5 years]) and group B (age, >/=60 years [n = 8; average age +/- SD, 67 +/- 4 years; average postirradiation time +/- SD, 2 +/- 2 years]) and were compared with age- and sex-matched healthy individuals in group C (age, <60 years [n = 20; average age +/- SD, 44 +/- 12 years] and group D (age, >/=60 years [n = 10; average age, 70 +/- 3 years]). Selective culture of oral rinse samples was carried out to isolate, quantify, and speciate yeast recovery. All test subjects underwent a 3-month comprehensive oral and preventive care regimen plus topical antifungal therapy, if indicated. A total of 12 subjects from group A and 5 subjects from group B were recalled for reassessment of yeast colonization. Sequential (pre- and posttherapy) Candida isolate pairs from patients were phenotypically (all isolate pairs; biotyping and resistotyping profiles) and genotypically (Candida albicans isolate pairs only; electrophoretic karyotyping by pulsed-field gel electrophoresis, restriction fragment length polymorphism [RFLP], and randomly amplified polymorphic DNA [RAPD] assays) evaluated. All isolates were Candida species. Irradiated individuals were found to have a significantly increased yeast carriage compared with the controls. The isolation rate of Candida posttherapy remained unchanged. A total of 9 of the 12 subjects in group A and 3 of the 5 subjects in group B harbored the same C. albicans or Candida tropicalis phenotype at recall. Varying degrees of congruence in the molecular profiles were observed when these sequential isolate pairs of C. albicans were analyzed by RFLP and RAPD assays. Variations in the genotype were complementary to those in the phenotypic characteristics for some isolates. In conclusion, irradiation-induced xerostomia seems to favor intraoral colonization of Candida species, particularly C. albicans, which appeared to undergo temporal modifications in clonal profiles both phenotypically and genotypically following hygienic and preventive oral care which included topical antifungal therapy, if indicated. We postulate that the observed ability of Candida species to undergo genetic and phenotypic adaptation could strategically enhance its survival in the human oral cavity, particularly when salivary defenses are impaired.

Adult↗
PubMed1998

Direct detection of Mycobacterium tuberculosis in respiratory specimens using an automated DNA amplification assay and a single tube nested polymerase chain reaction (PCR).

The performance of an automated DNA amplification assay (Roche Cobas Amplicor Mycobacterium Tuberculosis Test (aPCR) was compared with an in-house single tube nested polymerase chain reaction (nPCR) for the direct detection of Mycobacterium tuberculosis in respiratory specimens. Among 385 specimens, 56 were culture positive for mycobacteria (44 positive for Mycobacterium tuberculosis and 12 positive for non-tuberculosis mycobacteria). The diagnostic sensitivities of aPCR and nPCR were 86% and 91% whereas a 100% diagnostic specificity of both assays was attained. By aPCR, inhibitors were detected in 6% of the clinical samples. For nPCR, the usage of a new thermostable DNA polymerase facilitated pre-PCR decontamination using uracil-N-glycosylase and "hot start" in single step procedure. The results of the study indicated that DNA amplification assays, either manual or automated, were rapid and specific tools for diagnosing pulmonary tuberculosis.

Base Sequence↗
PubMed1997

Comparison of two automated DNA amplification systems with a manual one-tube nested PCR assay for diagnosis of pulmonary tuberculosis.

Eighty-four specimens of respiratory secretions culture positive for mycobacteria (70 positive for Mycobacterium tuberculosis and 14 positive for nontuberculous mycobacteria) and 120 culture-negative specimens were evaluated by three DNA amplification techniques: a manual in-house single-tube nested PCR (nPCR) and two commercial automated assays (the Cobas Amplicor System [aPCR-h] from Roche Diagnostic Systems and the Abbott LCx Probe System [aLCx-p] from Abbott Laboratories). The overall diagnostic sensitivities of the nPCR, aPCR-h, and aLCx-p were 77.1, 84.3, and 77.1%, respectively, and the sensitivities were 57.9, 57.9, and 36.8%, respectively, for smear-negative specimens. Specimens culture positive for nontuberculous mycobacteria were negative by all three assays. Eight culture-negative specimens which were positive by one or more assays had previously been documented by culture to be positive for M. tuberculosis and were taken from patients who were treated with antituberculosis agents. Retesting of specimens negative by one assay by the other two assays revealed that each test had its unique group of negative specimens. When considering the DNA extraction and amplification steps of these assays separately, it was found that extracts from aPCR-h and aLCx-p were compatible with nPCR amplication, while the two automated assays could only amplify extracts processed with their own reagents. Limiting dilution analysis revealed that the order of analytical sensitivity was nPCR, followed by aLCx-p and then aPCR-h. Comparison of the work flow of each assay revealed that although the aPCR-h demands the least specimen handling, the turnaround time of aLCx-p is the most favorable.

Antigens, Bacterial↗
PubMed1996

Single-tube nested PCR in the diagnosis of tuberculosis.

AIMS: To evaluate the usefulness of a single-tube nested polymerase chain reaction (PCR) assay in the diagnosis of tuberculosis in 1497 pulmonary and 536 extrapulmonary specimens. METHODS: A single-tube nested PCR, utilising two sets of primers with different melting temperatures (88 degrees C for external primers; 70 degrees C for internal primers) to augment sensitivity and specificity without increasing the risk of amplicon contamination, was evaluated. Specimens were initially tested for the repetitive IS6110 sequences and if negative, retested for the universal 38 kilodalton sequence and for inhibitors. dUTP/Uracil-N-glycosylase and Instagene treatment were used to minimise contamination and the effect of inhibitors, respectively. RESULTS: Using culture as the gold standard, the overall sensitivity of the assay was 89% for pulmonary and 42% for extrapulmonary specimens. Sensitivity varied greatly with respect to sample type (92% for follow up specimens from a chest hospital and 70% for non-follow up specimens from a general hospital). The smear positivity rates were 15% for extrapulmonary specimens, and 69% and 45%, respectively, for follow up and non-follow up specimens from pulmonary sites. Specificity was 99.7%. Inhibitors were present more frequently in extrapulmonary than in pulmonary specimens (13.4% v 2.7%). CONCLUSION: Despite the high sensitivity of the PCR assay for the diagnosis of tuberculosis in pulmonary specimens, it was less effective in the extrapulmonary samples. This is probably because of the lower bacterial load in extrapulmonary specimens, the presence of more inhibitors adversely affecting the PCR assay and the higher volume of specimens used for culture.

Base Sequence↗
PubMed1995

Adriamycin-inducible proteins associated with drug sensitivity in resistant immunoblastic B lymphoma cells.

We have previously established an immunoblastic B lymphoma cell line, designated HOB1. This cell line is hypersensitive to a wide spectrum of chemotherapeutic agents. Two co-regulated polypeptides around 64 kDa (termed p64) were induced 10-30-fold in response to adriamycin and some other drugs at the IC50 (the concentration inhibiting cell growth by 50%). These inducible proteins are localized as monomeric forms in the cytosolic fraction, with isoelectric points of pH = 6.2 (major protein) and pH = 7.0 (minor protein). An adriamycin-resistant cell line was established from HOB1 cells. The p64 inducibility was dramatically reduced in resistant HOB1 cells or unrelated cell lines which show phenotypic resistance to adriamycin toxicity. The loss of p64 inducibility in resistant cells is not due to a failure of cells to take up adriamycin since drug accumulation kinetics remained the same as in the parental cells.

Antibodies, Neoplasm↗
PubMed1995

IS6110 based amplityping assay and RFLP fingerprinting of clinical isolates of Mycobacterium tuberculosis.

AIMS: To evaluate the usefulness of two IS6110 based typing methods, an amplityping assay and restriction fragment length polymorphism (RFLP) analysis, for fingerprinting respiratory isolates of Mycobacterium tuberculosis. METHODS: For amplityping, a pair of primers which amplify the intervening sequence between the repetitive insertion sequence IS6110 was used to generate a banding pattern which was confirmed by hybridisation. This assay was compared with conventional chromosomal DNA RFLP typing in the evaluation of 110 epidemiologically diverse isolates. RESULTS: Polymerase chain reaction (PCR) amplityping generated a single pattern in Hong Kong Chinese strains, but two and four diverse patterns in Filipino and Vietnamese strains, respectively, and could be completed within four days. When compared with chromosomal DNA RFLP typing, which took three weeks to complete, four different RFLP patterns could be seen among the Chinese strains, while seven patterns were found in the Filipino and Vietnamese strains. No change in amplityping or RFLP patterns was found in 36 sequential isolates from the same patients after anti-tuberculosis treatment for up to 12 months, despite the emergence of resistance in three of these strains. No specific amplityping or RFLP pattern could be related to different patterns of drug susceptibility. CONCLUSION: PCR amplityping could be used initially as a rapid typing method to distinguish strains originating from different localities. This could be important for investigation of outbreaks of tuberculosis--for example, in refugee camps.

Bacterial Typing Techniques↗
PubMed1994

Changing epidemiology of human salmonellosis in Hong Kong, 1982-93.

A comprehensive analysis of the epidemiology of salmonellosis in a major hospital in Hong Kong from 1982-93 is reported. The trend of salmonella isolations over the past 12 years and changes in the occurrence of individual serotypes are delineated. A total of 5328 isolates were analyzed. Groups B (Salmonella typhimurium and S. derby) and E (S. anatum) were the commonest serogroups isolated from the intestinal tract in all age groups. A significant increase in the isolation of group D salmonellae has been observed since 1989. This is accounted for by a substantial rise in S. enteritidis isolation as seen in Western countries, despite a concomitant decrease of S. typhi. The extraintestinal isolation index (EII) is proposed as an index of the virulence potential of individual serotypes and serogroups. Group D salmonella was found to be the most invasive serogroup. While group D was the predominant serogroup isolated from extraintestinal sites in patients older than 1 year, group B serotypes (especially S. typhimurium) were more frequently seen in infants younger than 12 months.

Adolescent↗
PubMed1994

Genetic relationships and virulence factors among classical enteropathogenic Escherichia coli serogroup O126 strains.

Thirty-nine Escherichia coli strains of the enteropathogenic (EPEC) serogroup O126 isolated from sporadic and outbreak cases of infantile diarrhoea between 1982 and 1988 were studied. These strains consisted of four serotypes showing close genetic relationships between their virulence markers, outer-membrane protein and lipopolysaccharide profiles, and electrophoretic types by multilocus enzyme electrophoresis. None of these strains exhibited localised adherence to HEp-2 cells or the attaching-effacing properties of classical type I EPEC. Of the 39 strains, 31 were of serotype O126:H12 and enterotoxigenic; one strain was serotype O126:H10 and enteroaggregative. The remaining six strains of serotype O126:H21 and one strain of serotype O126:H8 harboured no known virulence factors for diarrhoeagenic E. coli.

Bacterial Outer Membrane Proteins↗