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Biomedical subjects

C G Clark

Publications and source records attributed to C G Clark.

At least 19 recordsLinked to original sources
PubMed1992

Entamoeba histolytica: is conversion of "nonpathogenic" amebae to the "pathogenic" form a real phenomenon?

Entamoeba histolytica isolates have been shown to fall into two groups based on isoenzyme analysis. These groupings ("pathogenic" and "nonpathogenic") correlate well with the clinical course of the infection. A controversy exists over whether isoenzyme patterns are stable or whether under certain circumstances an isolate can convert from one form to the other. Resolution of this uncertainty is of importance since the nonpathogenic pattern has never been observed in amebae isolated from cases of active disease. This implies that, if the patterns are stable, carriers of amebae with this nonpathogenic pattern may never develop invasive disease. Although we set out to study isoenzyme conversion, we have been unable to replicate the two published accounts of this phenomenon. We have examined all of the variables proposed to be involved in the triggering of conversion, both individually and in combination. In none of the experiments was an alteration in the isoenzyme pattern observed. We now believe that isoenzyme patterns are stable and that all available evidence, other than the reported conversions, points to pathogenic and nonpathogenic E. histolytica being distinct species.

Animals↗
PubMed1992

Colonization of the uterus by the oral protozoan Entamoeba gingivalis.

Pap smears occasionally reveal protozoa of the genus Entamoeba in the uterus of intrauterine device (IUD) users, but definitive identification of the species involved has not been possible. Using riboprinting, a technique that compares ribosomal RNA gene sequences, we present evidence that the organism is Entamoeba gingivalis, an inhabitant of the mouth. Colonization most likely occurs via orogenital contact and requires the presence of an IUD and a concomitant bacterial infection.

Actinomycosis↗
PubMed1992

Differentiation of pathogenic Entamoeba histolytica from other intestinal protozoa by riboprinting.

Differentiation of the pathogen Entamoeba histolytica from the variety of other amebas that can infect the human intestinal tract is vital for accurate diagnosis and treatment. Morphology and serology alone are not adequate for positive identification to be achieved. We have developed methods using the polymerase chain reaction to amplify amebal ribosomal RNA genes that allow either specific detection of E. histolytica or species identification.

Animals↗
PubMed1991

Synthesis and characterization of a Pertussis toxin-biotin conjugate.

We prepared a Pertussis toxin-biotin conjugate and found its biological properties to be similar to those of native Pertussis toxin with respect to the hemagglutination, Chinese hamster ovary cell, and lymphocyte proliferation assays. Direct binding to Chinese hamster ovary and Jurkat cells was observed using fluorescence microscopy. Pertussis toxin-biotin was also found to possess similar glycoconjugate binding specificities as those of 125I-labeled Pertussis toxin.

Animals↗
PubMed1991

Ribosomal RNA genes of 'pathogenic' and 'nonpathogenic' Entamoeba histolytica are distinct.

Most infections with Entamoeba histolytica are asymptomatic. Two forms of the organism can be distinguished biochemically, and this finding has been explained by two distinct hypotheses: (1) there are two morphologically indistinguishable species, one of which causes disease; (2) there is one species which exists in two interconvertible forms, one of which causes disease. Knowledge of which hypothesis is correct has major implications for evaluation and treatment of carriers. We have studied the ribosomal RNA genes of the two forms hypothesizing that, if E. histolytica is one species, there should be no differences between them. We have found that the ribosomal RNA genes of the two forms are quite distinct, which supports the hypothesis that E. histolytica is two species.

Animals↗
PubMed1991

The Laredo strain and other 'Entamoeba histolytica-like' amoebae are Entamoeba moshkovskii.

A small number of Entamoeba isolates from humans, the best known of which is the 'Laredo' strain, have the ability to grow at room temperature. This peculiarity, along with other characteristics, distinguishes the strains from the human pathogen E. histolytica despite their being morphologically inseparable. In contrast, these 'E. histolytica-like' strains share several features with E. moshkovskii, which is most frequently isolated from polluted water. To examine the taxonomic relationships among these morphologically similar organisms, we have used polymerase chain reaction amplification of the small subunit ribosomal RNA gene combined with restriction fragment length polymorphism analysis, 'riboprinting'. The results clearly show that the 'E. histolytica-like' amoebae are indeed strains of E. moshkovskii, and not closely related to E. histolytica.

Animals↗
PubMed1990

Lymphocyte receptors for pertussis toxin.

We have investigated human T-lymphocyte receptors for pertussis toxin by affinity isolation and photoaffinity labeling procedures. T lymphocytes were obtained from peripheral human blood, surface iodinated, and solubilized in Triton X-100. The iodinated mixture was then passed through pertussis toxin-agarose, and the fractions were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Autoradiography of the fixed, dried gels revealed several bands in the pertussis toxin-bound fraction that were not observed in fractions obtained from histone or fetuin-agarose. Further investigations employed a photoaffinity labeling reagent, sulfosuccinimidyl 2-(p-azido-salicylamido)-1,3'-dithiopropionate, to identify pertussis toxin receptors in freshly isolated peripheral blood monocytic cells, T lymphocytes, and Jurkat cells. In all three cell systems, the pertussis toxin affinity probe specifically labeled a single protein species with an apparent molecular weight of 70,000 that was not observed when the procedure was performed in the presence of excess unmodified pertussis toxin. A protein comparable in molecular weight to the one detected by the photoaffinity labeling technique was also observed among the species that bound to pertussis toxin-agarose. The results suggest that pertussis toxin may bind to a 70,000-Da receptor in human T lymphocytes.

Affinity Labels↗
PubMed1979

Dietary cholesterol is co-carcinogenic for human colon cancer.

Colorectal carcinoma is becoming the most common form of visceral cancer in Western populations. A fat-related dietary factor is implicated in its pathogenesis, and evidence in man suggests that this factor may be cholesterol. Dietary cholesterol is co-carcinogenic in animals with colon cancer, and there is indirect evidence for a similar role in man. It is proposed that prolonged exposure to dietary cholesterol is co-carcinogenic for human colon cancer in that it facilitates the development, growth, and spread of this disease.

Animals↗
PubMed1979

H2-antagonists in the treatment of recurrent ulceration after vagotomy.

Twenty-two patients with recurrent ulceration following vagotomy with and without a drainage procedure have been treated with histamine H2-antagonists and followed for up to 3 years. Four of a group of 6 patients responded to a single therapeutic course. Ten of 16 patients assigned to long term maintenance treatment remain symptom-free. Cimetidine may be an alternative to further surgery in the treatment of some patients with recurrent ulceration after vagotomy.

Adult↗
PubMed1979

Effects of bile acids on mucus secretion in the dog colon.

The effects of unconjugated bile acids on colonic mucus production have been studied in the dog. Isolated segments of both ascending and descending colon were simultaneously perfused with either chenodeoxycholic, deoxycholic or cholic acid and the mucus production estimated from the total hexose output. Each bile acid produced a significant increase in mucus output in both the right and left colonic segments. There were no differences between the right and left colon in terms of mucus output for any of the bile acids studied. Deoxycholic had a greater effect than chenodeoxycholic which had a greater effect than cholic acid in colonic mucus production. Further, these effects on mucus secretion were completely reversible. In diarrhoea associated with disease or resection of the ileum, the effect of bile acids on mucus secretion may be as important as their effects on the inhibition of water and electrolyte absorption.

Animals↗
PubMed1978

Failure of bran to protect against experimental colon cancer in rats.

The ability of dietary fibre to prevent colon cancer was tested in rats injected with 1,2-dimethylhydrazine (D.M.H.), a substance which induces cancer of the colon. 60 rats were fed the same formula solid diet but allocated three different amounts of dietary fibre--20 were given 4.8% w/w crude fibre, 20 were given 20% w/w bran, and 20 received no fibre. Half of the animals in each fibre group received a course of subcutaneous D.M.H. and half were given subcutaneous saline. After a year's observation, there was no significant difference in the incidence of, or mortality from, colonic carcinoma between the D.M.H.-treated groups.

Adenocarcinoma↗