Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “parasite”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 865 records · Page 48Linked to original sources

Amplified fragment length polymorphism measures proportions of malaria parasites carrying specific alleles in complex genetic mixtures.

We are interested in developing a method for the identification of those genes in malaria parasites which underlie a variety of selectable phenotypes of the parasites including drug resistance and strain-specific immunity. A key aspect of our approach is to subject a genetically mixed population of malaria parasites to a specific phenotypic selection pressure such as the administration of an antimalarial drug and then identify genetic markers affected by the selection. Our aim, therefore, is to be able to identify those genetic markers carried by sensitive parasites which disappear from the population after selection as they should be closely linked to the locus determining the phenotype involved. We have previously identified more than 800 amplified fragment length polymorphisms (AFLP) distinguishing two cloned strains of the rodent malaria parasite Plasmodium chabaudi chabaudi and distributed across the whole of the parasites' genome. Here we evaluate the possibility that the intensities of these AFLP bands are quantitatively related to the proportions of parasite DNA which bear these markers in mixtures of genetically different parasites. We prepared mixtures of DNA and parasitised blood from different mixtures of two genetically distinct clones (AS and AJ) of P. c. chabaudi and analysed AFLP markers amplified from them. The results show that the relative band intensities of AFLP markers are, indeed, linearly related to the proportions of parasite DNA in a genetically mixed sample. The precision of the method is sufficient to detect reliably the effects of phenotypic selection at loci closely linked to a genetic locus under selection.

Alleles↗

The immunogenetics of resistance to Trichostrongylus colubriformis and Haemonchus contortus parasites in sheep.

Three possible immunogenetic markers for resistance to intestinal parasites in sheep have been studied. Allotypes of the major histocompatibility complex (MHC) of the sheep have been investigated as markers, using serological typing or gene probes, for associations between allotypes and resistance to parasites in selected high responder and low responder lines of sheep. Only the serologically-determined class I ovine leucocyte antigen (OLA) types SY 1a and SY 1b have been found to be consistently associated with increased resistance to Trichostrongylus colubriformis, but this association has not extended to the immunologically distinct Haemonchus contortus parasite. Gene probes of the sheep DRB, DQB and DQA MHC class II loci have detected animals with increased susceptibility to T. colubriformis. Eosinophilia was investigated as a marker and found to be associated with increased resistance to parasites in lines of Australian Merinos and New Zealand Romneys selected for resistance on the basis of low faecal egg count. Blood eosinophilia was distinct from eosinophil infiltration of the gut which was poorly associated with resistance. The mechanism of parasite resistance appeared to involve the release of vasoactive amines and leukotrienes into intestinal mucus, since the selected high responder sheep to T. colubriformis and H. contortus had significantly increased amounts of these agents in their gut mucus, compared with selected low responder or random-bred sheep. Antibodies to T. colubriformis and H. contortus have also been used as markers to select high responder sire groups of lambs in contact with the parasites, for the first time, at weaning. This assay had the advantage of detecting distinct antigens for the two parasites, which would allow resistance to the species of parasite to be selected in the lambs. Vaccines have been developed against H. contortus using 'novel' gut antigens from the parasite, but variable responsiveness of the host sheep seemed to result in varying degrees of protection which were stimulated by these vaccines.

Animals↗

Effects of parasitism by Asobara tabida (Hymenoptera: Braconidae) on the development, survival and activity of Drosophila melanogaster larvae.

The impact of parasitism by Asobara tabida on Drosophila melanogaster larval development, survival features and larval activity has been investigated using two strains of the parasitoid. The successful parasitism rate of the A1 strain was four times greater than that of the WOPV strain. Both strains induced equivalent mortality rates but hosts parasitized by A1 predominantly died as pupae. The time necessary for the host pupariation and emergence, and the larval weight at 72, 96 and 120 h post-parasitization were measured. Parasitized larvae exhibited longer periods of development and lower weights than controls, especially when parasitized by A1. These results suggest that hosts underwent physiological costs varying with respect to the outcome of the parasitic relationship. Of the parasitoid factors possibly responsible for these costs, we examined venoms for their impact on host mortality. Artificial injections of WOPV venoms induced higher mortality rates than did A1 venoms. Venoms were also found responsible for the induction of a transient paralysis, naturally occuring after parasitization. Again, the strongest effect was observed after parasitization by WOPV or injections of its venoms. This study gives new insights into the intriguing features of A. tabida and constitutes the first report of the paralysing properties of the venoms.

Journal Article↗

Characterisation of a delta-COP homologue in the malaria parasite, Plasmodium falciparum.

The mature human erythrocyte is a simple haemoglobin-containing cell with no internal organelles and no protein synthesis machinery. The malaria parasite invades this cell and develops inside a parasitophorous vacuole (PV). The parasite exports proteins into the erythrocyte to bring about extensive remodelling of its adopted cellular home. Plasmodial homologues of two COPII proteins, PfSar1p and PfSec31p, are exported to the erythrocyte cytosol where they appear to play a role in the trafficking of proteins across the erythrocyte cytoplasm [Eur. J. Cell Biol. 78 (1999) 453; J. Cell Sci. 114 (2001) 3377]. We have now characterised a homologue of the COPI protein, delta-COP. A recombinant protein corresponding to 90% of the Pfdelta-COP sequence was used to raise antibodies. The affinity-purified antiserum recognised a protein with an apparent M(r) of 58 x 10(3) on Western blots of malaria parasite-infected erythrocytes but not on blots of uninfected erythrocytes. Pfdelta-COP was shown to be largely insoluble in non-ionic detergent, possibly suggesting cytoskeletal attachment. Confocal immunofluorescence microscopy of parasitised erythrocytes was used to show that, in contrast to the COPII proteins, Pfdelta-COP is located entirely within the parasite. The location of Pfdelta-COP partly overlaps that of the endoplasmic reticulum (ER)-located protein, PfERC, and partly that of the trans-Golgi-associated protein, PfRab6. Treatment of ring-stage plasmodium-infected erythrocytes with brefeldin A (BFA) inhibited development of the ER structure within the parasite cytosol and prevented the trafficking of the P. falciparum erythrocyte membrane protein-1, PfEMP1, to the erythrocyte cytosol. The Pfdelta-COP and PfSec31p populations each appear to be associated with the restricted ER structure in brefeldin-treated rings. When more mature stage parasites were treated with BFA, erythrocyte cytosol-located populations of parasite proteins were not reorganised, however, the overlap between Pfdelta-COP and PfERC in parasite cytosol was more complete suggesting a possible redistribution of the Golgi compartment into the ER. These data support the suggestion that both COPI and COPII proteins are involved in the trafficking of proteins within the parasite cytoplasm. However, only COPII proteins are exported to the erythrocyte cytosol to establish a vesicle-mediated protein trafficking pathway to the erythrocyte membrane.

Amino Acid Sequence↗

Environmental parasitology: What can parasites tell us about human impacts on the environment?

There are a variety of ways that environmental changes affect parasites, suggesting that information on parasites can indicate anthropogenic impacts. Parasitism may increase if the impact reduces host resistance or increases the density of intermediate or definitive hosts. Parasitism may decrease if definitive or intermediate host density declines or parasites suffer higher mortality directly (eg. from toxic effects on parasites) or indirectly (infected hosts suffer differentially high mortality). Although these scenarios are opposing, they can provide a rich set of predictions once we understand the true associations between each parasite and impact. In this review, Kevin Lafferty discusses how parasite ecologists have used and can use parasites to assess environmental quality.

Journal Article↗

Schistosoma mansoni: stage-dependent membrane vesiculation and parasite killing induced by a cytotoxin from Pseudomonas aeruginosa.

We have examined the interaction between Schistosoma mansoni and a cytotoxin from Pseudomonas aeruginosa known to affect a variety of cell types through the formation of membrane pores. The killing effect of the cytotoxin on S. mansoni in vitro was strongly dependent on the parasite developmental stage. Skin schistosomula were most sensitive, while 4-week-old or older parasites were less so. In contrast, lung schistosomula were relatively resistant and juvenile mesenteric forms younger than 28 days were almost completely refractory to the action of the toxin. A sharp increase in sensitivity to the toxin was observed between 27 and 28 days of parasite age with a parasite length of about 1.80 mm appearing as the threshold. Thus, the schistosome sensitivity to the cytotoxin changed during parasite development in a way similar to the known sensitivity of the worms to in vitro immune attack. Killing of the parasites by the cytotoxin was preceded by the formation of vesicles which, in the case of adult worms, detached easily from the parasites. Evidence obtained by electron microscopy and by immunofluorescent assays with living worms demonstrated that the outer parasite membrane participated in the formation of the vesicle membrane. Thus, the cytotoxin may be a useful tracer for changes in the properties of the schistosome surface membrane during parasite development.

Animals↗

Commitment of the malaria parasite Plasmodium falciparum to sexual and asexual development.

Blood-stage malaria parasites in the vertebrate host can develop either into the asexual, multiplying forms, called schizonts, or into gametocytes, the sexual stages of the parasite. In the present work we studied the differentiation into asexual parasites or gametocytes of the progeny of single, isolated schizonts of the clone 3D7A of Plasmodium falciparum, using monoclonal antibodies specific for the sexual or asexual stages of the parasite. We observed that schizonts obtained from a continuous culture undergoing serial cycles of growth and dilution with fresh red blood cells produced either only gametocytes or only asexual parasites, showing a high degree of commitment to one or the other developmental pathway. The relative proportion of schizonts which produced gametocytes was very low at low parasite densities in culture, while at high parasite densities a much greater proportion of schizonts produced gametocytes. Nevertheless, at both low and high parasite densities individual schizonts were almost always fully committed to producing only gametocytes or only asexual parasites.

Animals↗

Characterization of buffalo-derived theilerial parasites with monoclonal antibodies and DNA probes.

The characteristics of intra-lymphocytic Theileria isolated from African buffalo and from cattle that were infected with buffalo-derived parasites were evaluated using anti-schizont monoclonal antibodies (mAbs) and DNA probes. Antigenic differences were revealed by the reactivities of 27 mAbs with the buffalo-derived parasites isolated from different animals. Antigenic diversity was also seen with Theileria-infected lymphoblastoid cell isolates taken from the lymph nodes and lambda gt11, showed specific hybridization to parasite DNA in Southern blots of restriction enzyme-digested, lymphoblastoid cells infected with buffalo-derived theilerial parasites. Genotypic differences between the buffalo-derived parasites were revealed by the restriction fragment length polymorphisms seen with hybridization of those probes to DNA from cloned and uncloned Theileria-infected cell lines. The evaluation of theilerial parasites derived from buffalo and from cattle which underwent typical T. p. lawrencei reactions, after being infected with buffalo-derived theilerial parasites, did not show any specific phenotypic or genotypic characteristics of these parasites that would distinguish them from T. p. parva and T. p. bovis parasites. The validity of these subspecies distinctions is discussed.

Animals↗

The effect of parasite dose on disease severity in the rodent malaria Plasmodium chabaudi.

Experiments were designed to look at the relationship between infective dose and disease severity using 2 clones of Plasmodium chabaudi that differ in virulence. We asked whether there were dose-severity relationships, whether clone differences in virulence were maintained over a range of doses, and whether disease severity could be accounted for by parasite dynamics. Groups of mice were infected with parasite doses differing by an order of magnitude, ranging from 100 to 1 x 10(8) parasites. Infective dose affected the probability of death, but only with the more virulent clone. Dose also affected morbidity. For both clones, higher doses induced greater anaemia. Larger doses caused greater weight loss, but only for infections with the more virulent clone. Here, for a given dose, mice lost a fixed amount of weight, irrespective of their initial weight. Larger doses induced earlier mortality and morbidity than did lower dose treatments. Finally, dose affected parasite dynamics, with earlier and higher peak parasite densities in larger dose infections. All these effects were small relative to clone differences in disease severity, which were apparent across the range of doses. Dose effects were manifested through the timing and/or magnitude of peak parasite densities, broadly supporting the idea that dose affects disease severity by altering the time the host has to control parasite densities and ameliorate the effects of parasites. We discuss the possible efficacy of intervention strategies aimed at reducing human disease severity by reducing infective parasite dose.

Animals↗

Is body size or activity of copepods related to ingestion of parasite larvae?

This study shows that ingestion of Schistocephalus solidus coracidia was related to general activity of Macrocyclops albidus copepods at the time of exposure. The lower the activity of the host, the fewer parasites it ingested. In an earlier study it was shown that large M. albidus copepods were less likely to become infected with S. solidus than small copepods, which could potentially be caused by differential ingestion of parasites. However, the current study did not show any evidence for such an effect arising through differential ingestion. Body size was not related to ingestion of parasites, but was positively correlated to activity. So, even though size did not significantly relate to ingestion of parasites, if anything, through their higher activity large copepods rather than small copepods may have ingested more parasites. This study indicates that differences in resistance to this parasite do not come about through differential ingestion of parasites. Also, an earlier study failed to show differential elimination of the parasite from the haemocoel. This leaves avoidance of penetration through the gut wall as the most plausible candidate causing large copepods to be more resistant to this parasite than small copepods.

Animals↗

Plasmodium falciparum: selection of parasite subpopulations with decreased sensitivity for antibody-mediated growth inhibition in vitro.

The chronic, recrudescent nature of malaria has been linked to antigenic diversity of the parasite in which protective immunity against Plasmodium falciparum may be obtained after repeated exposure to infection during a long time. In this study we show that parasite populations with decreased sensitivity to antibody-mediated growth inhibition are readily generated in vitro. A laboratory strain, F32, was cultured for long periods (10-12 weeks) in the presence of suboptimal inhibitory antibody concentrations. The antibodies used were the human monoclonal antibody 33G2 reacting with a linear epitope of the P. falciparum blood-stage antigen 332 and rabbit antibodies to repeat sequences of the blood-stage antigen Pf155/RESA. Our data indicate that the P. falciparum parasites adapt to antibody pressure as reflected by their specifically decreased sensitivity to growth inhibition. A relative resistance of the parasite to growth inhibition mediated by the antibodies used in the culture developed successively, while the parasite remained sensitive to growth inhibition by other antibodies. When the antibody pressure was removed a successive return of sensitivity to growth inhibition developed. Immunoflourescence did not reveal any significant difference in antigen expression between the parasite populations. However, PCR analysis showed that a new population appeared in the parasites grown in the presence of mAb 33G2, while no such change was detected in those grown in the presence of the rabbit antibodies. Our data suggest that the specific decrease in sensitivity to growth inhibition may either be due to down-regulation of antigen synthesis or expression by antibody pressure or, that antibody pressure selects for parasites with low expression of a specific antigen from a heterogeneous parasite population.

Animals↗

Sodium-dependent uptake of inorganic phosphate by the intracellular malaria parasite.

As the malaria parasite, Plasmodium falciparum, grows within its host erythrocyte it induces an increase in the permeability of the erythrocyte membrane to a range of low-molecular-mass solutes, including Na+ and K+ (ref. 1). This results in a progressive increase in the concentration of Na+ in the erythrocyte cytosol. The parasite cytosol has a relatively low Na+ concentration and there is therefore a large inward Na+ gradient across the parasite plasma membrane. Here we show that the parasite exploits the Na+ electrochemical gradient to energize the uptake of inorganic phosphate (P(i)), an essential nutrient. P(i) was taken up into the intracellular parasite by a Na+-dependent transporter, with a stoichiometry of 2Na+:1P(i) and with an apparent preference for the monovalent over the divalent form of P(i). A P(i) transporter (PfPiT) belonging to the PiT family was cloned from the parasite and localized to the parasite surface. Expression of PfPiT in Xenopus oocytes resulted in Na+-dependent P(i) uptake with characteristics similar to those observed for P(i) uptake in the parasite. This study provides new insight into the significance of the malaria-parasite-induced alteration of the ionic composition of its host cell.

Animals↗

Involvement of delta-aminolaevulinate synthase encoded by the parasite gene in de novo haem synthesis by Plasmodium falciparum.

The malaria parasite can synthesize haem de novo. In the present study, the expression of the parasite gene for delta-aminolaevulinate synthase (Pf ALAS ) has been studied by reverse transcriptase PCR analysis of the mRNA, protein expression using antibodies to the recombinant protein expressed in Escherichia coli and assay of ALAS enzyme activity in Plasmodium falciparum in culture. The gene is expressed through all stages of intra-erythrocytic parasite growth, with a small increase during the trophozoite stage. Antibodies to the erythrocyte ALAS do not cross-react with the parasite enzyme and vice versa. The recombinant enzyme activity is inhibited by ethanolamine and the latter inhibits haem synthesis in P. falciparum and growth in culture. The parasite ALAS is localized in the mitochondrion and its import into mitochondria in a cell-free import assay has been demonstrated. The import is blocked by haemin. On the basis of these results, the following conclusions are arrived at: PfALAS has distinct immunological identity and inhibitor specificity and is therefore a drug target. The malaria parasite synthesizes haem through the mitochondrion/cytosol partnership, and this assumes significance in light of the presence of apicoplasts in the parasite that may be capable of independent haem synthesis. The Pf ALAS gene is functional and vital for parasite haem synthesis and parasite survival.

5-Aminolevulinate Synthetase↗

Transmission-blocking activity of a chitinase inhibitor and activation of malarial parasite chitinase by mosquito protease.

During development in the mosquito midgut, malarial parasites must traverse a chitin-containing peritrophic matrix (PM) that forms around the food bolus. Previously Huber et al. [Huber, M., Cabib, E. & Miller, L. H. (1991) Proc. Natl. Acad. Sci. USA 88, 2807-2810] reported that the parasite secretes a protein with chitinase activity, and they suggested that parasite chitinase (EC 3.2.1.14) plays an important role in the parasite's egress from the blood meal. We found that allosamidin, a specific inhibitor of chitinase, completely blocked oocyst development in vivo and thus blocked malaria parasite transmission. Addition of exogenous chitinase to the blood meal prevented the PM from forming and reversed the transmission-blocking activity of allosamidin. Using exogenous chitinase, we also found that the PM does not limit the number of parasites that develop into oocysts, suggesting that the parasite produces sufficient quantities of chitinase to penetrate this potential barrier. In addition, we found that treatment of parasite chitinase with a diisopropyl fluorophosphate-sensitive trypsinlike protease from the mosquito midgut or endoproteinase Lys-C increased its enzymatic activity. These results suggest that malaria parasite has evolved an intricate mechanism to adapt to the PM and the protease-rich environment of the mosquito midgut.

Acetylglucosamine↗

Continuous in vitro propagation of the malaria parasite Plasmodium vivax.

The difficulty in controlling Plasmodium vivax, the most common cause of human malaria, has been complicated by growing drug resistance. We have established a method to cycle parasite generations in continuous culture using human blood cells. Chesson strain parasites were passaged from owl monkey erythrocytes to human reticulocytes in McCoy's 5A medium modified with L-glutamine with 25 mM Hepes buffer supplemented with 20% AB+ human serum. Reticulocytes were separated by differential centrifugation in homologous plasma from the peripheral blood of a hemochromatosis patient. Parasites were grown during each 48-hr cycle in a static candle jar environment until the beginning of schizogony, at about 36-40 hr, when reticulocytes were added and cultures transferred to a shaker for 10-12 hr. The addition of a concentration of 10% reticulocytes resulted in stabilizing parasite densities between 0.28 and 0.57 after cycle 3 and increasing the total number of parasites at least 2-fold with each generational cycle. Cultured parasites successfully infected an owl monkey. The morphology of cultured parasites was typical of P. vivax, with highly ameboid trophozoites evident; however, infected erythrocytes were enlarged and distorted on thin film preparations. The species identity of cultivated parasites was confirmed by analysis of the A and C 18S rRNA genes from genomic DNA and expression of only the A gene during erythrocytic asexual growth. The ability to culture P. vivax opens new opportunities to develop vaccines, test drugs, and clone parasites for genome sequencing.

Animals↗

pH regulation in the intracellular malaria parasite, Plasmodium falciparum. H(+) extrusion via a V-type H(+)-ATPase.

The mechanism by which the intra-erythrocytic form of the human malaria parasite, Plasmodium falciparum, extrudes H(+) ions and thereby regulates its cytosolic pH (pH(i)), was investigated using saponin-permeabilized parasitized erythrocytes. The parasite was able both to maintain its resting pH(i) and to recover from an imposed intracellular acidification in the absence of extracellular Na(+), thus ruling out the involvement of a Na(+)/H(+) exchanger in both processes. Both phenomena were ATP-dependent. Amiloride and the related compound ethylisopropylamiloride caused a substantial reduction in the resting pH(i) of the parasite, whereas EMD 96785, a potent and allegedly selective inhibitor of Na(+)/H(+) exchange, had relatively little effect. The resting pH(i) of the parasite was also reduced by the sulfhydryl reagent N-ethylmaleimide, by the carboxyl group blocker N,N'-dicyclohexylcarbodiimide, and by bafilomycin A(1), a potent inhibitor of V-type H(+)-ATPases. Bafilomycin A(1) blocked pH(i) recovery in parasites subjected to an intracellular acidification and reduced the rate of acidification of a weakly buffered solution by parasites under resting conditions. The data are consistent with the hypothesis that the malaria parasite, like other parasitic protozoa, has in its plasma membrane a V-type H(+)-ATPase, which serves as the major route for the efflux of H(+) ions.

Adenosine Triphosphate↗

Selective disruption of phosphatidylcholine metabolism of the intracellular parasite Toxoplasma gondii arrests its growth.

Toxoplasma gondii is an intracellular protozoan parasite capable of causing devastating infections in immunocompromised and immunologically immature individuals. In this report, we demonstrate the relative independence of T. gondii from its host cell for aminoglycerophospholipid synthesis. The parasite can acquire the lipid precursors serine, ethanolamine, and choline from its environment and use them for the synthesis of its major lipids, phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho), respectively. Dimethylethanolamine (Etn(Me)(2)), a choline analog, dramatically interfered with the PtdCho metabolism of T. gondii and caused a marked inhibition of its growth within human foreskin fibroblasts. In tissue culture medium supplemented with 2 mm Etn(Me)(2), the parasite-induced lysis of the host cells was dramatically attenuated, and the production of parasites was inhibited by more than 99%. The disruption of parasite growth was paralleled by structural abnormalities in its membranes. In contrast, no negative effect on host cell growth and morphology was observed. The data also reveal that the Etn(Me)(2)-supplemented parasite had a time-dependent decrease in its PtdCho content and an equivalent increase in phosphatidyldimethylethanolamine, whereas other major lipids, PtdSer, PtdEtn, and PtdIns, remained largely unchanged. Relative to host cells, the parasites incorporated more than 7 times as much Etn(Me)(2) into their phospholipid. These findings reveal that Etn(Me)(2) selectively alters parasite lipid metabolism and demonstrate how selective inhibition of PtdCho synthesis is a powerful approach to arresting parasite growth.

Animals↗

Overexpression of leucyl aminopeptidase in Plasmodium falciparum parasites. Target for the antimalarial activity of bestatin.

Malaria aminopeptidases are important in the generation and regulation of free amino acids that are used in protein anabolism and for maintaining osmotic stability within the infected erythrocyte. The intraerythrocytic development of malaria parasites is blocked when the activity of aminopeptidases is specifically inhibited by reagents such as bestatin. One of the major aminopeptidases of malaria parasites is a leucyl aminopeptidase of the M17 family. We reasoned that, when this enzyme was the target of bestatin inhibition, its overexpression in malaria cells would lead to a reduced sensitivity to the inhibitor. To address this supposition, transgenic Plasmodium falciparum parasites overexpressing the leucyl aminopeptidase were generated by transfection with a plasmid that housed the full-length gene. Transgenic parasites expressed a 65-kDa protein close to the predicted molecule size of 67.831 kDa for the introduced leucyl aminopeptidase, and immunofluorescence studies localized the protein to the cytosol, the location of the native enzyme. The product of the transgene was shown to be functionally active with cytosolic extracts of transgenic parasites exhibiting twice the leucyl aminopeptidase activity compared with wild-type parasites. In vitro inhibitor sensitivity assays demonstrated that the transgenic parasites were more resistant to bestatin (EC50 64 microM) compared with the parent parasites (EC50 25 microM). Overexpression of genes in malaria parasites would have general application in the identification and validation of targets for antimalarial drugs.

Animals↗