Entamoeba histolytica: effect of growth conditions and bacterial associates on isoenzyme patterns and virulence

Attenuation of In Vitro and In Vivo Virulence Is Associated with Repression of Gene Expression of AIG1 Gene in Entamoeba histolytica

Entamoeba histolytica virulence results from complex host-parasite interactions implicating multiple amoebic components (e.g., Gal/GalNAc lectin, cysteine proteinases, and amoebapores) and host factors (microbiota and immune response). UG10 is a strain derived from E. histolytica virulent HM-1:IMSS strain that has lost its virulence in vitro and in vivo as determined by a decrease of hemolytic, cytopathic, and cytotoxic activities, increased susceptibility to human complement, and its inability to form liver abscesses in hamsters. We compared the transcriptome of nonvirulent UG10 and its parental HM-1:IMSS strain. No differences in gene expression of the classical virulence factors were observed. Genes downregulated in the UG10 trophozoites encode for proteins that belong to small GTPases, such as Rab and AIG1. Several protein-coding genes, including iron-sulfur flavoproteins and heat shock protein 70, were also upregulated in UG10. Overexpression of the EhAIG1 gene (EHI_180390) in nonvirulent UG10 trophozoites resulted in augmented virulence in vitro and in vivo. Cocultivation of HM-1:IMSS with E. coli O55 bacteria cells reduced virulence in vitro, and the EhAIG1 gene expression was downregulated. In contrast, virulence was increased in the monoxenic strain UG10, and the EhAIG1 gene expression was upregulated. Therefore, the EhAIG1 gene (EHI_180390) represents a novel virulence determinant in E. histolytica.

Pathology Associated With an Outbreak of Entamoebiasis in Wild Cane Toads (Rhinella marina) in Tropical Australia

Infection due to Entamoeba spp. is known to cause serious disease in primates (Entamoeba histolytica) and snakes (Entamoeba invadens), but there are no detailed descriptions of the pathology associated with Entamoeba spp. infection in amphibians. In 2014, an outbreak of entamoebiasis associated with a novel species of Entamoeba induced clinical illness and poor body condition in free-ranging cane toads in Australia's Northern Territory. Here, we describe the gross pathology, histology, and clinical pathology linked to the outbreak. The study compared 25 toads with invasive entamoebiasis, defined as histologically visible amoebas within tissue, and 12 toads without invasive entamoebiasis. Grossly, affected toads had mild to marked congestion of colonic serosal vasculature, with variable thickening of the intestinal wall and serosanguineous to hemorrhagic colonic content. Histologically, invasive entamoebiasis manifested primarily as moderate to severe, variably hyperplastic to ulcerative colitis. The small intestine was affected in 10 of 25 toads, and 5 of 25 toads also had gastric lesions. Amoebas consistent in morphology with Entamoeba sp. were commonly intermingled with mucosal epithelium, frequently along the basement membrane, with deeper invasion into the superficial lamina propria in only 5 toads. Toads with invasive entamoebiasis had neutrophilia, monocytosis, and lymphopenia, and thus elevated neutrophil to lymphocyte ratios, suggestive of an inflammatory and/or stress leukogram.

Entamoeba histolytica: protein arginine transferase 1a methylates arginine residues and potentially modify the H4 histone

BACKGROUND

In eukaryotes, histone arginine methylation associates with both active and repressed chromatin states depending on the residues involved and the status of methylation. Even when the amino-terminus of Entamoeba histolytica histones diverge from metazoan sequences, these regions contain arginine residues that are potential targets for methylation. However, histone arginine methylation as well as the activity of arginine methyltransferases (PRMTs) has not been studied in this parasite. The aim of this work was to examine the dimethylation of arginine 3 of H4 histone (H4R3me2) and to identify the parasite PRMT that could be responsible for this modification (EhPRMT1).

METHODS

To examine the presence of H4R3me2 in E histolytica, we performed Western blot and immunofluorescence assays on trophozoites using an antibody against this epigenetic mark. To recognize the PRMT1 enzyme of this parasite that possibly perform that modification, we first performed a phylogenetic analysis of E. histolytica and human PRMTs. RT-PCR assays were carried out to analyze the expression of the putative PRMT1 genes. One of these genes was cloned and expressed in Escherichia coli. The recombinant protein was tested by its recognition by an antibody against human PRMT1 and in its ability to form homodimers and to methylate commercial histones.

RESULTS

The arginine 3 of human H4, which is subjected to post translational methylation, was aligned with the arginine 8 of E. histolytica H4, suggesting that this residue could be methylated. The recognition of an 18 kDa nuclear protein of E. histolytica by an antibody against H4R3me2 confirmed this assumption. We found that this parasite expresses three phylogenetic and structural proteins related to PRMT1. Antibodies against the human PRMT1 detected E. histolytica proteins in cytoplasm and nuclei and recognized a recombinant PRMT1 of this parasite. The recombinant protein was able to form homodimers and homotetramers and displayed methyltransferase activity on arginine 3 of chicken H4.

CONCLUSION

All these results suggest that E. histolytica contains as a minimum one structural and functional protein ortholog to PRMT1, enzyme that potentially dimethylates H4R8. This modification may play an important role in the gene expression regulation of this microorganism.

Profiles of Entamoeba histolytica-specific immunoglobulins in human sera

OBJECTIVE

To determine the profiles of anti-Entamoeba histolytica (E. histolytica) IgA, IgG, and IgM in sera of diarrheic and non-diarrheic individuals and partially characterize target antigens.

METHODS

Serum samples from thirty diarrheic and thirty non-diarrheic individuals were subjected to IgA, IgG, and IgM profiling through enzyme-linked immunosorbent assay (ELISA), flow cytometry, and immunoblot.

RESULTS

ELISA titer results showed that both diarrheic and non-diarrheic individuals possess high levels of E. histolytica-specific IgG compared to IgA and IgM. Flow cytometry data showed that diarrheic serum samples had higher mean reaction percentages against E. histolytica cells compared to non-diarrheic samples. Immunoreactive E. histolytica proteins with molecular weights ranging between 7 kDa and 292 kDa were recognized by diarrheic serum IgG, and 170 kDa and 250 kDa by non-diarrheic serum IgG.

CONCLUSIONS

Our findings suggest that serum anti-E. histolytica IgG, compared with serum anti-E. histolytica IgA and IgM responses, was generally high in both diarrheic and non-diarrheic sera, indicating a past exposure to the organism both in symptomatic patients as well as in asymptomatic carriers, respectively. In addition, serum IgG from diarrheic and non-diarrheic patients were able to detect immunogenic E. histolytica proteins.

Glucose starvation boosts Entamoeba histolytica virulence

The unicellular parasite, Entamoeba histolytica, is exposed to numerous adverse conditions, such as nutrient deprivation, during its life cycle stages in the human host. In the present study, we examined whether the parasite virulence could be influenced by glucose starvation (GS). The migratory behaviour of the parasite and its capability to kill mammalian cells and to lyse erythrocytes is strongly enhanced following GS. In order to gain insights into the mechanism underlying the GS boosting effects on virulence, we analyzed differences in protein expression levels in control and glucose-starved trophozoites, by quantitative proteomic analysis. We observed that upstream regulatory element 3-binding protein (URE3-BP), a transcription factor that modulates E.histolytica virulence, and the lysine-rich protein 1 (KRiP1) which is induced during liver abscess development, are upregulated by GS. We also analyzed E. histolytica membrane fractions and noticed that the Gal/GalNAc lectin light subunit LgL1 is up-regulated by GS. Surprisingly, amoebapore A (Ap-A) and cysteine proteinase A5 (CP-A5), two important E. histolytica virulence factors, were strongly down-regulated by GS. While the boosting effect of GS on E. histolytica virulence was conserved in strains silenced for Ap-A and CP-A5, it was lost in LgL1 and in KRiP1 down-regulated strains. These data emphasize the unexpected role of GS in the modulation of E.histolytica virulence and the involvement of KRiP1 and Lgl1 in this phenomenon.

During infection, pathogens are exposed to different environmental stresses that are mostly the consequence of the host immune defense. The most studied of these environmental stresses are the response of pathogens to nitric oxide and to hydrogen peroxide, both produced by phagocytes. In contrast, the overall knowledge about the response of pathogens to metabolic stresses is scanty. Amebiasis is caused by the unicellular protozoan parasite Entamoeba histolytica, and has a worldwide distribution with substantial morbidity and mortality. During its journey in the host, the parasite is exposed to the host immune system and to variations in nutrient availability due to the host nutrition status and the competition with the bacterial flora of the large intestine. How E. histolytica responds to glucose starvation (GS) has never been investigated. Here, the authors report that the parasite virulence is boosted by GS. Paradoxically, two well accepted virulence factors, the amoebapore A and the cysteine protease A5 are less abundant in the glucose-starved parasites. This Accordingly, these proteins are not required for the boosting of the E. histolytica virulence, in contrast to KRiP1 and LgL1 that seem to be involved in this phenomenon.

Regulation of gene expression in protozoa parasites

Infections with protozoa parasites are associated with high burdens of morbidity and mortality across the developing world. Despite extensive efforts to control the transmission of these parasites, the spread of populations resistant to drugs and the lack of effective vaccines against them contribute to their persistence as major public health problems. Parasites should perform a strict control on the expression of genes involved in their pathogenicity, differentiation, immune evasion, or drug resistance, and the comprehension of the mechanisms implicated in that control could help to develop novel therapeutic strategies. However, until now these mechanisms are poorly understood in protozoa. Recent investigations into gene expression in protozoa parasites suggest that they possess many of the canonical machineries employed by higher eukaryotes for the control of gene expression at transcriptional, posttranscriptional, and epigenetic levels, but they also contain exclusive mechanisms. Here, we review the current understanding about the regulation of gene expression in Plasmodium sp., Trypanosomatids, Entamoeba histolytica and Trichomonas vaginalis.

Structural and functional analysis of the Entamoeba histolytica EhrabB gene promoter

BACKGROUND

The Entamoeba histolytica EhrabB gene encodes for a Rab GTPase involved in phagocytosis. It is located at a virulence locus where the Ehcp112 gene is in the complementary strand at 332 bp of EhrabB start codon, suggesting a finely regulated transcription of both genes. However, the transcription regulation in this parasite is poorly understood.

RESULTS

To initiate the knowledge of EhrabB gene expression regulation, here we studied the structural characteristics of its gene promoter and its control transcription elements. In silico searches of the EhrabB 5'-flanking region revealed that it contains a motif similar to the upstream regulatory element 1 (URE1) of the E. histolytica hgl5 gene. It also has sequences with homology to C/EBP and GATA1 binding sites, and heat shock elements (HSE). Primer extension experiments revealed that EhrabB has at least four transcription initiation sites. The elements at the 5'-flanking region that drive EhrabB gene expression were detected and characterized using transitory transfected trophozoites with a plasmid carrying the CAT reporter gene. EhrabB transcription is negatively regulated by a sequence located between positions -491 to -428 with respect to the first transcription initiation site. We also showed that the URE1-like motif activates EhrabB transcription. In addition, heat shock activated the EhrabB promoter in episomal constructs and lead to an increase in de novo EhrabB transcription.

CONCLUSION

The data suggest that EhrabB transcription is controlled negatively by an unidentified sequence, but it is activated by an URE1-like motif. Our analyses also revealed the presence of activator HSE that function under stress.

The pathogenesis of amoebiasis

Amoebiasis, the infection of humans with Entamoeba histolytica, has a worldwide distribution; humans are the main reservoir and source of infection(1), although some other primates can also be infected. The motile trophozoite of E. histolytica (Fig. 1) lives in the lumen of the large intestine where it multiplies and eventually differentiates into cysts which are shed in the faeces and are responsible for transmission of infection. Two forms of amoebiasis are recognized: luminal amoebiasis where no clinical signs or symptoms are apparent, and invasive amoebiasis where the trophozoites invade the intestinal mucosa to produce dysentery or amoeboma, and can spread in blood to give extraintestinal lesions such as liver abscess. Isoenzyme markers for pathogenic and non-pathogenic types of E. histolytica are well documented, but there is some debate (see Parasitology Today, vol. 3, 37-43) about whether the two types represent completely separate entities or if they can change from one type to the other under certain circumstances (Box 1). Nonpathogenic types produce no apparent symptoms; in this article Adolfo Martínez-Palomo discusses the pathology associated with pathogenic types.

Multilocus enzyme electrophoresis: a valuable technique for providing answers to problems in parasite systematics

The aim of this review is to highlight the effectiveness of the technique of multilocus enzyme electrophoresis in answering questions relating to the systematics of parasites and to highlight errors in the way the technique has been used and the results interpreted. We have approached this topic by answering specific questions that we have been asked by colleagues and students not necessarily familiar with the technique, the method of data analysis and its application. Although the technique has been applied to provide answers for taxonomic and population genetics studies, it remains under-utilised, perhaps because of recent advances in newer molecular technology. Rather than not acknowledge or dismiss the value of more traditional technology, we suggest that researchers examine problems in the systematics of parasites by the comparison of data derived from morphological, biochemical and molecular techniques.

Royal Society of Tropical Medicine and Hygiene meeting at Manson House, London, 19 February 1998. Amoebic disease. Entamoeba dispar, an organism reborn

Entamoeba dispar was described by Emile Brumpt in 1925, and was promptly dismissed as a synonym of E. histolytica by almost all his contemporaries. In the 1970s, however, evidence began to accumulate indicating that Brumpt may have been correct and E. dispar is now accepted as a distinct species. This review introduces the organism's history and briefly summarizes what we currently know about its biology. The acceptance of E. dispar has profound implications for our understanding of the epidemiology of amoebiasis and, ultimately, may give us an insight into what makes E. histolytica a pathogen.

Control of ferredoxin and Gal/GalNAc lectin gene expression in Entamoeba histolytica by a cis-acting DNA sequence

The ferredoxin (fdx) and lectin (hgl5) promoters of Entamoeba histolytica contain the DNA sequence motif TATTCTATT (URE3). Previously we showed that mutation of the URE3 motif in the hgl5 lectin promoter results in an increase in promoter reporter activity. Mutation of this motif in the fdx promoter led to a 40-to-50% decrease in fdx promoter activity as measured by reporter gene activity and abundance of mRNA. E. histolytica nuclear proteins exhibited sequence-specific binding to the URE3 motif in electrophoretic mobility shift assays. These results support the regulation of both ferredoxin and lectin promoters by a nuclear protein(s) which recognizes the URE3 motif.

Entamoeba dispar: cultivation with sterilized Crithidia fasciculata

Four isolates of Entamoeba dispar identified by their hexokinase and phosphoglucomutase isoenzyme profile and by their failure to react with Entamoeba histolytica-specific monoclonal antibody (4G6) could be grown in either Diamond's BI-S-33 medium, newly developed BCSI-S (Biosate cysteine starch iron-serum) medium, or casein-free YI-S medium in the presence of Crithidia fasciculata (ReF-1:PRR) sterilized by heating 56 degrees C for 30 min and subsequent incubation with 1% hydrogen peroxide for 24 hours at 4 degrees C. After the cultures were maintained for over 50 passages, the amebae were identified as E. dispar by isoenzyme analysis, polymerase chain reaction with E. histolytica- and E. dispar-specific primers, i.e. p11 plus p12 and p13 plus p14, respectively, and by negative reactivity with monoclonal antibody 4G6. The flagellates added to the culture were judged to be metabolically inactive based on the results of nuclear magnetic resonance spectroscopy, electron microscopy, and polarographic analysis. All of these findings suggest that E. dispar can grow in vitro with metabolically inactive C. fasciculata as a culture associate.

Identification of significant variation in the composition of lipophosphoglycan-like molecules of E. histolytica and E. dispar

The lipophosphoglycan-like (LPG-like) molecules of E. histolytica virulent strains are clearly distinct from those of the avirulent E. histolytica and E. dispar strains. Abundant 'LPG' levels are apparently limited to virulent strains, while lipophosphopeptidoglycans ('LPPG's) are common to both virulent and avirulent strains of E. histolytica and E. dispar. It is therefore conceivable that 'LPPG' performs a function that is essential to survival within the host, while the 'LPG' performs a more specific function related to virulence.

Entamoeba histolytica and Entamoeba dispar are distinct species; clinical, epidemiological and serological evidence

The name of the causative organism of invasive amoebiasis, Entamoeba histolytica, was first introduced in 1903, even though this intestinal amoeba had been recognised since 1875. The marked disparity between the number of infected individuals and those with invasive amoebiasis resulted in a number of explanatory hypotheses being proposed. Although none of these were universally accepted, Brumpt's concept of two morphologically identical species gained increasing acceptance 50-60 years later when technology became available to investigate this anomaly. Sargeaunt spear-headed this drive by establishing the value of isoenzyme electrophoresis for studying the host-parasite relationship. From this foundation, incorporation of clinical, epidemiological and serological parameters to studies of the parasite resulted in the conclusion that a species complex comprising two morphologically identical amoebae was implicated with the disease. The two organisms have been named E. histolytica and Entamoeba dispar. The former is a pathogen and is responsible for invasive amoebiasis, while the latter is a gut commensal. Demonstration of the existence of this species complex has subsequently been confirmed by studies on the nucleic acids from several independent laboratories. The acceptance of E. histolytica and E. dispar as distinct species has had a major impact on our understanding of amoebiasis and its clinical management.

Riboprinting: a tool for the study of genetic diversity in microorganisms

Classical morphology-based methods of taxonomic and phylogenetic analysis are inadequate in many groups of structurally simple eukaryotes. Molecular methods can generate data independently of the complexity of the organisms' morphology. Riboprinting is one such technique, and involves restriction enzyme analysis of polymerase chain reaction amplified small subunit ribosomal RNA genes. The utility of the method is illustrated with examples from several genera of intestinal and bloodstream parasites. Among the applications of riboprinting are the detection of cryptic genetic variation within species, organism misidentifications and culture mix-ups, independent verification of DNA sequences, and the rapid generation of data useful in phylogenetic analyses.

Preparation of a monoclonal antibody specific for Entamoeba dispar and its ability to distinguish E. dispar from E. histolytica

A monoclonal antibody (MAb), MAb ED17 (immunoglobulin G2a [IgG2a]), prepared against trophozoites of Entamoeba dispar SAW1734RclAR cultured monoxenically with Crithidia fasciculata, reacted with 25 of 26 isolates of E. dispar by an indirect fluorescent-antibody test. In contrast, the MAb failed to react with any of 20 isolates of E. histolytica or other enteric protozoan parasites. Western blot (immunoblot) analysis showed that the molecular mass of the E. dispar antigen recognized by the MAb was 160 kDa under reduced conditions. Immunoelectron microscopy revealed that the antigen was mainly located on digested C. fasciculata, but not on undigested organisms. Double staining with a mixture of MAb ED17 and MAb 4G6 (an IgG1 MAb which reacts exclusively with E. histolytica), followed by incubation with a mixture of fluorescein isothiocyanate-labeled anti-mouse IgG2a and tetramethylrhodamine isothiocyanate-labeled anti-mouse IgG1 antibodies, simultaneously identified mixed populations of E. dispar and E. histolytica. This method may prove to be useful for the accurate identification of E. dispar and E. histolytica, even in mixed infections.

Direct sequencing of the PCR amplified SSU rRNA gene of Entamoeba dispar and the design of primers for rapid differentiation from Entamoeba histolytica

Since 1993, strains of Entamoeba histolytica sensu lato have been assigned to 2 species on the basis of clinical, biochemical, immunological and genetic evidence: the pathogenic strains to E. histolytica sensu stricto, the non-pathogenic strains to Entamoeba dispar. Analysis of the gene encoding for the small subunit ribosomal RNA (SSU rDNA) supports the existence of 2 species. However, while 3 whole SSU rDNA sequences are available in the data bases for E. histolytica, only a partial sequence has been published for E. dispar. Here we report a SSU rDNA sequence for E. dispar. Compared to those of E. histolytica, this sequence shows 1.7% nucleotide substitutions. On the basis of our rDNA data, 2 primers were designed to produce polymerase chain reaction (PCR) amplification from both E. histolytica and E. dispar. Primer specificity for the 2 amoebae was assessed both theoretically against the data bases, and experimentally against a collection of eukaryotic and prokaryotic DNAs. The amplified stretch encompasses a polymorphic Dde I restriction site which allows, after cleavage of the fragment, E. histolytica and E. dispar to be distinguished. The reliability of this method of identification was assessed comparing the results with those based on classic isoenzyme analysis.

Differentiation of Korean isolates of Entamoeba histolytica from Entamoeba dispar

Cysts of Entamoeba histolytica are still found from humans in Korea, but not all of the cysts are known as pathogenic. The non-pathogenic strain is regarded as a different species, E. dispar. In this study, Korean isolates of conventional E. histolytica were subjected for the differentiation by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analysis. Human stools were screened by routine microscopic examination, and cyst or trophozoite positive stools were inoculated into Robinson media. The cultivated trophozoite positive stools were inoculated into Robinson media. The cultivated trophozoites were prepared for DNA extraction, and the DNAs were used for PCR with common primers of P1 gene. The PCR products were digested with 3 restriction enzymes and RFLP was observed. Also anti-sense primers containing the cleavage site of each restriction enzyme were designed for differentiation only by PCR. The PCR products of Korean isolates S9, S12, YS-6, and YS-27 were spliced by Taq I and Xmn I but not by Acc I, and the isolates S1, S3, S11, S15, S16, S17, S20, YS-17, and YS-44 were spliced by Acc I but not by Taq I and Xmn I. These RFLP pattern correlated well with PCR products by the species specific primers. The findings confirm that the Korean isolates S9, S12, YS-6, and YS-27 are E. histolytica and others are E. dispar. In Korea, most of the asymptomatic cyst carriers are infected by E. dispar, not by E. histolytica.

Influence of bacteria upon cytopathic effect and erythrophagocytosis of different axenic strains of Entamoeba histolytica

At this moment, the duality of species suggested for E. histolytica is being considered for discussion. In order to contribute to settling this question, we investigated the possibility of conversion of avirulent ameba to virulent ones, as well as, the possibility of increasing virulence of virulent strains, by means of association with bacteria. Five strains of E. histolytica were employed, two of them regarded as avirulent and three virulent ones. Amebas were associated with the bacteria Escherichia coli 055 and 0115, previously demonstrated as capable to modify the pathogenic behavior of E. histolytica. Changes in virulence of amebas were assessed by cytopathic effect upon cultured mammal cells and erythrophagocytosis. The virulence of pathogenic strains was significantly increased after bacteria association in opposition to what was observed for nonpathogenic ones, which were not influenced by bacteria association.

A redescription of Entamoeba histolytica Schaudinn, 1903 (Emended Walker, 1911) separating it from Entamoeba dispar Brumpt, 1925

Explaining the low incidence of invasive disease (10%) in humans infected with Entamoeba histolytica has occupied the attention of generations of both clinical and nonclinical investigators. One possible explanation would be the existence of two morphologically identical species-one an invasive pathogen, the other noninvasive. This was first proposed by Brumpt in 1925, but his explanation was virtually ignored until 1978 when the first of several publications appeared suggesting that E. histolytica did indeed consist of two species. We have reexamined Brumpt's claim in light of recent biochemical, immunological and genetic studies and conclude that the data derived from these investigations provide unequivocal evidence supporting his hypothesis. With this in mind, we redescribe the invasive parasite retaining the name Entamoeba histolytica Schaudinn, 1903 (Emended Walker, 1911), and set it apart from the noninvasive parasite described by Brumpt, Entamoeba dispar Brumpt, 1925.

Entamoeba histolytica: changes in the zymodeme of cloned nonpathogenic trophozoites cultured under different conditions

In this paper we report the occurrence of changes in the migration of certain isoenzymes of a cloned strain (MAV-CINVESTAV) of Entamoeba histolytica. This strain was isolated from an asymptomatic carrier and showed an initial nonpathogenic zymodeme I. The transfer of polyxenic trophozoites from Robinson's medium (7% serum) to Jones' medium (30% serum) provoked changes in isoenzyme expression, resulting in the conversion of zymodeme I to a zymodeme that was similar to the XVII zymodeme except for two slow-running bands and a single fast-running band that were detected for hexokinase (HK). This XVII-like zymodeme reverted to zymodeme I when trophozoites were cultured under monoxenic conditions in TY1-S-33 medium (10% serum). When we cultured cloned strain MAV-CINVESTAV under axenic conditions in TY1-S-33 medium, trophozoites expressed a pathogenic zymodeme with two fast-running HK bands and a beta-phosphoglucomutase band. In addition, phagocytosis and the ability of the trophozoites to destroy cell-culture monolayers were expressed only in trophozoites cultured under axenic conditions. The axenization procedure required the presence of live Fusobacterium symbiosum and is also described herein.

The amoeba enigma

After more than 70 years of intermittent debate over the true relationship between the 'pathogenic' and 'non-pathogenic' forms of Entamoeba histolytica, the application of molecular biology has finally yielded an unambiguous answer: these are not interconvertible phenotypes of the same parasite, a kind of unicellular Jekyll and Hyde, but two quite distinct genetic entities that just happen to look the same. But given the overwhelming evidence now available from gene sequences, pointing to an evolutionary divergence some tens of millions of years ago, why is it that certain eminent workers in the field are still claiming that, at least in vitro, conversion between the two phenotypes can take place? In this article Bill Spice and John Ackers review recent developments in the molecular biology of E. histolytica and assess the continuing controversy over the status of this enigmatic parasite.

Selection of specific genotypes of Giardia intestinalis by growth in vitro and in vivo

This study examined whether allelic changes observed when clinical isolates of Giardia intestinalis made in suckling mice were adapted to in vitro growth occurred as a result of gene switching (alternate isoenzymes) or through selection of organisms with different genotypes from mixed infections. Samples were compared electrophoretically at 20 enzyme loci. Marked allelic differences were detected between the uncloned clinical isolates grown in mice and the axenic cultures established from them. Furthermore, the allelic profiles of the uncloned isolates changed during the course of in vivo or in vitro growth. In contrast, all clones produced from each isolate retained identical allelic profiles, regardless of whether they were grown in vivo or in vitro. These findings argue against gene switching as an explanation for the observed allozyme changes and support preferential selection of organisms with specific genotypes by growth conditions. The data indicate the presence of at least 2 and possibly up to 4 distinct genotypes within each clinical isolate. The genetic differences detected between clinical isolates in suckling mice were of similar magnitude to those that separate different axenic isolates of G. intestinalis into cryptic species. Conversely, the genetic differences between the isolates were limited when sampled after establishment in vitro. These findings have significant implications for research on Giardia and other medically important parasites and raise the possibility that culture may exert a similar selective bias on the genotypes isolated from infections with other parasitic protozoa.

Intestinal parasites in returned travelers

Travelers returning from third-world countries may become infected with a variety of intestinal parasites. Although protozoan infections are more frequently seen, intestinal worms are also encountered. If considered in the differential diagnosis, these infections usually are readily diagnosed and treated.

Non-pathogenic entamoeba histolytica in Italian HIV-infected homosexuals

A cohort of 51 homosexuals who were either HIV-positive or had AIDS was followed prospectively with parasitologic stool examination and in vitro culture in order to determine the incidence of E. histolytica infection. Amoebic isolates were further characterized by electrophoretic isoenzyme study. Five subjects (9.8%) were found to be infected with E. histolytica. None of the amoebic isolates were found to be pathogenic by isoenzyme analysis.

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.

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.

Differences in genomic DNA sequences between pathogenic and nonpathogenic isolates of Entamoeba histolytica identified by polymerase chain reaction

A lambda gt11 cDNA library was constructed from the poly(A)+ RNA of trophozoites of Entamoeba histolytica HM-1:IMSS strain. The library was immunologically screened with monoclonal antibody 4G6, which is specific for the 30,000-Mr antigen of pathogenic isolates. A 0.7-kb clone was isolated, and its nucleotide sequence was determined. To examine whether this gene was specific for pathogenic isolates, a polymerase chain reaction was performed by using four sets of primers and the genomic DNA of pathogenic and nonpathogenic isolates as templates. Amplified DNAs were detected not only in pathogenic isolates but also in nonpathogenic isolates. However, when sequences of amplified DNA of these isolates were compared, minor differences were observed. By considering the presence or absence of recognition sites of some endonucleases, it was possible to distinguish between the pathogenic and nonpathogenic isolates. When various isolates with different zymodemes were examined by polymerase chain reaction and enzyme digestion, the results of typing were entirely in accord with those of zymodeme analysis. These results indicate that there is dimorphism in the genomic DNA coding the 30,000-Mr antigen of E. histolytica and that the combined use of the polymerase chain reaction and enzyme digestion is a useful strategy for identification of species and determination of pathogenicity.

Isoenzymes in Entamoeba as detected by isoelectrofocusing

Two major zymodemes from different Entamoeba histolytica strains were detected, despite different growth conditions, using isoelectrofocusing in ultrathin gels. One was associated with strains cultured from symptomatic patients, while the other was associated with microorganisms obtained from clinically symptomatic persons.

Leishmania major: resistance of promastigotes to paromomycin, and susceptibility of amastigotes to paromomycin-methylbenzethonium chloride ointment

Cutaneous lesions caused by Leishmania major in BALB/c mice were cured completely when treated topically with an ointment comprising 15% paromomycin sulphate and 1-2% methylbenzethonium chloride ointment in soft white paraffin twice daily for 10 days. No parasites were detected in tissue smears or in cultures from treated cutaneous lesions. Re-developing lesions, considered to be resulting from the migration of parasites from internal organs, showed almost the same response to topical treatment. Promastigotes of the virulent clone 121 of L. major LRC-L137 which were exposed to 100 micrograms ml-1 of paromomycin in RPMI medium at 28 degrees C developed resistance to the drug over 10 passages of exposure. Enzyme analysis of susceptible and resistant promastigotes of this clone showed no differences with regard to their profiles based on 11 enzymes.

Variations in cytotoxicity and isoenzyme patterns of uncloned and cloned cultures of axenic Entamoeba histolytica

Five clones of axenic Entamoeba histolytica (HMI) grown as discrete colonies in semisolid agar medium were adapted in liquid medium and labelled as HMI-C121, HMI-C131, HMI-C143, HMI-C144 and HMI-C145. The clone HMI-C121 was more cytotoxic to the cultured Baby Hamster Kidney (BHK) cells while all other clones were significantly (P less than 0.001) less cytotoxic as compared to the cloned HMI-C121 and uncloned E. histolytica (HMI). The uncloned Indian axenic E. histolytica (KCG:0986:11) as well as E. histolytica (NIH:200) cultures were significantly (P less than 0.001) less cytotoxic to cultured BHK cells. No difference in the electromobility of maleate NADP oxidoreductase (ME) or glucophosphate isomerase (GPI) isoenzyme in the lysates of all the cloned and uncloned cultures of E. histolytica was observed. The clones HMI-C121, HMI-C131, HMI-G143 and HMI-C144 had three bands of hexokinase (HK) while all uncloned cultures and one of clones, HMI-C145 had only two bands. Though cloned and uncloned cultures had a single PGM band, the relative electromobility (rf) of phosphoglucomutase (PGM) for clone HMI-C131, HMI-C143 HMI-C144 was relatively less (rf 0.075) and these were also significantly (P less than 0.001) less cytotoxic to BHK cells as compared to clone HMI-C121. It is felt that axenic E. histolytica culture consists of several populations (clones) and expression of isoenzymes pattern or cytotoxic potentials would depend upon the population which predominantly multiples and outgrows other populations in the culture system.

Isolation of a strain-specific Entamoeba histolytica cDNA clone

Entamoeba histolytica is an intestinal parasite causing significant morbidity and mortality worldwide. More tools are needed to understand the epidemiology and molecular pathogenesis of amebiasis. A cDNA library was constructed by using poly(A)+ RNA isolated from an axenic strain of E. histolytica, HM1:IMSS, which expresses a pathogenic isoenzyme pattern (zymodeme). Differential screening of the library yielded a strain-specific 3' polyadenylated cDNA clone, C2, possessing nine 26-nucleotide tandem repeats. RNA and DNA transfer blot analysis of four axenic strains of E. histolytica possessing the same pathogenic zymodeme revealed that the gene is present and expressed in pathogenic E. histolytica HM1:IMSS and 200:NIH but is not present in pathogenic strains HK-9 and Rahman. In addition, Southern blot analysis using the C2 clone showed heterogeneity of genomic organization between HM1:IMSS and 200:NIH. DNA dot blot hybridization analysis demonstrated that cDNA clone C2 was also able to distinguish axenically cultured E. histolytica strains possessing pathogenic zymodemes from those possessing nonpathogenic zymodemes and could detect as few as 100 amebic trophozoites. We conclude that C2 is a strain-specific E. histolytica cDNA clone that, in conjunction with other E. histolytica-specific probes, could serve as a useful epidemiologic tool.

DNA fingerprinting of the human intestinal parasite Giardia intestinalis with hypervariable minisatellite sequences

Individual isolates of the Giardia duodenalis group of protozoan intestinal parasites were identified by DNA fingerprinting with hypervariable minisatellite sequences. A morphologically identical parasite is found in some forty different animal species. Although the species name intestinalis is reserved for the human isolates, electrophoretic karyotyping suggests that most duodenalis isolates fall into the same species grouping. Distinction based upon morphology, restriction endonuclease cleavage of genomic DNA or isoenzyme analysis has not been adequate to identify individual strains. The successful use of hypervariable sequences in the identification of individual human genomes encouraged us to examine the use of these same sequences for the possible identification of parasite isolates. We initially use as a fingerprinting probe the genome of the bacteriophage M13, which has repeated sequences recognising homologous hypervariable sequences in the human genome. The M13 probe recognises a weakly homologous set of hypervariable sequences in Giardia. The number of informative bands is comparable to those seen in mammals, since the lower molecular weight bands are also useful. There is considerable divergence in the sequences of individual Giardia minisatellites. Some cloned Giardia hypervariable sequences are more homologous to M13 than they are to each other. Similar results were observed with the hypervariable repeat sequences 3' to the human alpha-globin gene when they were used as a probe to distinguish Giardia isolates. The poly(dA-dC).poly(dG-dT) probe which recognises frequent TG tracts in a number of organisms also detects a few variable bands amidst a hybridisation background in the Giardia genome. Thus Giardia isolates which could not be distinguished by restriction endonuclease cleavage, antibody typing or isoenzyme analysis have been identified by DNA fingerprinting procedures. Detailed analysis of strain movement, resurgence, variation, host range and drug resistance is now possible. Similar families of sequences may be widespread in lower eukaryotes and useful for generating individual specific fingerprints. A procedure for detecting individual parasites is also presented. Since Giardia is regarded as the most ancient eukaryote before the occurrence of symbiosis with purple non-sulphur bacteria to generate mitochondria, the identification of hypervariable sequences in the Giardia genome should also aid in understanding the mechanism of generation and evolution of these sequences.

Characterization of an axenic strain of Hartmannella vermiformis obtained from an investigation of nosocomial legionellosis

A free-living amoeba identified as Hartmannella vermiformis was isolated from a water sample obtained during an investigation of nosocomial legionellosis. Hartmannella vermiformis is known to support the intracellular multiplication of Legionella pneumophila. This strain of H. vermiformis, designated CDC-19, was cloned and established in axenic culture to develop a model for the study of the pathogenicity of legionellae. Isoenzyme patterns of axenically-cultivated strain CDC-19 were compared with two strains of H. vermiformis derived from the type strain, one axenic (ATCC 50236) and the other grown in the presence of bacteria (ATCC 30966). Enzyme patterns suggested that all three strains are assignable to the species H. vermiformis. Axenic H. vermiformis strain CDC-19 has been deposited with the American Type Culture Collection (ATCC 50237) and should prove useful in the study of protozoan-bacterial interaction.

Autonomous replication sequences in an extrachromosomal element of a pathogenic Entamoeba histolytica

Entamoeba histolytica possesses a 24.5 kilobase plasmid-like molecule which encodes for the organism's ribosomal RNAs. Sequence analysis of this extrachromosomal element revealed the presence of AT rich sequences which show homology to the origin of replication of other lower eucaryotes. An 802 bp fragment containing these sequences was cloned into a yeast shuttle vector lacking the origin of replication and the construct tested for its ability to replicate autonomously in yeast. Mitotic stability tests as well as evidence for plasmid maintenance indicate that the transformed cells contained self-replicating episomes and not stably integrated molecules. The nucleotide sequence of this ARS-containing fragment is presented.

Characterization of an immuno-dominant variable surface antigen from pathogenic and nonpathogenic Entamoeba histolytica

A 125-kD surface antigen of Entamoeba histolytica is recognized by 73% of immune sera from patients with amoebic liver abscesses. Using pooled human immune sera a cDNA clone (lambda cM17) encoding this antigen (M17) has been isolated from a lambda gt11 expression library of the virulent stain E. histolytica HM1:IMSS. Monospecific antibodies, purified by binding to phage lysate of lambda cM17, and mAb FA7 reacted exclusively with the 125-kD antigen by Western blot analysis. Surface binding and cap formation are observed with patient sera, purified monospecific antiserum, and mAb FA7. Corresponding genomic clones (pBSgM17-1/2/3) were isolated by hybridization with the cDNA clone. These contained an open-reading frame of 3345 bp, which is in good agreement with the mRNA size of approximately 3.0 kb as revealed by Northern hybridization with lambda cM17. The inferred amino acid sequence predicts a 125,513 dalton protein that contains 17 potential N-linked glycosylation sites and is unusually rich in tyrosine and asparagine residues. A distinctly hydrophobic NH2-terminal region may serve as membrane anchor or signal sequence. In contrast to conservation of an immunodominant epitope recognized in pathogenic and nonpathogenic strains by monoclonal FA7 and human immune sera, amplification and sequence analysis of a 1,4000-bp fragment of this gene from a fresh nonpathogenic isolate by use of the PCR demonstrate regions of significant sequence divergence in this antigen. A 1% sequence variability among different isolates of the pathogenic strain HM1:IMSS and a 12-13% variability between pathogenic and nonpathogenic strains are revealed by comparison to published partial amino acid sequences (Tannich, E., R.D. Horstmann, J. Knobloch, and H.H. Arnold. 1989. Proc. Natl. Acad. Sci. USA. 86:5118). Some restriction enzymes were found that allowed PCR diagnosis of nonpathogenic and pathogenic isolates with the exclusion of E. histolytica-like Laredo, suggesting that a detailed study of nonpathogenic and pathogenic isolates in relation to the M17 antigen sequence will provide a basis of differentiating isolates.

Repetitive DNA elements characteristic of pathogenic Entamoeba histolytica strains can also be detected after polymerase chain reaction in a cloned nonpathogenic strain

Strains of Entamoeba histolytica which were isolated from symptomatic patients and which possess a characteristic pathogenic isoenzyme pattern (zymodeme) have extrachromosomal circular DNA molecules containing RNA genes and clusters of tandemly reiterated PvuI elements. The nucleotide sequence of comparable reiterated BamHI elements present in amebae with nonpathogenic zymodemes differs from that found in pathogenic ones. By using the polymerase chain reaction, it was demonstrated that the cloned, nonpathogenic E. histolytica strain SAW 1734R clAR also contains one or few of the tandemly repeated DNA PvuI elements characteristic of the pathogenic amebae. Sequences were detected by hybridization with the P-145 probe after in vitro amplification. Because of technical difficulties, it was impossible to resolve whether single copies of the nonpathogenic BamHI repetitive elements are present in pathogenic amebae. Our findings suggest that in the nonpathogenic amebae, the signal to start amplifying the PvuI-type elements may be induced during the process of elimination of bacterial associates from their growth environment.

Identification of a pathogenic isolate-specific 30,000-Mr antigen of Entamoeba histolytica by using a monoclonal antibody

A monoclonal antibody (MAb) produced against trophozoites of Entamoeba histolytica strain HM-1:IMSS, reacted with all of 42 isolates and 4 clones showing pathogenic zymodeme (Z) patterns, i.e., Z-II, Z-II alpha-, Z-II (glucose phosphate isomerase: gamma +), Z-VII, Z-VII (glucose phosphate isomerase: alpha lack, gamma +), Z-XI, Z-XIV, and Z-XIX, regardless of culture conditions, geographical origins, or host symptoms in an indirect fluorescence antibody test. In contrast, the MAb failed to react with 14 isolates possessing nonpathogenic zymodemes Z-I and Z-VIII and did not react with other enteric protozoan parasites, such as E. histolytica-like Laredo, Entamoeba hartmanni, Entamoeba coli, Endolimax nana, Dientamoeba fragilis, Trichomonas hominis, and Giardia lamblia. Western immunoblotting analysis showed that the molecular weight of the antigenic component recognized by the MAb was exclusively 30,000 in pathogenic isolates of different zymodemes. These results suggest that the 30,000-molecular-weight antigen is a marker of pathogenic isolates and that the indirect fluorescent-antibody test with the MAb is useful for the accurate discrimination of pathogenic amebae.

Differentiation of clinical isolates of Entamoeba histolytica by using specific DNA probes

Most individuals infected with Entamoeba histolytica are reported to be clinically asymptomatic. On the basis of the electrophoretic migration of hexokinase and phosphoglucomutase isoenzymes, two groups of E. histolytica isolates have been classified. Those derived from symptomatic cases were found to have fast-migrating hexokinase bands and were labeled pathogenic. The others, isolated from cyst passers, had (in most cases) slow-migrating bands and were called nonpathogenic. Differences between these two groups of E. histolytica were found recently at the DNA level. Two sets of different DNA probes derived from tandemly repeated sequences present in extrachromosomal circular DNA elements in each group of E. histolytica were characterized. Using these probes with procedures for direct hybridization of trophozoites on nylon membranes, we could correctly correlate hexokinase electromobility with the DNA hybridization signal of 81 different isolates of E. histolytica. The advantages of using DNA probes lie in their sensitivity (fewer than 200 trophozoites can be detected) and specificity. The probes hybridized only with amebae from the E. histolytica species and not with other enteric protozoa and can be useful as a diagnostic tool.

Zymodeme conversion of isolates of Entamoeba histolytica

A non-cloned and a cloned isolate of non-pathogenic (zymodeme I) Entamoeba histolytica derived from homosexual males were incubated in Diamonds' TP-S-1 medium in the presence of Crithidia for 52 d. A single addition of Escherichia coli irradiated at 1000 Gy rendered the culture axenic. Analysis of zymodeme patterns established that both isolates had transformed to zymodeme II. The re-establishment of cultures in the presence of microflora from the original cultures resulted in a return to zymodeme I, whereas incubation with microflora from an amoebic isolate exhibiting a zymodeme II pattern maintained the pathogenic pattern.

A genetic approach to species criteria in the amoeba genus Naegleria using allozyme electrophoresis

The present study employs allozyme electrophoresis to characterize and inter-relate 61 isolates of Naegleria. Diploidy was confirmed, with heterozygotes observed at 29 of the 33 loci established and in all but two isolates. With a single exception, isolates clustered at two levels of similarity, either below 21% or above 52%. It is argued that such a major discontinuity provides a sound biological basis for a species concept in Naegleria. On this basis the present species-level taxonomy does not reflect the genetic diversity of the genus. The study recognized 18 genetic groups of species rank. The subspecies N. australiensis italica deserves specific rank; additional thermophilic species not closely related to N. fowleri and N. lovaniensis are recognized; and N. gruberi as currently conceived is a complex of 10 species, at least five of which are represented in the formal culture collections. Most species are genetically too different for relationships to be elucidated by allozyme electrophoresis, supporting the view that some of the times of divergence within the genus are extremely ancient.

Genomic DNA differences between pathogenic and nonpathogenic Entamoeba histolytica

cDNA libraries were constructed from pathogenic (HM-1:IMSS) and nonpathogenic (SAW 1734) isolates of Entamoeba histolytica. A cDNA clone (cEH-P1) specific for pathogenic amoebae was identified by screening with a pool of sera from patients with invasive amoebiasis that had been absorbed with nonpathogenic amoebae. This clone was used for the identification of a homologous clone (cEH-NP1) in the cDNA from nonpathogenic amoebae. Sequence analysis and comparison of the predicted amino acid sequences for both clones disclosed 12% evolutionary divergence in structure. Hybridization of both cDNA probes to genomic DNA from four pathogenic and five nonpathogenic E. histolytica isolates revealed two distinct Southern blot patterns, one characteristic for pathogenic amoebae and the other for nonpathogenic amoebae. Further, the complex pattern of restriction fragments hybridizing to an actin cDNA probe was also different between pathogenic and nonpathogenic isolates but was conserved within each group of amoebae. The results indicate that pathogenic isolates of E. histolytica are genetically distinct from nonpathogenic isolates.

Nondysenteric intestinal amebiasis. Colonic morphology and search for Entamoeba histolytica adherence and invasion

There is controversy regarding the presence of colonic mucosal abnormalities or mucosal invasion by Entamoeba histolytica in patients with "nondysenteric intestinal amebiasis." To determine the role of E. histolytica in causing symptoms and mucosal changes and to detect if mucosal invasion by E. histolytica is present in nondynsenteric intestinal amebiasis, we evaluated 24 E. histolytica-infected patients (stool microscopy positive for E. histolytica) and 12 noninfected controls who presented with chronic gastrointestinal symptoms, but without dysentery, to a clinic in Calcutta. The colonic mucosa was evaluated at colonoscopy, and mucosal biopsies obtained from the cecum, sigmoid colon, and rectum were evaluated by light microscopy, indirect immunofluorescence microscopy, and scanning electron microscopy. At colonoscopy mucosal ulcerations were absent in all the controls and all except one of the E. histolytica-infected patients. E. histolytica trophozoites or cysts were not seen in the lamina propria or on the luminal surface in any infected patient by light and immunofluorescence microscopy. On scanning electron microscopy, structures that resembled rounded E. histolytica trophozoites were seen on the luminal surface in two of 19 cecal specimens from the infected patients. Moderate or severe mucosal inflammation was frequent on light microscopy in both the E. histolytica-infected patients and the noninfected controls with the cecum involved in two thirds of both groups. Antibodies to E. histolytica were detected in serum of 25% of study patients and 58% of controls. Mucosal inflammation did not correlate with stool positivity for E. histolytica or seropositivity for ameba antibody.(ABSTRACT TRUNCATED AT 250 WORDS)