Changes in isoenzyme patterns of a cloned culture of nonpathogenic Entamoeba histolytica during axenization

Intestinal amoebiasis: 160 years of its first detection and still remains as a health problem in developing countries

Amoebiasis is a parasitic disease caused by Entamoeba histolytica (E. histolytica), an extracellular enteric protozoan. This infection mainly affects people from developing countries with limited hygiene conditions, where it is endemic. Infective cysts are transmitted by the fecal-oral route, excysting in the terminal ileum and producing invasive trophozoites (amoebae). E. histolytica mainly lives in the large intestine without causing symptoms; however, possibly as a result of so far unknown signals, the amoebae invade the mucosa and epithelium causing intestinal amoebiasis. E. histolytica possesses different mechanisms of pathogenicity for the adherence to the intestinal epithelium and for degrading extracellular matrix proteins, producing tissue lesions that progress to abscesses and a host acute inflammatory response. Much information has been obtained regarding the virulence factors, metabolism, mechanisms of pathogenicity, and the host immune response against this parasite; in addition, alternative treatments to metronidazole are continually emerging. An accesible and low-cost diagnostic method that can distinguish E. histolytica from the most nonpathogenic amoebae and an effective vaccine are necessary for protecting against amoebiasis. However, research about the disease and its prevention has been a challenge due to the relationship between E. histolytica and the host during the distinct stages of the disease is multifaceted. In this review, we analyze the interaction between the parasite, the human host, and the colon microbiota or pathogenic microorganisms, which together give rise to intestinal amoebiasis.

Pathogenicity of Entamoeba dispar under xenic and monoxenic cultivation compared to a virulent E. histolytica

Two xenic isolates and cloned cultures of Entamoeba dispar were submitted to monoxenization using Crithidia fasciculata as the associated organism. Growth in monoxenic cultivation and ability of xenic and monoxenic trophozoites to destroy VERO cells and produce lesions in hamster livers were compared to those of a virulent E. histolytica. Parental and cloned E. dispar under monoxenic cultivation showed a remarkable lower growth than the monoxenic E. histolytica and were avirulent in both in vivo and in vitro tests. When xenically cultured, trophozoites of E. dispar showed a moderate lytic activity against VERO cells (1.5 to 41.8% of destruction) but caused severe hepatic lesions in hamsters as those caused by the virulent E. histolytica (29 to 100% in prevalence and 0.86 to 4.00 in lesion degree). Although E. dispar has not been associated with invasive disease in men, the ability of xenic trophozoites to produce prominent tissue damage in experimental conditions has indicated that some strains have a considerable pathogenic potential when in presence of bacteria.

Immunopathology of Entamoeba histolytica infections

Entamoeba histolytica infects almost 10% of the world's population and results in about 100 000 deaths annually(1). Relatively little information is available concerning the immune response and the immunopathology elicited by this parasite, probably due in part to the lack of a truly appropriate animal model(2-4). However, there has been some progress - particularly concerning the interaction of this parasite with cells of the immune system(5,6). This review summarizes the salient features of the cellular immune response and immunopathology, largely from in vitro studies and studies using the gerbil model for invasive amoebiasis(7,8). Overall, the results suggest that invasive amoebtasis induces profound immune dysfunction both at the effector level of macrophages and on their accessory cell potential.

AXENIC CULTIVATION OF ENTAMOEBA DISPAR IN NEWLY DESIGNED YEAST EXTRACT–IRON–GLUCONIC ACID–DIHYRDOXYACETONE–SERUM MEDIUM

Yeast extract-iron-gluconic acid-dihydroxyacetone-serum medium that allows axenic cultivation of Entamoeba dispar was designed based on casein-free yeast extract-iron-serum (YI-S) medium, and the usefulness of the medium was assessed. The main differences from YI-S medium are replacement of glucose by gluconic acid, addition of dihydroxyacetone and D-galacturonic acid monohydrate, and sterilization by filtration. This medium promoted the axenic growth of 5 strains of E. dispar (2 strains of nonhuman primate isolates and 3 strains of human isolates). In addition, to clarify the biological basis for the growth of E. dispar in this medium, analyses of relevant enzymes on the glycolytic pathway of the amoebae as well as of the protozoans that are the best culture supplement for amoebae are being performed.

Remarkable genetic polymorphism among Entamoeba histolytica isolates from a limited geographic area

In order to understand genetic polymorphisms among Entamoeba histolytica strains in a limited geographic area and among restricted social populations, we studied nucleotide polymorphism in DNA regions that do not encode proteins (locus 1-2 and locus 5-6) and in genes coding for chitinase and for serine-rich E. histolytica protein. Thirty E. histolytica isolates from domestically infected Japanese amebiasis patients (male homosexuals and residents in institutions for the mentally handicapped) and four reference strains were examined. PCR revealed remarkable polymorphisms in both the number and size of the PCR fragments containing these loci. Polymorphisms in lengths, types, and numbers of internal repeat units were observed in locus 1-2 and the repeat-containing region of serine-rich E. histolytica protein among the Japanese isolates. In contrast, polymorphism at locus 5-6 was observed almost exclusively in the number of repeats of a 16-nucleotide unit. The repeat-containing region of chitinase appeared to be the least polymorphic among the four loci with a single dominant genotype representing 66% (20 out of 30) of all of the isolates. Isolates obtained from male homosexuals showed a more complex genetic polymorphism than those from residents in institutions. Considering all four polymorphic loci together, all 19 Japanese isolates from male homosexuals were distinct. In contrast, all isolates obtained from mass-infection cases at a single institution had an identical genotype, suggesting that these cases were caused by a single E. histolytica strain. No significant correlation was found between genotypes and zymodemes or between genotypes and clinical presentations, e.g., colitis or liver abscess. Certain genotypes were observed with higher frequencies in male homosexuals or residents of institutions. These data indicate that genotyping of the E. histolytica isolates by using these four polymorphic loci could serve as a tool to fingerprint individual isolates. We propose that genotyping of ameba isolates should help to determine geographic origins of isolates and routes of transmission.

Recent developments in amoebiasis:the Gal/GalNAc lectins of Entamoeba histolytica and Entamoeba dispar

Amoebiasis is responsible for 50000-100000 deaths annually. Invasive amoebic disease begins with the attachment of Entamoeba histolytica trophozoites to colonic mucin, a process mediated by the amoebic Gal/GalNAc lectin. The non-pathogenic counterpart, E. dispar, is morphologically identical but genetically distinct. Investigations comparing the Gal/GalNac lectin from these two organisms are under way.

Characterization of the gene encoding serine acetyltransferase, a regulated enzyme of cysteine biosynthesis from the protist parasites Entamoeba histolytica and Entamoeba dispar. Regulation and possible function of the cysteine biosynthetic pathway in Entamoeba

The enteric protist parasites Entamoeba histolytica and Entamoeba dispar possess a cysteine biosynthetic pathway, unlike their mammalian host, and are capable of de novo production of L-cysteine. We cloned and characterized cDNAs that encode the regulated enzyme serine acetyltransferase (SAT) in this pathway from these amoebae by genetic complementation of a cysteine-auxotrophic Escherichia coli strain with the amoebic cDNA libraries. The deduced amino acid sequences of the amoebic SATs exhibited, within the most conserved region, 36-52% identities with the bacterial and plant SATs. The amoebic SATs contain a unique insertion of eight amino acids, also found in the corresponding region of a plasmid-encoded SAT from Synechococcus sp., which showed the highest overall identities to the amoebic SATs. Phylogenetic reconstruction also revealed a close kinship of the amoebic SATs with cyanobacterial SATs. Biochemical characterization of the recombinant E. histolytica SAT revealed several enzymatic features that distinguished the amoebic enzyme from the bacterial and plant enzymes: 1) inhibition by L-cysteine in a competitive manner with L-serine; 2) inhibition by L-cystine; and 3) no association with cysteine synthase. Genetically engineered amoeba strains that overproduced cysteine synthase and SAT were created. The cysteine synthase-overproducing amoebae had a higher level of cysteine synthase activity and total thiol content and revealed increased resistance to hydrogen peroxide. These results indicate that the cysteine biosynthetic pathway plays an important role in antioxidative defense of these enteric parasites.

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.

Entamoeba dispar: ultrastructure, surface properties and cytopathic effect

The cytological features of Entamoeba dispar, recently recognized by biochemical and molecular biology criteria as a distinct species, were compared to those of Entamoeba histolytica When cultured under axenic conditions, living trophozoites of E. dispar strain SAW 76ORR clone A were more elongated in form, had a single frontal pseudopodium, and showed a noticeable uroid. In sections of E. dispar trophozoites stained with Toluidine blue, characteristic areas of cytoplasmic metachromasia were seen due to the presence of large deposits of glycogen, seldom found in E. histolytica strain HM1:IMSS. Under the light microscope the periphery of the nucleus in E. dispar was, lined by finer, more regularly distributed dense granules. With transmission electron microscopy the surface coat of E. dispar was noticeable thinner. In addition. E. dispar had a lower sensitivity to agglutinate with concanavalin A and a higher negative surface charge, measured by cellular microelectrophoresis. The cytopathic effect of E. dispar was much slower, analyzed by the gradual loss of transmural electrical resistance of MDCK epithelial cell monolayers mounted in Ussing chambers. Whereas in E. histolytica phagocytosis of epithelial cells plays an important role in its cytopathic effect. E. dispar trophozoites placed in contact with MDCK cells showed only rare evidence of phagocytosis. The results demonstrate that the morphology of E. dispar is different to that of E. histolytica, both at the light microscopical and the ultrastructural levels. In addition they show that E. dispar in axenic culture has a moderate cytopathic effect on epithelia] cell monoLayers. However, when compared to E. histolytica, the in vitro lytic capacity of E. dispar is much slower and less intense.

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.

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.

Protozoa. Amebiasis

The intestinal protozoan parasite Entamoeba histolytica causes amebic dysentery and amebic liver abscess, and ranks third worldwide among parasitic causes of death. The application of molecular techniques to the study of this organism have led to major advances in understanding the pathophysiology of amebic infection. This article reviews what is currently known about the pathogenesis, clinical manifestations, diagnosis, and treatment of amebiasis.

Genomic variation in Trypanosoma cruzi clonal cultures

Spontaneous changes in restriction DNA profiles and pulsed-field gel electrophoresis (PFGE) patterns, along with a concomitant loss of infectivity, were observed in infective clones of Trypanosoma cruzi strain Y either following a number of passages during the exponential growth phase of after subcloning in liver infusion tryptone (LIT) medium using as the probe a genomic fragment of the parasite (pMYP16), indicating naturally occurring rearrangements of DNA sequences. No variation could be detected when the genomic DNA was probed with conserved T. cruzi tubulin and actin genes. There was no correlation between such rearrangements and the life-cycle forms of the parasites, since trypomastigote forms showed the same karyotype and hybridization patterns as did epimastigote forms. The variations observed could be reverted and infectivity, recovered after inoculation of the parasites in newborn mice.

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.

Recent advances in amebiasis

Advancements in our understanding of amebiasis have been rapid over the decade that I have followed this field. What was identified morphologically for years as Entamoeba histolytica has been redescribed with modern techniques as a complex of two species, the commensal parasite E. dispar and the pathogenic parasite E. histolytica that is the cause of colitis and liver abscess. Antigen detection tests are now available for the rapid detection in stool of the pathogenic species E. histolytica. New understandings of the importance of luminal as well as tissue-active antimebic medications in the treatment of invasive disease have been reached. The groundwork is being laid for an understanding of the protective immune responses to infection, and at the lab bench DNA transfection of the parasite has opened studies of pathogenesis to genetic analysis. While necessarily an incomplete sketch of the field, I have attempted here to highlight some recent and important developments of interest to clinicians and microbiologists.

Rapid diagnosis of Entamoeba infection by using Entamoeba and Entamoeba histolytica stool antigen detection kits

Humans are infected by two morphologically identical species of Entamoeba: Entamoeba histolytica causes amebic colitis and liver abscess, and Entamoeba dispar is noninvasive. Several weeks of culture and isoenzyme (zymodeme) analysis are required to differentiate E. histolytica from E. dispar. Here we report a field trial of commercial antigen detection kits designed to rapidly detect and differentiate E. histolytica from E. dispar in stool specimens. Stool specimens from 202 patients with diarrhea were examined for E. histolytica and E. dispar by microscopy, culture, and antigen detection. Compared with culture, microscopic identification of the E. histolytica-E. dispar complex was 60% sensitive and 79% specific, while the screening antigen detection test for the E. histolytica-E. dispar complex was 80% sensitive and 99% specific. Differentiation of E. dispar from E. histolytica by the E. histolytica-specific test was 95% sensitive and 93% specific compared with zymodeme analysis. We conclude that the antigen detection test for the E. histolytica-E. dispar complex is more sensitive and specific than microscopy and that the E. histolytica-specific antigen detection test is as reliable and much more rapid than zymodeme analysis for the differentiation of E. histolytica from E. dispar.

Molecular analysis of two hexokinase isoenzymes from Entamoeba histolytica

The zymodemes, electrophoretic patterns of hexokinase, phosphoglucomutase and glucose phosphate isomerase isoenzymes, have been widely used to determine the pathogenicity of Entamoeba histolytica isolates. Although pathogenic and nonpathogenic forms of E. histolytica differ clearly in sequences of many homologous genes, a conversion between pathogenic and nonpathogenic zymodemes has been reported by several laboratories. To approach the question what might be the basis for the observed conversion, we examined the molecular biology of the hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) isoenzymes in pathogenic E. histolytica. We isolated two different cDNAs pHXK1 and pHXK2 coding for polypeptides with significant sequence similarity to hexokinases and deduced molecular masses of 49.8 kDa and 49.4 kDa. The two hexokinase sequences differed by 11% on the amino acid and by 8% on the nucleotide level. Expression of the cDNAs in Escherichia coli as nonfusion proteins gave two polypeptides with hexokinase activity. The recombinant Hxk1 and Hxk2 polypeptides comigrated with the more basic and more acidic isoforms of pathogenic amoebae in starch gel electrophoresis, as well as in low and high resolution isoelectric focussing gels. This identified the observed hexokinase isoenzymes of pathogenic E. histolytica as the products of two genes, hxk1 and hxk2.

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.

Molecular cloning of a 30-kilodalton lysine-rich surface antigen from a nonpathogenic Entamoeba histolytica strain and its expression in a pathogenic strain

A monoclonal antibody (MAb), 318-28, that specifically reacts with a 30-kDa antigen present on membrane surfaces of all nonpathogenic (NP) Entamoeba histolytica strains tested and which did not react with pathogenic (P) strains was used for the isolation of the cDNA coding for this antigen from an expression library of an NP E. histolytica strain. The deduced amino acid composition was rich in lysine residues (14.5%), with some sequence similarity to a polyadenylate-binding protein. Southern and Northern (RNA) blot analyses, as well as amplifications of DNA segments by PCR, indicate that a very similar gene (identity of 96.5%) exists in P strains of E. histolytica. Unexpectedly, the NP-specific antigen was also identified by MAb 318-28 on the surfaces of a cloned, xenically cultivated and well-characterized P strain (BNI:0591) that was recently isolated from a human liver abscess. Binding of the MAb, both to the cell surfaces and to Western blots (immunoblots), was abolished, however, upon axenization of the BNI:0591 cultures. Oligonucleotide primers, designed to anneal only to specific DNA sequences of the NP 30-kDa protein gene copy, amplified a DNA segment from P strain BNI:0591 which was identical in sequence to that of the NP 30-kDa protein gene. Our findings indicate that a P strain of E. histolytica can possess and express, under certain growth conditions, an antigen that is usually detected only in NP strains.

Characterization of two distinct gene transcripts for ribosomal protein L21 from pathogenic and nonpathogenic strains of Entamoeba histolytica

A second gene (rp-L21) copy, clone g34, coding for ribosomal (r-) protein L21, was isolated from the pathogenic (P) strain HM-1:IMSS cl6 of the intestinal parasite Entamoeba histolytica (Eh). The gene was compared to the previously isolated copy, gLE3 [Petter et al., Mol. Biochem. Parasitol. 56 (1992) 329-334], with respect to its primary structure, mRNA levels and binding to the r-complex during translation. Unlike the gLE3 gene copy [Petter et al., Mol. Biochem. Parasitol. 56 (1992) 329-334], g34 was found not to be physically connected to an actin gene copy. Homologous copies of the two rp-L21 genes were also characterized from the nonpathogenic (NP) strain SAW1734R clAR, as well as from its P derivative. Sequence comparison of the coding regions of the two rp-L21 revealed almost full identity. Significant differences were found, however, within their 3' and 5' flanking regions. Using the 3' rapid amplification of cDNA ends (3' RACE) method [Frohman et al., Proc. Natl. Acad. Sci. USA 85 (1988) 8998-9002], as well as Northern and slot blot hybridizations, it was demonstrated that both rp-L21 mRNAs are found in similar amounts. However, as was shown by differential hybridization, the relative binding of each transcript to the r-complex varied somewhat between P and NP strains. This finding suggests that the control of expression of rp-L21 in Eh may involve regulation at the post-transcriptional level.

Changes in Trypanosoma cruzi kinetoplast DNA minicircles induced by environmental conditions and subcloning

Reversible changes in kinetoplast DNA (kDNA) minicircles sequences were observed in clones of Trypanosoma cruzi strain Y, following a number of passages during exponential growth phase or after subcloning in blood-free medium. kDNA restriction patterns of clones were similar to those of the original uncloned strain, while subclones presented distinct kDNA restriction patterns. Homology experiments demonstrated strong hybridization between kDNA with the same electrophoretic mobility patterns while only weak signals were observed with kDNA of different patterns. The changes observed, which are unprecedented in T. cruzi clones, characterize transkinetoplastidy, and seem to be associated with similarly reversible changes both in zymodeme and in infectivity.

An attempt at reversibility and increase of the virulence of axenic strains of Entamoeba histolytica

In this study we have tried to verify whether the interaction "in vitro" with bacteria or small pieces of normal hamster liver would modify the pathogenic behavior of axenic strains of E. histolytica: avirulent ones (ICB-32 and ICB-RPS), of attenuated virulence (ICB-CSP and HM1) and of mean virulence (ICB-462). Every attempt to render virulent, recover or increase the virulence of axenic strains of E. histolytica has failed.

Electrophoretic karyotype and chromosome assignments for a pathogenic and a nonpathogenic strain of Entamoeba histolytica

The electrophoretic karyotypes of a pathogenic and a nonpathogenic strain of Entamoeba histolytica were determined by pulsed-field gel electrophoresis. A number of previously isolated genes were assigned to specific chromosomal bands. Significant differences between the chromosomal patterns of these strains as well as in the assignment of most genes were found.

Zymodeme alteration of Entamoeba histolytica isolates under varying conditions

Cysts of Entamoeba histolytica obtained from 2 asymptomatic subjects were cloned in vitro and the isoenzyme patterns and virulence of the cultures derived from them were determined. Incubation in Diamond's TYI-S-33 medium with Crithidia sp. for 80 d resulted in a change from zymodeme III to zymodeme II, with consistent virulence. Axenization by replacing Crithidia sp. with crithidial lysate had no further effect on the zymodeme but the virulence of the amoebae was attenuated. A reversal to zymodeme III, together with augmentation of virulence, was observed on the 20th day after transfer of axenic amoebae to Diamond's TYSGM-9 medium containing the original bacterial flora. The possibility that the isoenzyme pattern determining the zymodeme and the virulence behave as 2 independent variables in E. histolytica is discussed.

A family of transcripts (K2) of Entamoeba histolytica contains polymorphic repetitive regions with highly conserved elements

Sera from patients with invasive amebiasis were used to identify a cDNA clone (K2p-1) encoding a commonly recognized, repeat-containing antigen of the pathogenic Entamoeba histolytica HM-1:IMSS. K2p-1 was used to isolate 3 cDNA clones (K2 clones); one K2p-1 related clone from the same pathogenic E. histolytica strain and 2 from the nonpathogenic E. histolytica strain SAW-142. The nucleotide sequence and predicted amino acid sequence revealed a closely related family of transcripts differing mainly in the extent and arrangement of an internal region consisting of tandemly arranged repetitive elements. The repetitive units encoding either 12 or 8 amino acids were found to be highly conserved in all the K2 clones analyzed so far, suggesting that the repeat motifs perform functions common to both pathogenic and nonpathogenic E. histolytica. The genomic organization of the K2 genes was different when compared in pathogenic and nonpathogenic E. histolytica and may therefore be used to discriminate between pathogenic and nonpathogenic E. histolytica strains.

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.

Cloning of a virulence factor of Entamoeba histolytica. Pathogenic strains possess a unique cysteine proteinase gene

Cysteine proteinases are hypothesized to be important virulence factors of Entamoeba histolytica, the causative agent of amebic dysentery and liver abscesses. The release of a histolytic cysteine proteinase from E. histolytica correlates with the pathogenicity of both axenic strains and recent clinical isolates as determined by clinical history of invasive disease, zymodeme analysis, and cytopathic effect. We now show that pathogenic isolates have a unique cysteine proteinase gene (ACP1). Two other cysteine proteinase genes (ACP2, ACP3) are 85% identical to each other and are present in both pathogenic and nonpathogenic isolates. ACP1 is only 35 and 45% identical in sequence to the two genes found in all isolates and is present on a distinct chromosome-size DNA fragment. Presence of the ACP1 gene correlates with increased proteinase expression and activity in pathogenic isolates as well as cytopathic effect on a fibroblast monolayer, an in vitro assay of virulence. Analysis of the predicted amino acid sequence of the ACP1 proteinase gene reveals homology with cysteine proteinases released by activated macrophages and invasive cancer cells, suggesting an evolutionarily conserved mechanism of tissue invasion. The observation that a histolytic cysteine proteinase gene is present only in pathogenic isolates of E. histolytica suggests that this aspect of virulence in amebiasis is genetically predetermined.

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.

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.

Amebiasis

Entamoeba histolytica, the causative agent of amebiasis, was first described in 1875. Although a large number of people throughout the world are infected with this organism, only a small percentage will develop clinical symptoms. Morbidity and mortality due to E. histolytica vary from area to area and person to person. Recent findings have suggested that there are pathogenic and nonpathogenic strains of E. histolytica that can be differentiated by isoenzyme (zymodeme) analysis, monoclonal antibodies, and DNA probes. Whether pathogenicity is a genotypic trait or can be changed by environmental influences has not been resolved. Exchange of genetic material between strains of amebae can influence zymodeme patterns. Currently, detection of E. histolytica infections depends on examinations for ova and parasites and on serologic tests; however, the development of monoclonal antibodies and DNA probes specific for pathogenic zymodemes may be beneficial for clinical laboratory testing and therapeutic decisions when approved tests become available. A better understanding of the mechanisms of pathogenicity at the molecular level is evolving and should promote the development of vaccines and better target selection for therapeutic agents.

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.

Modulation of a surface antigen of Entamoeba histolytica in response to bacteria

Changes in the cell surface of Entamoeba histolytica, a human intestinal parasite and the causative agent of amebic dysentery, were examined with a monoclonal antibody, 2D7.10, which selectively recognizes carbohydrate epitopes in some axenic amebic strains. While high-level expression of this epitope was observed in axenic amebae, it was either absent or present only in small amounts in xenic amebae. Furthermore, reassociation of the axenic amebae with intestinal flora resulted in loss of the 2D7.10 epitope. Our data suggest that surface antigens of E. histolytica can be modulated in response to bacteria and may provide an explanation for the observed influence of bacteria on amebic virulence.

Ribosomal DNA sequences in the differentiation of pathogenic and non-pathogenic isolates of Entamoeba histolytica

Recombinant ribosomal DNA sequences were amplified by PCR and used as probes to perform a fingerprint analysis of total DNA from different Entamoeba histolytica isolates. RFLPs obtained with one of the probes, R-1, support previous proposals that pathogenic and non-pathogenic E. histolytica are closely related, yet genotypically distinct. Another probe, R-2, while not distinguishing between the two forms of E. hystolytica, was able to differentiate between them and E. moshkovskii, which has morphologically identical cysts and trophozoites. A third probe, BR-1, identified strain-specific RFLPs.

Molecular and cellular characterization of the 29-kilodalton peripheral membrane protein of Entamoeba histolytica: differentiation between pathogenic and nonpathogenic isolates

To further characterize the 29-kDa surface antigen of Entamoeba histolytica, we analyzed the complete nucleotide sequence and compared the immunoreactivity of this antigen in pathogenic and nonpathogenic strains. Five cDNA clones (one 1.0-kb full-length clone, designated p13, and four partial-length clones) encoding the antigen were analyzed for allelic variation. Comparison of the nucleotide sequences revealed several single-nucleotide substitutions in all five cDNAs, two of which resulted in amino acid differences. Localization of the antigen to the amebic surface in a previous report (B. E. Torian, B. M. Flores, V. L. Stroeher, F. S. Hagen, and W. E. Stamm, Proc. Natl. Acad. Sci. USA 87:6358-6362, 1990) was corroborated by transmission electron microscopy showing colloidal gold particles on the surface of the trophozoites. Computer analysis of the deduced amino acid sequence predicted that the protein encoded by p13 was a hydrophilic peripheral membrane protein, and these data were confirmed by a Triton X-114 membrane extraction showing the presence of the 29-kDa antigen primarily in the aqueous phase of the detergent partition. Monoclonal antibodies to a fusion peptide differentiated between pathogenic and nonpathogenic clinical strains of E. histolytica in immunoblots. Although no immunoreactive epitopes were detected on nonpathogenic strains, Northern (RNA) analysis and DNA-DNA hybridization with a 700-bp cDNA probe demonstrated that mRNA and the gene encoding the 29-kDa surface antigen were present in both pathogenic and nonpathogenic clinical isolates.

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.

Characterization of 20 Entamoeba histolytica strains isolated from patients with HIV infection

Twenty Entamoeba histolytica strains from patients with HIV-1 infection were isolated and compared with E. histolytica strains from patients without HIV infection. The isoenzyme pattern of the hexokinase as well as the hybridization with known DNA-probes were used as markers for pathogenicity. According to these markers, all 20 strains could be regarded as being nonpathogenic. Direct measurements of the virulence were carried out: destruction of monolayer tissue culture cells, capacity of phagocytosis and the ability to induce liver abscesses in the hamster. The virulence of strains from HIV patients was comparable to that of E. histolytica strains which had been isolated from HIV-negative asymptomatic carriers. In agreement with this, none of the HIV-positive patients showed symptoms of an invasive amebiasis. By PRC, no HIV-1 proviral DNA could be evidenced in the E. histolytica strains which had been isolated from HIV patients.