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

Patogenicidade de Entamoeba dispar em cultivo polixênico e monoxênico comparada a uma cepa virulenta de E. histolytica

Abstracts

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.

Entamoeba dispar; Entamoeba histolytica; Cultivation; Pathogenicity


Dois isolados de Entamoeba dispar em cultivo polixênico e culturas clonadas deles obtidas foram submetidos à monoxenização utilizando Crithidia fasciculata como organismo associado. O crescimento em cultivo monoxênico dos isolados e clones, bem como sua capacidade de destruir células VERO (efeito citopático) e de produzir lesões hepáticas em hamster foram comparados a uma cepa virulenta de E. histolytica. Os trofozoítos de E. dispar em cultivo monoxênico apresentaram um crescimento nitidamente menor que o de E. histolytica e foram avirulentos tanto no teste in vivo quanto in vitro. Entretanto, isolados e clones de E. dispar em cultivo polixênico exibiram uma atividade lítica moderada sobre as células VERO (1,5 to 41,8% de destruição) e causaram lesões hepáticas em hamster (29 a 100% em prevalência e 0,86 a 4,00 no grau de lesão) tão extensas quanto aquelas causadas pela E. histolytica. Embora E. dispar não seja associada à doença invasiva no homem, a ocorrência de lesões teciduais significativas, causadas por trofozoítos em condições experimentais, indica que esta espécie pode apresentar potencial patogênico considerável quando em presença de bactérias intestinais.


PARASITOLOGY

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

Patogenicidade de Entamoeba dispar em cultivo polixênico e monoxênico comparada a uma cepa virulenta de E. histolytica

Adriana Oliveira CostaI; Maria Aparecida GomesII; Orivaldo Alves RochaIII; Edward Felix SilvaII

IDepartamento de Patologia Básica, Setor de Ciências Biológicas, Universidade Federal do Paraná, Curitiba, PR, Brasil

IIDepartamento de Parasitologia, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brasil

IIIDepartamento de Patologia Geral, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brasil

Correspondence to Correspondence to: Adriana Oliveira Costa Departamento de Patologia Básica, Setor de Ciências Biológicas Centro Politécnico s/n, Jardim das Américas 81531-990 Curitiba, PR, Brasil Tel: +55 41 33611697 E-mail: adrincosta@ufpr.br

SUMMARY

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.

Keywords: Entamoeba dispar; Entamoeba histolytica; Cultivation; Pathogenicity.

RESUMO

Dois isolados de Entamoeba dispar em cultivo polixênico e culturas clonadas deles obtidas foram submetidos à monoxenização utilizando Crithidia fasciculata como organismo associado. O crescimento em cultivo monoxênico dos isolados e clones, bem como sua capacidade de destruir células VERO (efeito citopático) e de produzir lesões hepáticas em hamster foram comparados a uma cepa virulenta de E. histolytica. Os trofozoítos de E. dispar em cultivo monoxênico apresentaram um crescimento nitidamente menor que o de E. histolytica e foram avirulentos tanto no teste in vivo quanto in vitro. Entretanto, isolados e clones de E. dispar em cultivo polixênico exibiram uma atividade lítica moderada sobre as células VERO (1,5 to 41,8% de destruição) e causaram lesões hepáticas em hamster (29 a 100% em prevalência e 0,86 a 4,00 no grau de lesão) tão extensas quanto aquelas causadas pela E. histolytica. Embora E. dispar não seja associada à doença invasiva no homem, a ocorrência de lesões teciduais significativas, causadas por trofozoítos em condições experimentais, indica que esta espécie pode apresentar potencial patogênico considerável quando em presença de bactérias intestinais.

INTRODUCTION

The existence of Entamoeba dispar, a species morphologically similar to E. histolytica, was first proposed by BRUMPT in 19254 to explain asymptomatic cases of amoebiasis. Most researchers, however, rejected this idea at that time, mainly because the proposed organism could not be differentiated from E. histolytica. Some decades later, studies on the isoenzymatic profile (zymodemes) of several isolates22,23 allowed E. histolytica to be classified into pathogenic and non-pathogenic forms. Correlation of zymodemes with other biological and molecular data8 led researchers to reconsider Brumpt's theory. The non-pathogenic form of E. histolytica was then renamed E. dispar, a non-invasive parasite of the human gut28.

It is known that under certain conditions E. dispar may be pathogenic8, based on studies that demonstrated trophozoites ability to produce focal lesions in experimental animals6,27. A moderate lytic activity of an axenic E. dispar strain was also detected11. Nevertheless, studies on E. dispar pathogenicity remain scarce. Isolation of new E. dispar strains is necessary for a better characterization of this species.

In the present study, two xenically cultured E. dispar isolates and cloned cultures obtained from them were subjected to monoxenization using Crithidia fasciculata as the associated organism. Growth of monoxenic trophozoites, cytopathic effect and ability to produce lesions in hamster livers of either xenic and monoxenic cultures were compared to a virulent isolate of E. histolytica.

MATERIAL AND METHODS

Parasites: The Entamoeba isolates ICB-MGL2 (MGL2) and ICB-ADO (ADO) were obtained from cysts from an asymptomatic carrier and a symptomatic patient, respectively, both residents in Brazil. A physician examined the symptomatic patient in order to determine the clinic form. Cultures were established xenically in Pavlova's medium modified by SILVA13 with bacterial flora. As soon as sufficient growth had occurred, trophozoites were grown in culture flasks to produce lysates for isoenzyme typing22. Genetic identification was also confirmed by polymerase chain reaction through analyses of conformational polymorphism in the 482 bp fragment, as previously described14. Trophozoites of each isolate were cloned using a dilution method26 and the clones ADO-A3, ADO-B3 and MGL2-A3 were used in the experiments. The same bacterial flora from the parental strains was maintained in the clones. The E. histolytica strain ICB-EGG (zymodeme XIX), used as the virulent reference strain, was obtained from a symptomatic patient24.

Monoxenization: Trophozoites of xenic cultures were washed three times in sterile PBS (pH 7.2) and transferred to YI-S medium9 containing Crithidia fasciculata as the associated organism (C. fasciculata Anopheles American Type Culture Collection [ATCC] Accession Number 11745, stock maintained in YI-S at room temperature). Bacteria were eliminated by addition of ceftazidime (Fortaz®, Glaxo Wellcome), ofloxacin (Floxtat®, Janssen-Cilag) or amicacine sulphate (Amicacin®, Neoquimica) to the medium at final concentrations of 660 µg/mL, 40 µg/mL and 333 µg/mL, respectively. Antibiotic use was halted when absence of bacteria was confirmed by microbiological assays. Viable C. fasciculata was added only on the day of sub-culturing. Monoxenic samples were also submitted to isoenzyme typing and analysis of conformational polymorphism in the 482 bp fragment14.

Growth curves: After more than a year of monoxenic cultivation, the growth of Entamoeba was compared. Sixteen to 18 tubes containing 6.5 mL of YI-S medium were inoculated with 700 trophozoites/mL and 1.4 x 106 forms of Crithidia fasciculata. Every 24 hours, trophozoites from two tubes were quantified and the average was recorded. Growth curves were delineated with the average of three independent experiments. Differences in growth among the samples were evaluated by the Kruskal-Wallis test.

Cytopathic effect: The cytopathic effect of trophozoites on VERO cells monolayers was measured by a quantitative assay3. Briefly, confluent VERO cells monolayers (2 x 105 cells) were incubated for one h at 37 ºC in a 5% CO2 atmosphere with 1 x 105 trophozoites washed twice in PBS (pH 7.2) and transferred to DMEM (Dulbecco's Modified Eagle Medium). The interaction was stopped by ice bath for 10 min, the wells were carefully washed with cold saline, the remaining cells were fixed with 4% formaldehyde for 10 min and washed again with saline (two times). Methylene blue 0.1% in borate buffer (0.1 M pH 8.9) was added to the wells to stain the cells for 10 min. The wells were washed three times with borate buffer and the stain of each well was extracted with 1 mL of HCl 0.1 M. The solution was spectrophotometrically measured at 660 nm; the dye extracted from cells that did not interact with trophozoites (0% of destruction) was used as control. The bacterial flora of each xenic strain (seeded in 10 mL of Pavlova medium, grown for 24 hour at 37 ºC) and C. fasciculata (1 x 106 grown in YI-S medium) were also tested. The Kruskal-Wallis test was used to determine the significance of differences in the destruction.

Hepatic inoculation in hamsters: Thirty-day-old hamsters (Mesocricetus auratus) obtained from a biotery (Biotério do Departamento de Parasitologia ICB/UFMG, Belo Horizonte, Brazil) were anesthetized with sodium pentobarbital and subjected to laparotomy. Trophozoites (2.5 x 105 or 5.0 x 105) washed twice in PBS (pH 7.2) were inoculated directly into the left lobe of the liver. Bacterial flora (seeded in 10 mL of Pavlova medium, grown for 24 hour at 37 ºC) and C. fasciculata (1 x 106 grown in YI-S medium) were inoculated separately as controls. Animals were sacrificed six days later and results were determined by prevalence (number of animals with lesions divided by number of animals inoculated, expressed as a percentage) as well as the individual and average severity of lesions, based on criteria described previously10, with some modifications. Fragments of the lesion were inoculated into culture medium or macerated in PBS for microscopic examination in order to confirm the presence of amoebae. Fixation and processing for haematoxylin and eosin staining (H&E) were performed to allow histopathological examination of the material. Statistical significance of prevalence and severity of lesions were determined by Pearson's X2 and Kruskal-Wallis tests, respectively.

RESULTS

Zymodeme, specific characterization by PCR and clinical form of the subjects from whom the strains were isolated are shown in Table 1. Complete monoxenization of E. dispar and E. histolytica cultures was accomplished after treatment with antibiotics for three or four days. At this stage, the majority of trophozoites was rounded and did not attach to the glass of the culture tubes. After approximately two months, the monoxenic cultures became stable and trophozoites multiplied adhered to the glass. Cultures did not grow in YI-S without C. fasciculata, even after a year had passed. No alterations in the zymodemes of the parental and cloned E. dispar were detected after monoxenization, and neither did the amplification profile 482 pb fragment vary (Fig. 1).


Exponential growth of monoxenic cultures is represented in Fig. 2. After 48 hours of cultivation, the E. histolytica EGG strains surpassed the E. dispar strains, the difference reaching a significant point at 72 hours, a significance level of 10% (p = 0.089). While the E. dispar strains suffered growth decrease between 48 and 72 hours, the EGG E. histolytica kept its exponential growth until 120 hours of cultivation.


There was no significant growth difference when the E. dispar original and cloned strains were compared.

Most xenic E. dispar cultures presented a low to mild lytic activity against VERO cells (1.5 to 41.8% destruction), whereas the monoxenic cultures caused very little or no damage (Fig. 3). Destruction rates of E. dispar were significantly lower than those of E. histolytica, regardless the culture condition (p < 0.05). Bacterial flora or C. fasciculata alone did not destroy VERO cells.


Xenically cultivated trophozoites of both E. histolytica and E. dispar were able to produce lesions in hamster livers, with variable prevalence and severity (Table 2). Parental xenic strains of E. dispar showed similar prevalence and lesion grades (p = 0.396) to those of the xenic strain of E. histolytica and higher values than their respective clones (p < 0.05). Macroscopically, the lesions caused by xenic E. histolytica and E. dispar were similar, characterized by a yellowish necrotic area with a friable aspect. Microscopic examination of unfixed fragments revealed the presence of bacteria and motile trophozoites, the lesions being characterized as mixed. H&E stained slides demonstrated areas with complete destruction of parenchyma and predominantly chronic inflammatory infiltrate on the border between necrotic and preserved parenchymal tissue (Figs. 4 A-B). Trophozoites were barely detected in H&E slides of xenic E. dispar, whereas they were abundant in slides of xenic E. histolytica (Fig. 4 C). Hamsters inoculated with bacterial flora presented normal livers or bacterial lesions smaller than 4 mm in diameter, limited by a fibroid wall enclosing purulent material (Fig. 4 D); perivascular and periportal, predominantly chronic, infiltrate was observed in less damaged areas of tissue.



Monoxenic E. dispar cultures did not cause lesions in hamster livers, even when higher concentrations (5.0 x 105) of trophozoites were used. Hamsters inoculated with the monoxenic E. histolytica presented lesions when 5.0 x 105 trophozoites were inoculated, but not with 2.5 x 105 (Table 2).

DISCUSSION

The two isolates obtained in this study, identified as E. dispar by PCR and isoenzyme typing, were monoxenized in YI-S medium, which has been demonstrated as being appropriate for this species15 . Unlike previous reports showing zymodeme alterations after modification of the cultivation system1,17,18,26, monoxenization did not affect this parameter in our E. dispar isolates.

When it comes to the clinical form of the ADO strain carrier, other causes for the symptoms - such as pathogenic bacteria, virus and Blastocystis hominis - should be considered. The latter has been associated to symptoms in some infections21 and was detected in the parasitological examination of the patient (data not shown). However, considering that the isolation of strains with nonpathogenic zymodeme from symptomatic patients has already been described in Brazil2, the hypothesis that the symptoms could be due to E. dispar ought not to be discarded. More clinical and biological studies are necessary in order to further investigate such possibility.

The E. dispar growth in monoxenic cultivation was clearly lower than the E. histolytica strain utilized for comparison. Maximal density obtained for E. dispar occurred in one of the experimental repetitions of ADO strain, which reached 9,000 trophozoites/mL at 72 hours (data not shown). This density was six times lower than the density of E. histolytica EGG at the same time. Some hypothesis suggested explaining the difficulty in axenizing E. dispar strains are the inability of trophozoites to obtain enough nutrients through pinocytose or their lower tolerance to toxic metabolites produced in the medium7. These characteristics could also explain the lower duration time of the E. dispar growth curves in monoxenic cultivation, compared to the E. histolytica.

As expected, E. histolytica exhibited high lytic activity and caused lesions in hamster livers, regardless the cultivation condition. Hepatic lesions by monoxenic E. histolytica were induced only when inocula containing the highest concentrations of the parasite were used, suggesting a decrease in virulence in vivo due to axenic cultivation. The phenomenon has usually been observed in studies with axenic E. histolytica cultures16.

When monoxenic E. dispar were tested, biological tests were practically negative. Concerning to the hepatic inoculation, we also used 106 trophozoites of the parental monoxenic strains ADO and MGL2 (data not shown) and obtained no lesion whatsoever. These results are similar to those of ESPINOSA-CANTELLANO et al.12, who showed that an axenic E. dispar was unable to cause lesions in hamster livers. Therefore, the monoxenic cultures can exhibit behavior similar to the axenic ones. Since axenization is a time-consuming and difficult process and Crithidia fasciculata has no pathogenic effect, monoxenic cultures could be an alternative for the biological evaluation of E. dispar in the absence of bacteria.

Nevertheless, the interaction of trophozoites and bacteria must be considered in the determination of the pathogenic potential of E. dispar. In our experiments, the cytopathic effect of E. dispar was moderate for xenic cultures, in contrast to the absent or minimal destruction when monoxenic strains or bacteria were used alone. Differences in results obtained from parental and cloned cultures had also occurred in the hepatic inoculation test, and can indicate population variability, which has already been detected in E. histolytica20. No correlation was found between the in vitro and in vivo assays with xenic E. dispar, suggesting that the mechanisms of pathogenicity in vivo can be quite different from the in vitro ones when bacteria are present in the culture.

Concerning the hepatic inoculation, the prominent lesions induced by xenic E. dispar could not be attributed only to associated bacteria, since bacterial flora alone did not cause important lesion. Thus, a synergistic effect between E. dispar and bacteria must be considered. Presence of inflammatory cells around the necrotic foci, lysis of these cells by trophozoites and release of lytic enzymes could be involved in the tissue damage. This mechanism has been shown in experimental hepatic amoebiasis induced by axenic E. histolytica25. Another report also showed the migration of inflammatory cells around trophozoites in the hepatic parenchyma inoculated with an axenic E. dispar12, but no necrotic lesion was developed. In our experiments, it is possible that the presence of bacteria could enhance the inflammatory process, maintaining a propitious environment that allows trophozoites to multiply and spread along the hepatic parenchyma.

Although E. dispar cannot cause invasive disease to man, the experimental induction of lesions in animals is a way to access its pathogenic potential. Previous reports showed the ability of xenic strains with nonpathogenic zymodeme in causing lesions in hamster liver5,19. Our studies evaluated quantitatively not only the prevalence, but also the degree of the lesion, showing that some E. dispar can experimentally cause severe tissue damage in vivo as that caused by a virulent E. histolytica. It remains to be investigated whether the pathogenic potential of some strains of E. dispar is enough to determine symptoms on the human host.

ACKNOWLEDGMENTS

This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG). We thank João da Costa Viana and Edna Maria Pires for their technical assistance. Part of the results reported here was presented at the XIV Seminar on Amebiasis held in Mexico City, November 27-30, 2000.

Received: 15 March 2006

Accepted: 7 June 2006

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  • Correspondence to:

    Adriana Oliveira Costa
    Departamento de Patologia Básica, Setor de Ciências Biológicas
    Centro Politécnico s/n, Jardim das Américas
    81531-990 Curitiba, PR, Brasil
    Tel: +55 41 33611697
    E-mail:
  • Publication Dates

    • Publication in this collection
      30 Oct 2006
    • Date of issue
      Oct 2006

    History

    • Accepted
      07 June 2006
    • Received
      15 Mar 2006
    Instituto de Medicina Tropical de São Paulo Av. Dr. Enéas de Carvalho Aguiar, 470, 05403-000 - São Paulo - SP - Brazil, Tel. +55 11 3061-7005 - São Paulo - SP - Brazil
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