Genetic diversity of Blastocystis hominis sensu lato isolated from humans in Poland

INTRODUCTION AND OBJECTIVES

Blastocystis hominis s. l. is one of the most commonly detected protozoa in the human large intestine. The aim of the study was to determine the genetic subtypes of Blastocystis hominis s. l. occurring in humans in Poland.

MATERIALS AND METHODS

Stool samples from patients diagnosed in the Laboratory of the Department of Parasitology, National Institute of Public Health – National Institute of Hygiene (NIZP-PZH) and in the Parasitology Laboratory of the Hospital for Infectious Diseases in Warsaw were examined. Blastocystis subtypes were assayed based on the fragment of small-subunit ribosomal RNA gene sequences (SSU rDNA).

RESULTS

The examined isolates were classified into five Blastocystis subtypes (STs), fifteen of which belonged to ST3, three to ST1, two to ST2, two to ST6, and one isolate belonged to ST7. In three cases the subtype of isolate was not identified.

DISCUSSION AND CONCLUSIONS

In Poland, the subtypes ST1, ST2, ST3, ST4, ST6 and ST7 have been reported in humans so far. The ST6 and ST7 subtypes are rarely detected in humans in Europe. In Poland, the ST6 subtype was previously described in chickens. On the basis of the studies, it was found that Blastocystis isolated from humans in Warsaw show high genetic diversity. In order to determine the possible pathogenic potential of individual Blastocystis subtypes, special epidemiological studies are required.

[Blastocystis hominis- parasites or commensals? ]

Blastocystis hominis (B. hominis) is a cosmopolitan pro- tozoa which parasitizes the human large intestine. This parasite had been considered to be commensal of the large intestine for a long time, because even an intense invasion may be asymptomatic. However, this species is now being regarded as a parasitic organism. In this paper the latest data concerning the epidemiology, diagnostics and treatment of B. hominis invasion have been cited and discussed.

[Is Blastocystis hominis an opportunist agent?]

Despite its high prevalence throughout the world, major issues about Blastocystis hominis remain unresolved, including fundamental areas such as taxonomy and pathogenicity. Sequences of the SSUrRNA gene place Blastocystis in the stramenophiles. Analysis of the elongation factor 1-alpha gene, however, indicates similarity to Entamoeba histolytica. There is considerable morphological variability and karyotype diversity, and it appears that more than one species is present in humans and animals. In culture, three major forms predominate: vacuolar, granular, and ameboid. The vacuolated form (usually 10 to 30 mum) was most frequently detected in fecal specimens. The prevalence of Blastocystosis in humans appears to be higher in developing countries (30% to 50%) than in developed countries (1.5% to 10%), and has been associated with travel. B. hominis is the most common parasite isolated from stool specimens in symptomatic and asymptomatic persons in a variety of settings. Isolates resembling B. hominis have been described in a variety of mammals, birds, reptiles, and even insects. The significance of this human infection is uncertain.

Infectivity of different genotypes of human Blastocystis hominis isolates in chickens and rats

Most Blastocystis hominis isolates from humans are believed to be potentially zoonotic. This is because B. hominis isolates found in a variety of other host species have been found to have identical or relatively similar genotypes to those found in human isolates. However, the transmission of human B. hominis isolates to other animals has not been confirmed experimentally. In this study, the infectivity associated with several unique human Blastocystis genotypes (subtypes 2, 3, 4 and 7) was therefore investigated by infecting chickens and rats with two isolates of each subtype experimentally. The results showed that one isolate of subtype 4 and one isolate of subtype 7 was capable of infecting both chickens and rats, while two isolates of subtype 2, another isolate of subtype 4, and another isolate of subtype 7 could only infect chickens. Conversely, two isolates of subtype 3 failed to infect either of the animals. These results confirmed that several genotypes from human isolates could infect chickens and/or rats, indicating that chickens and rats are suitable experimental animal models for studying the zoonotic potential of human Blastocystis isolates.

PCR fingerprinting of Blastocystis isolated from symptomatic and asymptomatic human hosts

Genomic DNA from 16 Blastocystis hominis isolates comprising of eight asymptomatic isolates (A1-A8) and eight symptomatic isolates (S1-S8) was amplified by arbitrarily primed polymerase chain reaction (AP-PCR) using 38 arbitrary 10-mer primers. Six primers (A10, B5, C20, D1, F6, and F10) generated reproducible DNA fingerprints. AP-PCR amplification revealed similar DNA fingerprints among all symptomatic isolates (S1-S8) with common bands at 850 bp using primer A10, 920 bp using primer B5, and 1.3 kbp using primer D1. Isolates A1, A3, A4, A5, A6, and A7 showed similar DNA banding patterns and all asymptomatic isolates (A1-A8) shared a major band at 1 kbp using primer B5. Isolates A2 and A8 showed distinct DNA banding patterns that differed from the remainder of the isolates. The results of the phylogenetic analyses showed that all symptomatic isolates (S1-S8) formed a clade with >70% similarity among the isolates and which were clearly separate from asymptomatic isolates A1, A3, A4, A5, A6, and A7. Asymptomatic isolates A2 and A8 formed two distinct and separate clades. AP-PCR revealed higher genetic variability within the asymptomatic isolates than within the symptomatic isolates. The present study suggests that AP-PCR can be a valuable method for differentiating between isolates of B. hominis and our results support the hypothesis that our asymptomatic and symptomatic B. hominis isolates may represent two different strains/species with varying pathogenic potential.

Molecular characterization of Blastocystis isolates in the Philippines by riboprinting

Extensive genomic polymorphism has been demonstrated among morphologically identical Blastocystis isolates. To this end, 32 Blastocystis isolates from the Philippines (12 from humans, 12 from pigs and 8 from chickens) were analyzed genetically by riboprinting or restriction fragment length polymorphism (RFLP) analysis of polymerase chain reaction (PCR) amplified small subunit rDNA. Three distinct riboprint patterns were observed from the HinfI digestion, while four patterns resulted from the RsaI digestion of Blastocystis SSU rDNA. Restriction fragment profiles between Blastocystis isolates from different hosts were generally different from each other. However, Blastocystis isolates within each host group were practically the same. Cluster analysis of the riboprint patterns revealed seven distinct groups of the Blastocystis isolates, including a zoonotic strain. These results demonstrate the genetic heterogeneity of Blastocystis in the Philippines and a support to the idea of the organism's zoonotic potential.

Fecal-oral transmission of the cyst form of Blastocystis hominis in rats

The infectivity of two Blastocystis hominis strains, RN94-9 and NIH:1295:1, was examined in 3-week-old SPF Wistar rats. The NIH:1295:1 strain, originally isolated from a guinea pig, was only able to infect rats via intracecal inoculation of the cultured organisms, while the RN94-9 strain, originally isolated from a laboratory rat, was able to infect rats by oral inoculation of the cultures due to the presence of a cystic form in the in vitro culture. Since many cysts were discharged in the feces of the infected rats, the infectivity of the concentrated cysts was compared between the two strains. Successful oral infection was observed in rats inoculated with 1 x 10(2)-1 x 10(6) cysts of the RN94-9 and NIH:1295:1 strains. The infectivity of the ten cysts varied in the three experiments of ten rats, being 20-100% and 30-100% in the RN94-9 and NIH:1295:1 strains, respectively. When an uninfected normal rat was housed with five experimentally inoculated rats, the normal rat became infected, demonstrating the fecal-oral transmission of the cyst form of this parasite. These results show that the Wistar rat is an ideal host for the propagation of strains RN94-9 and NIH:1295:1 of B. hominis, and demonstrate that the cyst form is the only transmissible form of this parasite.

Molecular and phylogenetic analysis of Blastocystis isolates from various hosts

Blastocystis hominis is a protozoan parasite found in humans. B. hominis-like organisms have been found in a variety of animals, but have been called Blastocystis sp. because the isolates from animals were indistinguishable from B. hominis morphologically. Recent molecular studies show that some isolates from animals have genetic similarity with B. hominis. However, it has been unclear whether the isolates from animals have zoonotic potential or not. In the present study, the SSUrDNA of 19 Blastocystis isolates from these animals was sequenced in its entirety, and the phylogenetic relationship among isolates from humans and animals was clarified using available nucleotide sequences of the same locus. Phylogenetic analysis showed that the 19 isolates analyzed in the present study could be classified into seven groups (I-VII): Group I consisted of the isolates from humans, primates, cattle, pigs and birds; Group II of the isolates from humans and primates; Group III of the isolates from humans, cattle and pigs; Group IV of the isolates from primates, birds and rodents; Group V of the isolates from cattle and pigs; Groups VI and VII of the isolates from humans and birds. These results indicate that many of the isolates harboring in animals have zoonotic potential, or have cross-transmissibility among heterogeneous hosts.

Polymerase chain reaction-based genotype classification among human Blastocystis hominis populations isolated from different countries

Since the genotype of human Blastocystis hominis isolates is highly polymorphic, PCR-based genotype classification using known sequenced-tagged site (STS) primers would allow the identification or classification of different genotypes. Five populations of human B. hominis isolates obtained from Japan, Pakistan, Bangladesh, Germany, and Thailand were subjected to genotype analysis by using seven kinds of STS primers. Ninety-nine out of 102 isolates were identified as one of the known genotypes, while one isolate from Thailand showed two distinct genotypes and two isolates from Japan were negative with all the STS primers. The most dominant genotype among four populations, except for all four isolates from Thailand, was subtype 3 and it varied from 41.7% to 92.3%. The second most common genotype among four populations was either subtype 1 (7.7-25.0%) or subtype 4 (10.0-22.9%). Subtype 2, subtype 5, and/or subtype 7 were only rarely detected among the isolates from Japan and Germany, while subtype 6 was not detected. The phylogenetic position of the two isolates which were negative with all STS primers, was inferred from the small subunit rRNA (SSU rRNA) genes with the known sequence data of 20 Blastocystis isolates. Since the two isolates were positioned in an additional clade in the phylogenetic tree, this suggested they were a new genotype. These results demonstrated that PCR-based genotype classification is a powerful tool with which to analyse genotypes of Blastocystis isolates obtained from clinical samples. In addition, two groups of the isolates from 15 symptomatic and 11 asymptomatic patients in Bangladesh were compared with the PCR-based subtype classification. Since both groups were only classified into two distinct genotypes of subtype 1 or subtype 3 and no statistically significant difference was observed between the two groups, in this study it could not be shown that the specific genotype correlated with the pathogenic potential of B. hominis.

Recent advances in Blastocystis hominis research: hot spots in terra incognita

Despite being discovered more than 80 years ago, progress in Blastocystis research has been gradual and challenging, due to the small number of laboratories currently working on this protozoan parasite. To date, the morphology of Blastocystis hominis has been extensively studied by light and electron microscopy but all other aspects of its biology remain little explored areas. However, the availability of numerous and varied molecular tools and their application to the study of Blastocystis has brought us closer to understanding its biology. The purpose of this review is to describe and discuss recent advances in B. hominis research, with particular focus on new, and sometimes controversial, information that has shed light on its genetic heterogeneity, taxonomic links, mode of transmission, in vitro culture and pathogenesis. We also discuss recent observations that B. hominis has the capacity to undergo programmed cell death; a phenomenon similarly reported for many other unicellular organisms. There are still many gaps in our knowledge of this parasite. Although there is a growing body of evidence suggesting that B. hominis can be pathogenic under specific conditions, there are also other studies that indicated otherwise. Indeed, more studies are warranted before this controversial issue can be resolved. There is an urgent need for the identification and/or development of an animal model so that questions on its pathogenesis can be better answered. Another area that requires attention is the development of methods for the transfection of foreign/altered genes into B. hominis in order to facilitate genetic experiments.

Ribodemes of Blastocystis hominis isolated in Japan

To determine if genetic diversity of Blastocystis hominis exists in Japan, we monitored 64 B. hominis-infected people: 39 asymptomatic people whose infections were detected during routine medical check-ups (32 Japanese and 7 non-Japanese) and 25 patients with gastrointestinal symptoms who visited the outpatient clinics of St. Luke's International Hospital (19 Japanese and 6 non-Japanese). We detected 6 known and 2 new riboprint patterns in isolates from the infected people. There were no differences in the distribution of ribodemes between isolates from Japanese and non-Japanese people, similar to that in other countries. However, we noted a possible relationship between ribodeme type and pathogenicity. The results suggest that ribodemes I, III, and VI may be responsible for gastrointestinal symptoms.

Blastocystis elongation factor-1alpha: genomic organization, taxonomy and phylogenetic relationships

The elongation factor-1 alpha (EF-1alpha) is a highly conserved ubiquitous protein that is involved in translation and is desirable for use in phylogenetic studies on Blastocystis, an enigmatic intestinal parasite with a contentious taxonomic position. In the present study, a PCR product (BEalpha) that codes for a major part of the coding region of the EF-lalpha protein was amplified. Genome walking experiments together with cloning were implemented to elucidate the 5' and 3' ends of the EF-1alpha gene of the human isolate, Blastocystis hominis C. The genomic organization and the potential transcription factor binding sites of the 5' end of B. hominis C EF-1alpha were identified. A comparative study on the deduced amino acid sequences of BEalpha of 13 Blastocystis isolates from various hosts was done to evaluate the phylogenetic relationship among the species. A phylogenetic reconstruction analysis with other eukaryotic EF-1alpha sequences was carried out to trace the phylogenetic position of Blastocystis among eukaryotic organisms.

Genomic analysis of Blastocystis hominis strains isolated from two long-term health care facilities

The genotype Blastocystis hominis is highly polymorphic. Therefore, a genetic marker would be a powerful tool for the identification or classification of B. hominis subtypes and could be used as a means to resolve the transmission route or origin of the parasite. To this end, 32 B. hominis isolates were collected from patients and/or staff members of two long-term health care facilities (facilities A and B), and these organisms were subjected to genotype analysis based on diagnostic PCR primers and restriction fragment length polymorphism (RFLP) of small subunit rRNA gene (rDNA). Based on PCR amplification using diagnostic primers which were developed from randomly amplified polymorphic DNA analysis of known strains of B. hominis, the 32 isolates of B. hominis were classified into three different subtypes. Thirty isolates, including twenty-four that were isolated from patients and a staff member, from facility A and all isolates isolated from six patients from facility B showed the same genotype. Two of six patients of facility B had been transferred from facility A, and these two patients also had the same-genotype B. hominis that corresponded to 24 isolates from facility A. This genotype strain may have been transmitted by these two patients from facility A to facility B, suggesting human-to-human transmission. In contrast, 2 of 26 isolates from facility A showed distinct genotypes, suggesting that the colonization by these two isolates is attributable to another infectious route. These different subtypes were subjected to RFLP analysis, and the RFLP profiles were correlated with the results obtained by diagnostic PCR primers. This study presents the first molecular evidence of possible human-to-human B. hominis infection between and/or among two small communities.

Intestinal amebae

Although the intestinal amebae that infect humans are not thought of as classic agents of food-borne disease, food plays an important role in the transmission of these protozoa. This is particularly true for areas of the world where the organisms are endemic. Transmission of most intestinal protozoa occurs by the fecal-oral route via contaminated food or water. Among the four genera of amebae that infect man, only Entamoeba histolytica and Blastocystis hominis are causes of disease. This article focuses on E. histolytica because of the organism's medical and economic impact on humans. In addition, the epidemiology, control, and laboratory diagnosis of these protozoa are addressed.

Genomic polymorphism among Blastocystis hominis strains and development of subtype-specific diagnostic primers

Genomic polymorphisms among nine strains of Blastocystis hominis were examined by random amplified polymorphic DNA (RAPD) using four different arbitrary polymerase chain reaction (PCR) primers. Based on the RAPD patterns, nine strains were classified into three groups. The specific primers designed from the unique bands yielded a single band from within each same group, but did not amplify between all the groups examined. Specificity of these diagnostic primers was tested against several common intestinal parasites and a yeast, and no amplification was confirmed. Since the current criteria indicates that Blastocystis organisms isolated from humans are designated as B. hominis, the authors propose to classify several subtypes among B. hominis groups based on the difference of genomic DNA using three diagnostic primers.

Blastocystis hominis in animals: incidence of four serogroups

In toto, 520 faecal samples from mammals, birds, reptiles, amphibians, fish, and snails were investigated (see Table 1). 91 strains of Blastocystis hominis could be isolated by culture. However, only 48 of them were suitable for axenisation. 96 percent of samples belonged to four serogroups detected in humans but two strains, one from a pig and another from a duck, could not be classified, suggesting the existence of one or two further serogroups. While humans showed mainly serogroups I and II, pigs harboured serogroups III and IV. Four serogroups were isolated from monkeys. The question whether the genus Blastocystis consists of one or more species is discussed.

Five subgroups of Blastocystis hominis from symptomatic and asymptomatic patients revealed by restriction site analysis of PCR-amplified 16S-like rDNA

DNA polymorphism of Blastocystis hominis isolates was examined by the amplification of a gene fragment coding for the 16S-like rDNA. Using identical primers, fragments of approximately 850 bp were amplified from 110 B. hominis isolates and fragments of 1.1 kbp were amplified from 48 isolates. Digestion of the amplification products with the restriction enzymes HinfI, RsaI, and AluI revealed different profiles for each fragment length. Subgroup I and II, resulting from digestion of the smaller 850 bp fragment, have identical HinfI and AluI restriction bands, subgroup III and IV have identical RsaI fragments after digestion of the 1.1 kbp DNA. Subgroup V resembles subgroup III in a few bands after the RsaI and AluI restriction, respectively. Ninety (54%) of the isolates studied were assigned to subgroup I, 20 (12%) to subgroup II, 35 (21%) to subgroup III, 12 (7%) to subgroup IV, and 1 (1%) to subgroup V. Five (3%) of the examined people were coinfected with B. hominis of the subgroup III, 3 (2%) carried B. hominis of the subgroup I and II. These results show that there are 5 B. hominis subgroups none of which was found to be significantly correlated with the reported disease.

Four serologically different groups within the species Blastocystis hominis

61 strains of Blastocystis hominis were isolated from 340 stool specimens coming from 140 patients with intestinal symptoms and from 200 healthy persons. The overall female to male ratio was 1.9 to 1. Classification of the 61 isolates of B. hominis was determined with the aid of the immunodiffusion assay. Four serologically different groups could be identified. Their frequency ratio in the population studied was 39.5%:39.5%:18%:3%. Group 3 was found more often than the other three groups in stool samples of patients. However, within the statistical deviation, none of the four immunological different groups was found to be correlated with the disease or the sex of the hosts.

[Blastocystis hominis: in search of a disease, a misunderstood organism]

Blastocystis hominis is a micro-organism which remains somewhat mysterious. Having defined its present position in the classification of Protozoa, the authors describe its ultrastructure and morphology. Its epidemiology and pathogenicity are discussed in the light of experimental studies and human clinical data, especially in AIDS patients. Metronidazole seems to be the most active drug against this organism, but extreme caution must be exerted when the possible pathogenic property of B. hominis is discussed.

Protein and DNA evidence for two demes of Blastocystis hominis from humans

Analysis of 10 stocks of Blastocystis hominis isolated from human stools revealed two discrete groups of organisms. Proteins of the two groups were immunologically distinct and hybridization with random probes generated from the DNA of one stock showed that the DNA content of the two groups was different. Further studies are required to determine whether these should be classified as discrete species or whether these groups are epidemiologically significant.