Giardia sp.: comparison of electrophoretic karyotypes

The compact genome of Giardia muris reveals important steps in the evolution of intestinal protozoan parasites

Diplomonad parasites of the genus Giardia have adapted to colonizing different hosts, most notably the intestinal tract of mammals. The human-pathogenic Giardia species, Giardia intestinalis, has been extensively studied at the genome and gene expression level, but no such information is available for other Giardia species. Comparative data would be particularly valuable for Giardia muris, which colonizes mice and is commonly used as a prototypic in vivo model for investigating host responses to intestinal parasitic infection. Here we report the draft-genome of G. muris. We discovered a highly streamlined genome, amongst the most densely encoded ever described for a nuclear eukaryotic genome. G. muris and G. intestinalis share many known or predicted virulence factors, including cysteine proteases and a large repertoire of cysteine-rich surface proteins involved in antigenic variation. Different to G. intestinalis, G. muris maintains tandem arrays of pseudogenized surface antigens at the telomeres, whereas intact surface antigens are present centrally in the chromosomes. The two classes of surface antigens engage in genetic exchange. Reconstruction of metabolic pathways from the G. muris genome suggest significant metabolic differences to G. intestinalis. Additionally, G. muris encodes proteins that might be used to modulate the prokaryotic microbiota. The responsible genes have been introduced in the Giardia genus via lateral gene transfer from prokaryotic sources. Our findings point to important evolutionary steps in the Giardia genus as it adapted to different hosts and it provides a powerful foundation for mechanistic exploration of host-pathogen interaction in the G. muris-mouse pathosystem.

Development of Species-specific rDNA Probes for Giardia by Multiple Fluorescent In Situ Hybridization Combined with Immunocytochemical Identification of Cyst Wall Antigens

In this study, we describe the development of fluorescent oligonucleotide probes to variable regions in the small subunit of 16S rRNA in three distinct Giardia species. Sense and antisense probes (17–22 mer) to variable regions 1, 3, and 8 were labeled with digoxygenin or selected fluorochomes (FluorX, Cy3, or Cy5). Optimal results were obtained with fluorochome-labeled oligonucleotides for detection of rRNA in Giardia cysts. Specificity of fluorescent in situ hybridization (FISH) was shown using RNase digestion and high stringency to diminish the hybridization signal, and oligonucleotide probes for rRNA in Giardia lamblia, Giardia muris, and Giardia ardeae were shown to specifically stain rRNA only within cysts or trophozoites of those species. The fluorescent oligonucleotide specific for rRNA in human isolates of Giardia was positive for ten different strains. A method for simultaneous FISH detection of cysts using fluorescent antibody (genotype marker) and two oligonucleotide probes (species marker) permitted visualization of G. lamblia and G. muris cysts in the same preparation. Testing of an environmental water sample revealed the presence of FISH-positive G. lamblia cysts with a specific rDNA probe for rRNA, while negative cysts were presumed to be of animal or bird origin.

Sequence map of the 3-Mb Giardia duodenalis assemblage A chromosome

The genome of the gut protozoan parasite Giardia duodenalis (assemblage A) has been sequenced and compiled as contigs and scaffolds (GiardiaDB- http://GiardiaDB.org ), but specific chromosome location of all scaffolds is unknown. To determine which scaffolds belong to the 3-Mb chromosome, a library of probes specific for this chromosome was constructed. The probes were hybridised to NotI-cleaved whole chromosomes, and the combined size of different NotI segments identified by the probes was 2,225 kb indicating the probes were well distributed along the 3-Mb chromosome. Six scaffolds (CH991814, CH991779, CH991793, CH991763, CH991764, and CH991761) were identified as belonging to the 3-Mb chromosome, and these scaffolds were ordered and oriented according to scaffold features including I-PpoI sites and hybridisation pattern. However, the combined size of scaffolds was more than 4 Mb. Approximately, 1 Mb of scaffold CH991763 carrying previously identified sequences specific for the 1.5-Mb chromosome(s) including subtelomeric sequence was reassigned, and several other anomalies were addressed such that the final size of the apparently 3-Mb chromosome is estimated to be 2,885 kb. This work addresses erroneous computer-based assignment of a number of contigs and emphasises the need for alternative and confirmatory methods of scaffold construction.

Distribution of Giardia duodenalis genotypes and subgenotypes in raw urban wastewater in Milwaukee, Wisconsin

Giardia cysts in 131 raw wastewater samples from Milwaukee, Wis., were genotyped by sequence analysis of the triosephosphate isomerase gene which showed the presence of two distinct genotypes (assemblages A and B) of Giardia duodenalis. Of the 131 samples, 111 belonged to assemblage A, and the remaining samples belonged to assemblage B. A high degree of genetic polymorphism was evident within the assemblage B cluster, with 10 distinct subgenotypes identified, eight of which have not been reported before.

A 63 kDa VSP9B10A-like protein expressed in a C-8 Giardia duodenalis Mexican clone

BACKGROUND

It is well documented that Giardia duodenalis undergoes surface antigenic variation both in vivo and in vitro. Proteins involved have been characterized and referred to as VSP (variable surface protein).

METHODS

Two cloned cDNA inserts of 0.45 and 1.95 kb were obtained from G. duodenalis expression library and sequenced. Comparison sequence analyses were made against Genbank. PCR analysis was performed on G. duodenalis isolates to identify isolates bearing genes encoding such a peptide. Specific antiserum was prepared against 450-bp encoded peptide and tested by Western blot, immunofluorescence, and inhibition of adhesion of G. duodenalis to target cells.

RESULTS

We cloned and characterized a G. duodenalis 450-bp DNA fragment; its DNA sequence analysis revealed that this fragment displayed 99% identity with vsp9B10A gene. Predicted amino acid sequence for this fragment also had significant (99%) identity to VSP9B10A. A second 1.95-kb insert, which encompassed the 450-bp cDNA fragment, was also isolated; its DNA and amino acid sequence displayed 99.5% identity with vsp9B10A gene and 99.2% with the corresponding inferred protein, respectively. This inferred protein contained 24 Cys-X-X-Cys motifs and long ORF of 642 aminoacids. PCR analysis showed that DNA sequence encoding a fragment of this gene was present in P1, CIEA:0487:2-C-8 clone and in INP:180800-B2 G. duodenalis human isolates, while it was absent in sheep isolate of G. duodenalis INP:150593-J10.

CONCLUSIONS

Immunofluorescence analysis using antibodies raised against the peptide encoded by 450-bp fragment showed that expression of this epitope varies on trophozoite surface of the C-8 Mexican clone and is involved in parasite adhesion to target epithelial cells.

Triosephosphate isomerase gene characterization and potential zoonotic transmission of Giardia duodenalis

To address the source of infection in humans and public health importance of Giardia duodenalis parasites from animals, nucleotide sequences of the triosephosphate isomerase (TPI) gene were generated for 37 human isolates, 15 dog isolates, 8 muskrat isolates, 7 isolates each from cattle and beavers, and 1 isolate each from a rat and a rabbit. Distinct genotypes were found in humans, cattle, beavers, dogs, muskrats, and rats. TPI and small subunit ribosomal RNA (SSU rRNA) gene sequences of G. microti from muskrats were also generated and analyzed. Phylogenetic analysis on the TPI sequences confirmed the formation of distinct groups. Nevertheless, a major group (assemblage B) contained most of the human and muskrat isolates, all beaver isolates, and the rabbit isolate. These data confirm that G. duodenalis from certain animals can potentially infect humans and should be useful in the detection, differentiation, and taxonomy of Giardia spp.

Presence of human Giardia in domestic, farm and wild animals, and environmental samples suggests a zoonotic potential for giardiasis

Giardia lamblia which parasitize humans belong to either of two genotypes, A or B, based on specific signature sequences in the 5' end of the small subunit (16S) ribosomal RNA (rRNA) gene. These two genotypes also were found in cysts from fecal samples of animal origin such as dogs, cats, some farm animals and wild animals. In addition, trophozoites recovered from cysts obtained from environmental samples belonged to these two genotypes as well, suggesting that the G. lamblia genotypes A and B are widespread and possibly zoonotic. Trophozoites were recovered from rats and these isolates might belong to another genotype of G. lamblia. Deer mice and one dog appeared to be parasitized by genotypes of Giardia with close affinity to G. microti. This species, therefore, also consists of a genotype complex.

Biology of Giardia lamblia

Giardia lamblia is a common cause of diarrhea in humans and other mammals throughout the world. It can be distinguished from other Giardia species by light or electron microscopy. The two major genotypes of G. lamblia that infect humans are so different genetically and biologically that they may warrant separate species or subspecies designations. Trophozoites have nuclei and a well-developed cytoskeleton but lack mitochondria, peroxisomes, and the components of oxidative phosphorylation. They have an endomembrane system with at least some characteristics of the Golgi complex and encoplasmic reticulum, which becomes more extensive in encysting organisms. The primitive nature of the organelles and metabolism, as well as small-subunit rRNA phylogeny, has led to the proposal that Giardia spp. are among the most primitive eukaryotes. G. lamblia probably has a ploidy of 4 and a genome size of approximately 10 to 12 Mb divided among five chromosomes. Most genes have short 5' and 3' untranslated regions and promoter regions that are near the initiation codon. Trophozoites exhibit antigenic variation of an extensive repertoire of cysteine-rich variant-specific surface proteins. Expression is allele specific, and changes in expression from one vsp gene to another have not been associated with sequence alterations or gene rearrangements. The Giardia genome project promises to greatly increase our understanding of this interesting and enigmatic organism.

Giardia duodenalis: inter-strain variability of proteins, antigens, proteases, isoenzymes and nucleic acids

Giardia duodenalis isolates from asymptomatic or symptomatic patients and from animals present similarities and differences in the protein composition, antigenic profile, pattern of proteases and isoenzymes, as well as in nucleic acids analysis. In the present overview, these differences and similarities are reviewed with emphasis in the host-parasite interplay and possible mechanisms of virulence of the protozoon.

Cloning of two putative Giardia lamblia glucosamine 6-phosphate isomerase genes only one of which is transcriptionally activated during encystment

The biosynthesis of the carbohydrate component of the cyst wall of the protozoan parasite Giardia lamblia, a polymer of N-acetylgalactosamine (GalNac), is by a pathway that is initiated with the conversion of fructose 6-phosphate to glucosamine 6-phosphate by an aminating isomerase, glucose 6-phosphate isomerase. This enzyme appears only after Giardia trophozoites are induced to start the production of cyst wall components after bile is added. To investigate whether induction of glucosamine 6-phosphate isomerase is by protein modification or by transcription activation, its gene was cloned and sequenced. Two genes, gpi1 and gpi2, encoding putative glucosamine 6-phosphate isomerases were identified but one, gpi1 was expressed. The transcript for gpi1 appeared not earlier than 6 h after cells were induced with bile salts. These results show that the first enzyme in the pathway leading to GalNac synthesis in encysting Giardia cyst wall biosynthesis is under transcriptional control.

Comparison of the levels of intra-specific genetic variation within Giardia muris and Giardia intestinalis

The extent of intra-specific genetic variation between isolates of Giardia muris was assessed by allozyme electrophoresis. Additionally, the levels of allozymic variation detected within G. muris were compared with those observed between members of the two major assemblages of the morphologically distinct species Giardia intestinalis. Four isolates of G. muris were analysed. Three (Ad-120, -150, -151) were isolated from mice in Australia, while the fourth (R-T) was isolated from a golden hamster in North America. The 11 isolates of G. intestinalis (Ad-1, -12, -2, -62, representing genetic Groups I and II of Assemblage A and BAH-12, BRIS/87/HEPU/694, Ad-19, -22, -28, -45, -52, representing genetic Groups III and IV of Assemblage B) were from humans in Australia. Intra-specific genetic variation was detected between G. muris isolates at four of the 23 enzyme loci examined. Similar levels of variation were found within the genetic groups that comprise Assemblages A and B of G. intestinalis. These levels of intra-specific variation are similar to those observed within other morphologically-distinct species of protozoan parasites. We suggest that the magnitude of the genetic differences detected within G. muris provides an indication of the range of genetic variation within other species of Giardia and that this can be used as a model to delineate morphologically similar but genetically distinct (cryptic) species within this genus.

Formation of the Giardia cyst wall: studies on extracellular assembly using immunogold labeling and high resolution field emission SEM

Encystment of the intestinal protozoan, Giardia, is a key step in the life cycle that enables this parasite to be transmitted from host to host via either fecal oral, waterborne, or foodborne transmission. The process of encystment was studied by localizing cyst wall specific antigens with immunofluorescence for light microscopy and immunogold staining for field emission scanning electron microscopy. Chronological sampling of Giardia cultures stimulated with endogenous bile permitted identification of an intracellular and extracellular phase in cyst wall formation, a process which required a total of 14-16 h. The intracellular phase lasted for 8-10 h, while the extracellular phase, involved the appearance of cyst wall antigen on the trophozoite membrane, and the assembly of the filamentous layer, a process requiring an additional 4-6 h for completion of mature cysts. The extracellular phase was initiated with the appearance of cyst wall antigen on small protrusions of the trophozoite membrane (approximately 15 nm), which became enlarged with time to caplike structures ranging up to 100 nm in diameter. Caplike structures involved with filament growth were detected over the entire surface of the trophozoite including the adhesive disc and flagella. Encysting cells rounded up, lost attachment to the substratum, and became enclosed in a layer of filaments. Late stages in encystment included a "tailed" cyst in which flagella were not fully retracted into the cyst. Clusters of cysts were seen in which filaments at the surface of one cyst were connected with the surface of adjacent cysts or the "tailed" processes of adjacent cysts, suggesting that the growth of cyst wall filaments may be at the terminal end. In conclusion, the process of encystment has been shown to consist of two morphologically different stages (intracellular and extracellular) which requires 16 h for completion. Further investigation of the extracellular stage with regard to assembly of the filamentous layer of the cyst wall may lead to innovative methods for interfering with production of an intact functional cyst wall, and thereby, regulation of viable Giardia cyst release from the host.

Molecular genetic analysis of Giardia intestinalis isolates at the glutamate dehydrogenase locus

Samples of DNA from a panel of Giardia isolated from humans and animals in Europe and shown previously to consist of 2 major genotypes--'Polish' and 'Belgian'--have been compared with human-derived Australian isolates chosen to represent distinct genotypes (genetic groups I-IV) defined previously by allozymic analysis. Homologous 0.52 kilobase (kb) segments of 2 trophozoite surface protein genes (tsa417 and tsp11, both present in isolates belonging to genetic groups I and II) and a 1.2 kb segment of the glutamate dehydrogenase (gdh) gene were amplified by the polymerase chain reaction (PCR) and examined for restriction fragment length polymorphisms (RFLPs). Of 21 'Polish' isolates that were tested, all yielded tsa417-like and tsp11-like PCR products that are characteristic of genetic groups I or II (15 and 6 isolates respectively) in a distinct assemblage of G. intestinalis from Australia (Assemblage A). Conversely, most of the 19 'Belgian' isolates resembled a second assemblage of genotypes defined in Australia (Assemblage B) which contains genetic groups III and IV. RFLP analysis of gdh amplification products showed also that 'Polish' isolates were equivalent to Australian Assemblage A isolates (this analysis does not distinguish between genetic groups I and II) and that 'Belgian' isolates were equivalent to Australian Assemblage B isolates. Comparison of nucleotide sequences determined for a 690 base-pair portion of the gdh PCR products revealed > or = 99.0% identity between group I and group II (Assemblage A/'Polish') genotypes, 88.3-89.7% identity between Assemblage A and Assemblage B genotypes, and > or = 98.4% identity between various Assemblage B/'Belgian' genotypes. The results confirm that the G. duodenalis isolates examined in this study (inclusive of G. intestinalis from humans) can be divided into 2 major genetic clusters: Assemblage A (= 'Polish' genotype) containing allozymically defined groups I and II, and Assemblage B (= 'Belgian' genotype) containing allozymically defined groups III and IV and other related genotypes.

A three nucleotide signature sequence in small subunit rRNA divides human Giardia in two different genotypes

The nucleotide sequence of the 16S rRNA gene and the space DNA region was determined for Giardia duodenalis, obtained from humans in The Netherlands (AMC-4) and Washington State (CM). These rDNA sequences differ from other G. duodenalis isolates (Portland-1 and BRIS/83/HEPU/ 106) both of which have virtually identical rDNA sequences. The most characteristic feature was found close to the 5' end of the 16S rRNA. The Portland-1 -Bris/83/HEPU/ 106 type has GCG in position 22-24, while AMC-4 and CM have AUC in this position. These two sequences, present in an otherwise conserved region of the 16S rRNA, are "signature" sequences, which divide Giardia isolates into two different groups.

Giardia intestinalis: detection of major genotypes by restriction analysis of gene amplification products

The polymerase chain reaction (PCR) has been used to amplify a 0.52 kb segment of Giardia intestinalis DNA, using primers specific for nucleotide sequences conserved within two genes (tsp11 and tsa417) that encode homologous, cysteine-rich trophozoite surface proteins. Using products amplified from axenic isolates belonging to genetic groups I and II (defined on the basis of allozyme electrophoresis data), restriction endonuclease analysis revealed both tsp11-like and tsa417-like fragments within all samples. The study also identified among the amplification products of group II organisms an additional fragment, containing a novel PstI site, that is not detected in the reaction products of group I isolates. The recovery of three distinct PCR products from each group II isolate was verified by cloning the fragments into the plasmid vector pGEM-7. Fragments containing the new PstI site possess the ClaI site common to both tsp11 and tsa417-like fragments, but they lack the HindIII site which characterizes tsp11-like fragments and also lack the PstI and KpnI sites which characterize tsa417-like fragments. Spot-blot analyses using cloned fragments of all three types as probes showed strong homologous hybridization but weak heterologous hybridization, indicating that each type differs substantially in nucleotide sequence from the others. Because the samples of Giardia DNA used in the PCR were purified from cultures that had been established from single trophozoites, the data indicate that individual trophozoites belonging to genetic group II possess three homologous genes defined by these related fragments. The presence of a PstI site in the amplified segment of the newly-discovered third gene of group II organisms provides a simple diagnostic means of differentiating group I and II isolates.

Chromosomal duplication in Giardia duodenalis

Strains of Giardia duodenalis from diverse parts of the world have three or four chromosomal bands in the range of 650-800 kb as defined by field inversion gel electrophoresis. The extra chromosome band in this range defined a group of strains which are geographically distinct from other strains missing this band. The cloned line WB-1B has three chromosome bands in this size range and chromosome band 3 was used to construct a library of chromosome-specific probes. In some strains examined, including BRIS/83/HEPU/106, a subset of these WB-1B probes hybridized to chromosome band 3 and to the extra chromosome band 4, indicating a partial duplication of chromosome 3 in BRIS/83/HEPU/106. This duplication was estimated to be at least 500 kb when the sizes of NotI chromosome segments which hybridized with chromosome band 3-specific probes were added. A second subset of WB-1B chromosome 3-specific probes hybridized to a fifth chromosome of strain BRIS/83/HEPU/106, in the size range 650-800 kb, which was not visible by ethidium bromide staining. The two subsets of WB-1B probes hybridized to a variety of chromosome combinations in this size range in other Giardia strains and included apparent reversal of chromosome 3 and 4 mobility as well as identification of other minor chromosomes. These data indicate that chromosome band 3 of the line WB-1B is a cluster of at least two different chromosomes that cannot be electrophoretically separated but genetic rearrangements in other strains allow separation of linkage groups carried by chromosome band 3 of WB-1B.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular variation in Giardia

Molecular characterisation of species within the genus Giardia has revealed that much of the phenotypic heterogeneity, particularly within the species G. duodenalis, has a genetic basis. The source of this genetic variation appears to arise from predominantly asexual, clonal reproduction, although occasional bouts of sexual reproduction cannot be ruled out. Genetic variation is extensive with some clones widely distributed and others seemingly unique and localised to a particular endemic focus. Little attention has been given to the molecular epidemiology of Giardia infections. Future studies should be directed at studying the ecology and dynamics of transmission of Giardia clones, particularly in localised areas, and to evaluating the factors that serve to maintain genetic diversity between clones, especially the role of inter-clonal competition. Future research using molecular techniques should aim to identify and follow Giardia clones in nature and correlate genetic typing with important clinical and epidemiological characteristics such as virulence, drug sensitivity and zoonotic potential.

Differentiation of major genotypes of Giardia intestinalis by polymerase chain reaction analysis of a gene encoding a trophozoite surface antigen

The polymerase chain reaction (PCR) has been used to amplify in vitro a semi-conserved region of a gene encoding an M(r) 68-72,000 surface antigen of Giardia intestinalis trophozoites. Using primers specific for conserved nucleotide sequences identified within the promoter-distal portion of two homologous genes (tsp11 and tsa417) cloned previously from the G. intestinalis isolates Ad-1 (from Australia) and WB (from Afghanistan), a single PCR-amplified DNA fragment of the expected size (0.52 kilobases) was obtained in high yield from either purified DNA or whole trophozoites of the Ad-1 isolate and from every 1 to 9 other axenic G. intestinalis isolates belonging to genetic groups I and II (defined previously on the basis of allozyme electrophoresis data--Andrews et al. 1989). Discernible product was recovered from as few as 2-4 trophozoites. In contrast, 6 G. intestinalis isolates that were assigned by allozymic analysis to genetic groups III/IV yielded small amounts of a 0.37-kilobase (kb) amplification product (with evidence in some samples of an additional 0.4 or 0.18 kb fragment) but no 0.52 kb product. Two animal-derived isolates of G. duodenalis (one from an Australian native rodent, Notomys alexis, the other from a domestic cat) also yielded a single 0.37 kb PCR-amplified fragment, whereas an isolate from another cat produced a 0.34 kb fragment. No product was recovered from G. muris, a morphologically distinct species of Giardia. The results demonstrate that different genotypes of G. duodenalis can be distinguished using this assay and that it is diagnostic for isolates belonging to two major clusters (groups I/II and III/IV) of G. intestinalis. The amplified DNA segment appears to be relatively conserved among group I and group II isolates of G. intestinalis. A related but clearly distinct sequence seems to be conserved among group III/IV isolates of G. intestinalis and some isolates of G. duodenalis.

The nucleotide sequence of the entire ribosomal DNA operon and the structure of the large subunit rRNA of Giardia muris

The total nucleotide sequence of the rDNA of Giardia muris, an intestinal protozoan parasite of rodents, has been determined. The repeat unit is 7668 basepairs (bp) in size and consists of a spacer of 3314 bp, a small-subunit rRNA (SSU-rRNA) gene of 1429, and a large-subunit rRNA (LSU-rRNA) gene of 2698 bp. The spacer contains long direct repeats and is heterogeneous in size. The LSU-rRNA of G. muris was compared to that of the human intestinal parasite Giardia duodenalis, to the bird parasite Giardia ardeae, and to that of Escherichia coli. The LSU-rRNA has a size comparable to the 23S rRNA of E. coli but shows structural features typical for eukaryotes. Some variable regions are typically small and account for the overall smaller size of this rRNA. The structure of the G. muris LSU-rRNA is similar to that of the other Giardia rRNA, but each rRNA has characteristic features residing in a number of variable regions.

Classification of subgroups of Giardia lamblia based upon ribosomal RNA gene sequence using the polymerase chain reaction

A sensitive and specific polymerase chain reaction-based assay has been developed to detect and analyze polymorphism in the Giardia lamblia 18S ribosomal RNA gene. Efficient amplification required the inclusion of cosolvents (glycerol and dimethyl sulfoxide) in the reaction. Following the optimization of conditions for amplification and subsequent hybridization of amplified product with radiolabeled oligonucleotide probe, a detection limit of less than one organism's worth of DNA was achieved. Thirty-five different G. lamblia strains obtained from various human and animal host types and geographic locations were analyzed by this method. The strains could be divided into 3 groups on the basis of defined nucleotide substitutions within the 183-bp amplified DNA fragment of the 18S ribosomal RNA gene. The groupings based upon the 18S ribosomal RNA gene sequence correlated with groupings previously assigned based upon patterns of surface antigens and restriction enzyme analysis. Analysis of the G. lamblia 18S ribosomal RNA gene sequences present in fecal specimens obtained from giardiasis patients revealed the presence of the different sequence types in these specimens. Some specimens contained more than one sequence type. The identification of subgroups of G. lamblia may facilitate studies of virulence, infectivity, and the epidemiology of giardia infection.

Investigation of human giardiasis by karyotype analysis

The patterns of transmission of Giardia lamblia and the potential contribution of strain differences to pathogenicity of infection is poorly understood. We used pulsed field gradient gel electrophoresis (PFGE) to separate chromosome-sized DNA molecules of 22 stocks of G. lamblia isolated from 13 individuals (6 symptomatic, 7 asymptomatic) living in Jerusalem. PGFE gels run under a variety of conditions revealed up to nine ethidium bromide-stained bands per isolate ranging in size from 0.7 to greater than 3 megabasepairs. Relative staining intensities indicated that some bands contained multiple chromosomes. Major differences in the number, size, and intensity of bands allowed a clear differentiation of the karyotypes of isolates from each of the different individuals. This is in contrast to previous studies where the karyotype of different isolates have been strikingly homogeneous. Hybridization of Southern blots with surface antigen, beta-tubulin, and ribosomal RNA genes revealed that these gene families were distributed to different sized chromosomes amongst the different isolates. PFGE thus revealed major differences in the karyotypes of different G. lamblia isolates that were obtained over a short period of time from a relatively confined geographic area. In contrast, karyotypes of isolates established either by direct cultivation of duodenal trophozoites or by excystation of stool cysts from the same individuals were almost identical. Also, isolates from the same individuals obtained over a prolonged period of time revealed only minor differences in their karyotype, suggesting that recurrent infection can be caused by genetically similar organisms. We conclude that chronic giardiasis can result from recurrence of occult infection or reinfection from a common source.

Differentiation of Giardia duodenalis from other Giardia spp. by using polymerase chain reaction and gene probes

Giardia spp. are waterborne organisms that are the most commonly identified pathogenic intestinal protozoans in the United States. Current detection techniques for Giardia species in water include microscopy and immunofluorescence techniques. Species of the genus Giardia are classified on the basis of taxonomic criteria, such as cell morphology, and on host specificity. We have developed a polymerase chain reaction- and gene probe-based detection system specific for Giardia spp., which can discriminate between the relevant species of the G. duodenalis type pathogenic to humans and other Giardia species that are not human pathogens. This method can detect a single Giardia cyst and is therefore sensitive enough for environmental monitoring.

The biology of Giardia spp

Gardia spp. are flagellated protozoans that parasitize the small intestines of mammals, birds, reptiles, and amphibians. The infectious cysts begin excysting in the acidic environment of the stomach and become trophozoites (the vegetative form). The trophozoites attach to the intestinal mucosa through the suction generated by a ventral disk and cause diarrhea and malabsorption by mechanisms that are not well understood. Giardia spp. have a number of unique features, including a predominantly anaerobic metabolism, complete dependence on salvage of exogenous nucleotides, a limited ability to synthesize and degrade carbohydrates and lipids, and two nuclei that are equal by all criteria that have been tested. The small size and unique sequence of G. lamblia rRNA molecules have led to the proposal that Giardia is the most primitive eukaryotic organism. Three Giardia spp. have been identified by light lamblia, G. muris, and G. agilis, but electron microscopy has allowed further species to be described within the G. lamblia group, some of which have been substantiated by differences in the rDNA. Animal models and human infections have led to the conclusion that intestinal infection is controlled primarily through the humoral immune system (T-cell dependent in the mouse model). A major immunogenic cysteine-rich surface antigen is able to vary in vitro and in vivo in the course of an infection and may provide a means of evading the host immune response or perhaps a means of adapting to different intestinal environments.

Cloning and restriction enzyme mapping of ribosomal DNA of Giardia duodenalis, Giardia ardeae and Giardia muris

In an attempt to study Giardia at the DNA sequence level, the rRNA genes of three species, Giardia duodenalis, Giardia ardeae and Giardia muris were cloned and restriction enzyme maps were constructed. The rDNA repeats of these Giardia show completely different restriction enzyme recognition patterns. The size of the rDNA repeat ranges from approximately 5.6 kb in G. duodenalis to 7.6 kb in both G. muris and G. ardeae. These size differences are mainly attributable to the variation in length of the spacer. Minor differences exist among these Giardia in the sizes of their small subunit rRNA and the internal transcribed spacer between small and large subunit rRNA. The genetic maps were constructed by sequence analysis of the DNA around the 5' and 3' ends of the mature rRNA genes and between the rRNA covering the 5.8S rRNA gene and internal transcribed spacer. Comparison of the 5.8S rDNA and 3' end of large subunit rDNA from these three Giardia species showed considerable sequence variation, but the rDNA sequences of G. duodenalis and G. ardeae appear more closely related to each other than to G. muris.