Determination of genotypes of Toxoplasma gondii strains isolated from patients with toxoplasmosis

To determine the genotypes of Toxoplasma gondii strains associated with human toxoplasmosis, we developed a sensitive approach for typing parasites grown from clinical samples by short-term in vitro culture. A newly described nested PCR assay was capable of amplifying genomic DNA from as few as five parasites in the presence of host tissues. Typing was based on DNA polymorphisms at the SAG2 locus, encoding tachyzoite surface antigen p22. Restriction fragment length polymorphisms in PCR-amplified SAG2 products were used to classify strains into one of the three major lineages of T. gondii. This approach was successfully used to determine the genotypes of 68 of 72 samples that had been previously isolated from patients with congenital, cerebral, and disseminated toxoplasmosis. Type II strains of T. gondii were found in a majority of the samples, accounting for 55 (81%) of the 68 toxoplasmosis cases. In contrast, type I and III strains were found in only 7 (10%) and 6 (9%) of the 68 cases, respectively. The results of this study support the previous finding that type II strains are most often associated with human toxoplasmosis. Nested PCR analysis at the SAG2 locus provides rapid assignment of T. gondii to a specific genotype that should be useful in analyzing a variety of clinical samples.

Expression of Toxoplasma gondii genes in the closely-related apicomplexan parasite Neospora caninum

We have established the feasibility of using Neospora caninum as a heterologous system for the expression of genes from the closely-related parasite Toxoplasma gondii. Plasmid construct containing the lacZ gene from Escherichia coli driven by T. gondii promoters were electroporated into N. caninum parasites, and expression of beta-galactosidase (beta-Gal) activity was assayed enzymatically. In transient assays, expression of beta-Gal driven by the GRA1 promoter was approximately 10-fold higher than the expression obtained with the SAG1 promoter. Enzyme activity was not detected when N. caninum parasites were transfected with a promoterless lacZ construct. Transfection of N. caninum with complete genomic clones of SAG1 or GRA2 from T. gondii yielded parasites that transiently expressed the respective gene products, as detected by immunofluorescence and Western blot. Additionally, these transiently expressed T. gondii proteins appeared by immunofluorescence localization and Triton X-114 partitioning to be correctly targeted in both extracellular and intracellular N. caninum parasites. Heterologous gene expression should be useful for studying the function of specific gene products and may facilitate the identification of genes responsible for the phenotypic differences observed between these two closely-related apicomplexan parasites.

Identification of two genetic markers that distinguish pathogenic and nonpathogenic strains of Acanthamoeba spp

Species-level identification of Acanthamoeba isolates is difficult and gives little or no indication of the isolate's pathogenicity. We identified two amplification-based genetic markers that were highly correlated with pathogenicity in Acanthamoeba spp. One marker, designed to amplify a 485-bp fragment of the small-subunit ribosomal RNA gene (ssrDNA), was preferentially amplified from the nonpathogenic strains; amplifications from the pathogenic strains yielded anomalous fragments of 650 and 900 bp. A second marker was developed on the basis of the anomalous 650-bp fragment. Primers to this sequence preferentially amplified a noncoding locus (called Ac6) only from the pathogenic strains. These two genetic markers may be useful for identification of pathogenic Acanthamoeba spp. strains.

Acute virulence in mice is associated with markers on chromosome VIII in Toxoplasma gondii

Toxoplasma gondii has an unusual population structure consisting of three widely distributed clonal lineages. Acute virulence in mice is strictly observed in type I strains, indicating that a genetic determinant(s) unique to this lineage controls acute pathogenesis. We have analyzed several naturally occurring recombinant strains of T. gondii that carry allele 1 at the SAG1 locus; this allele is characteristic of the type I strains and was previously found to be 100% correlated with the acute virulence phenotype. Recombinant strains G622-M and ROD both had a predominantly type III genotype, with the significant exception of allele 1 at the SAG1 locus. Although these two strains had virtually identical multilocus genotypes, they differed in their virulence in mice. Strain ROD was virulent whereas strain G622-M was nonvirulent, thus demonstrating that the presence of allele 1 at SAG1 is not alone sufficient to confer acute virulence. Several sequence polymorphisms upstream of SAG1 were found to be highly correlated with the acutely virulent lineages. Collectively, these results suggest that acute virulence is regulated by a region linked to the SAG1 locus on chromosome VIII in T. gondii.

Toxoplasma gondii comprises three clonal lineages: correlation of parasite genotype with human disease

The population genetic structure of Toxoplasma gondii was determined by multilocus restriction fragment length polymorphism analysis at 6 loci in 106 independent isolates from humans and animals. Phylogenetic and statistical analyses indicated a highly unusual population structure consisting of 3 widespread clonal lineages. Extensively mixed genotypes were only apparent in 4 strains, which indicated that, while not separate species, sexual recombination between the 3 lineages is exceedingly rare in natural populations. T. gondii is a major cause of subclinical human infection and an important opportunistic pathogen that causes severe disease in immunocompromised patients. While strains from all 3 lineages were isolated from humans, the majority of human toxoplasmosis cases were associated with strains of a type II genotype. The correlation of specific clonal lineages with human toxoplasmosis has important implications for development of vaccines, drug treatments, and diagnostic protocols.

Amplification of rDNA loci to detect and type Neisseria meningitidis and other eubacteria

In 1991-92, Neisseria meningitidis group C was isolated from the blood of eight students in Urbana, Illinois, USA, and from the cerebrospinal fluid of one student from a nearby community, Decatur, Illinois. These and other bacterial species were analysed by PCR fingerprinting using primers selected from the ribosomal (r)DNA loci. A rDNA primer pair spanning a region within the 16S rDNA amplified a predicted 280 base pair (bp) DNA fragment from Neisseria spp. and fragments of different sizes for other genera. This primer pair specifically detected a carrier of N. meningitidis in a small clinical battery. Identity of the fragment was confirmed by restriction endonuclease analysis. A 600 bp fragment was also amplified from the 16S-23S internal transcribed spacer (ITS) of N. meningitidis; amplification from six other genera yielded different-sized fragments. Digestion of the ITS fragment from N. meningitidis with Alu I revealed three patterns; pattern I was found only for serogroup C isolates, and it was the dominant pattern among recent isolates with the exception of the one from Decatur. The isolate from Decatur yielded pattern III which suggested a non-clonal relationship to the seven isolates from Urbana. Patterns II and III were more prevalent in isolates from the 1960's and 1980's. PCR-based analysis of these loci can complement the techniques which are currently used for the detection and typing of these and other eubacteria.

Identification of Acanthamoeba at the generic and specific levels using the polymerase chain reaction

We have adapted the polymerase chain reaction to identify strains of Acanthamoeba. Using computer-assisted analysis, primers were designed from an anonymous repetitive sequence and from published sequences of 18S and 5S ribosomal RNA genes of A. castellanii. Amplification of a short ribosomal DNA target (272 base pairs) at restrictive annealing conditions (greater than 50 degrees C) resulted in a single band that was unique for the genus and distinguished Acanthamoeba from Naegleria. This assay functioned with fresh and formalin-fixed cells as starting material. Amplification of longer targets (400-700 base pairs) at less restrictive annealing conditions (less than 47 degrees C) led to more than one band. This multiple banding pattern could reproducibly classify Acanthamoeba at the strain level and was, in certain cases, diagnostic for known pathogenic strains. However, these assays need to be further refined to make them relevant for clinical purposes.

Use of the polymerase chain reaction for the sensitive detection of St. Louis encephalitis viral RNA

Polymerase chain reaction (PCR) assays were developed for the detection of RNA from the St. Louis encephalitis (SLE) virus. Using computer-assisted analysis of the MSI-7 strain SLE virus genome, two primer pairs were selected from the capsid-coding and the membrane associated protein-coding genes, and one from the envelope-coding gene. Reverse transcription was primed with either specific oligomers or with random hexamers; these methods were compared for cDNA synthesis and subsequent PCR amplification with the oligomeric pairs. Random hexamers provided more sensitive detection of viral RNA. Each primer pair specifically amplified the expected sized fragment from the Parton SLE strain grown in Aedes albopictus cells, but did not amplify Aedes albopictus cell RNA controls. The technique also detected SLE virus RNA in 1 pg of total cellular RNA added to a background of 1 microgram boiled brain tissue, and in 0.5 pg of total RNA added to homogenized mosquito abdomen. PCR-based assays may be adaptable to detect SLE virus RNA in naturally infected mosquitoes, birds, and human cerebrospinal fluid and brain.