Student Research: Paul R. Seidel

Intraspecies Differentiation of Giardia lamblia by Amplified Sequence Polymorphisms


Giardia lamblia is a flagellated protozoan parasite of mammals with a broad host range which appears to consist of numerous genetically distinguishable subtypes/ It is widely distributed in the environment and contributes significantly to morbidity from enteric disease. It is now estimated to be the leading cause of waterborne gastrointestinal illness in the United States. Additionally, the parasite is estimated to have a low infective dose and is resistant to chemical disinfection methods commonly used in municipal water supply.

Differences between strains are important because they are likely to differ with respect characteristics of biological and public health significance. Differentiation between strains with respect to important epidemiological characteristics has been complicated by the fact that they are morphologically identical. Commonly used molecular methods of differentiation have included haryotyping, isoenzyme electrophoresis, comparison of restriction fragment length polymorphisms, surface antigens and ribosomal DNA sequences. These methods have been used successfully in dividing G. lamblia into two to five apparently consistent subgroups.

This study used two types of oligonucleotide sequences one a random ten base oligonucleotide, the other specific 20 base oligonucleotides, both types homologous to repetitive sequences in the Giardia genome to amplify sequences unique to the specific subtypes using the polymerase chain reaction. The amplified markers were then used to differentiate between different environmental isolates from a variety of sources. The test isolates included recently derived and older long-term propagated isolates. Considerable genetic diversity was demonstrated between the isolates examined. The specific primers allowed differentiation of the isolates into 14 unique types which were further grouped into five groups based on the similarity of amplified patterns. This differentiation is consistent with what has been observed by other investigators. A primer of arbitrary sequence demonstrated the greatest degree of heterogeneity among the isolates and no groupings were possible.