Human gastrointestinal illness due to Giardia duodenalis infection continues to be a public health problem. The ability to detect Giardia cysts is important in assessing the public health risk associated with environmental contamination. The low concentration of cysts in natural waters makes the use of the polymerase chain reaction (PCR) desirable in DNA probe-based detection methods. To date, amplification of DNA in environmental samples has been difficult to achieve. Humic compounds and/or other PCR inhibitors co-extract with nucleic acids, interfering with the polymerase reaction and lowering sensitivity. All of our attempts to separate humic compounds from DNA were unsuccessful. A substantial increase in the magnesium concentration in the reaction alleviated some of the inhibition, indicating that humics interfere by chelation of magnesium. PCR inhibition with environmental samples is most likely caused by a variety of contaminants.

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