Coxiella burnetii, the etiologic agent of Q fever, is an obligate intracellular bacterium. The reservoirs are extensive but partially known, and include mammals, birds and arthropods. Farm animals (i.e., cattle, sheep, goats) are identified as principal sources of human infection 1. Routine diagnosis of Q fever in veterinary medicine is usually performed by Stamp staining of the placenta or serological tests. Isolation and quantitation of C. burnetii are difficult, time consuming and require confined level L3 laboratories. Detection of C. burnetii is possible by using cell culture (shell vial culture system), but essentially it is used in clinical practice 5. Polymerase chain reaction has become a useful tool for the detection of C. burnetii in biological samples 6. However, this necessitates the extraction of the genomic DNA of C. burnetii without purification of bacteria. Thus, the samples contain gDNA of the bacteria and the eukaryotic cells. Different methods are reported in the literature 2, 4, but the sensitivity is very low or the method is not adapted to samples containing blood.
In this article, we report that the Perfect gDNA Blood Mini Kit from Eppendorf is highly adapted to the C. burnetii gDNA extraction when the bacterium is present in goat placenta (blood), mouse spleen or milk.
Materials and Methods
Total DNA was directly extracted from 20 l and 50 l of homogenized goat placenta naturally contaminated by C. burnetii (genome size: 2,103 kb), 10 l, 30 l and 50 l of homogenized mouse spleen (mouse injected intraperitoneally with C. burnetii) and 100 l of bovine milk contaminated by infective suspension of C. burnetii and containing 108 or 107 bacteria/ml). The extraction was carried out using the Eppendorf Perfect gDNA Blood Mini Kit as recommended in the manual with the exception of two steps: the lysis with Proteinase K was performed at 70C in a water bath over a thirty minute period (and could now be frozen at 20C before continuing the DNA extraction), and the final step: gDNA was eluted with 150 l of Elution Buffer.
Two primers, Trans1 and Trans2, derived from a transposon-like repetitive region of the C. burnetii genome, were used to amplify the gDNA of the bacterium (Trans-PCR) 2, 3. The expected product of amplification of the target sequence with these primers was 687 bp in length. Two other primers, O1 (CGGGAAGCTGTGGCGTGATG) and O2 (CTTGGCAGGTTTCTCCAGG), derived from the ovine g3pdh gene, were used to amplify gDNA of the eukaryotic cells. The expected amplification product of the target sequence with these primers was 168 bp in length.
The PCR reaction was performed on 2.5 l of each prepared sample in a total volume of 25 l. The final reaction mixture contained 1 M of each primer, 200 M of each deoxynucleoside triphosphate, 2.5 mM MgCl2 and 0.5 U of Taq DNA Polymerase. The Trans-PCR thermal program6 was modified:
1 cycle at 96C for five minutes, followed by 35 cycles at 96C for thirty seconds, 61C for one minute, 72C for two minutes and 1 final cycle at 96C for thirty seconds, 61C for one minute and 72C for ten minutes. DNA extracted from yolk sacs of chick embryos inoculated with CbO1 strain of C. burnetii was used as a positive control and sterile water as a negative control.Fig. 1: Amplification of a 168 bp fragment of eukaryotic gDNA. Genomic DNA was isolated from placenta (goat), spleen (mouse) and milk (bovine) with the Eppendorf Perfect gDNA Blood Mini Kit. Gel electrophoresis (1% agarose gel, x l reaction volume) shows the result of the PCR reaction.
As expected, the O1-O2 fragment of the eukaryotic cells could be amplified after extraction of the total gDNA of goat placenta, mouse spleen or bovine milk contaminated with C. burnetii (Fig. 1). As the quantity of cells in milk is low and variable, the PCR product obtained with these samples is weak. In addition, the Trans1-Trans2 fragment from C. burnetii could also be amplified successfully in Trans-PCR (Fig. 2).
Thus, this kit can be used for the detection of C. burnetii in different samples such as placenta, spleen, milk or blood (data not shown). Similar results were found with spleens of mice contaminated with Chlamydia and treated with this kit (data not shown).