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ABSTRACT |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The rapid diagnosis of a rickettsial illness is important for appropriate antibiotic treatment to be given promptly. Traditionally, the diagnosis of a rickettsial illness has been based on serological tests. Of the serological tests, the indirect microimmunofluorescence assay has been the most sensitive and specific, but usually it is not positive when the patient is acutely unwell.4 Culture techniques can be used for diagnosis and are very sensitive but can require up to 60 days to yield a positive result, limiting their clinical usefulness.5
In the late 1980s, polymerase chain reaction (PCR) detection of rickettsial nucleic acid became available as a quick and reliable method for the diagnosis of rickettsioses. The first of these assays detected the rickettsial 17-kDa gene6 with later tests detecting the rickettsial citrate synthase and ompA genes.7 These assays, although specific for rickettsia, were not very sensitive until nested procedures were introduced.8 Recently, assays involving Orientia tsutsugamushi real-time PCR,9 Anaplasma phagocytophilium and Borrelia burgdorferi multiplex real-time PCR,10 and Rickettsia prowazekii and Borrelia recurrentis real-time duplex PCR11 have been developed. They are highly sensitive and specific, with the potential for quantifying DNA copy numbers.
In this manuscript, we describe a real-time assay based on the rickettsial citrate synthase gene. This assay is both highly specific and extremely sensitive for the diagnosis of rickettsioses and can also be used to quantify rickettsial DNA copy numbers.
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MATERIALS AND METHODS |
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A portion of the "Rickettsia marmionii" gltA was amplified using the above protocol without probe. The resulting 74 base pair fragment was ligated into a pCR 2.1 plasmid (ligated plasmid size; 4,003 base pairs) and subsequently cloned into One Shot Top10 chemically competent Escherichia coli with a TA Cloning kit (Invitrogen, Australia) using the manufacturer’s instructions. Transformed E. coli were grown overnight in a shaking water bath at 37°C in Luria Bertani broth (Oxoid, Hampshire, England) supplemented with ampicillin (CSL, Melbourne, Australia). The E. coli were pelleted and had plasmids extracted and purified with the FastPlasmid Mini kit (Eppendorf, Hamburg, Germany) using the manufacturer’s instructions. Five microliters of purified plasmid solution was diluted (1:100) and the DNA quantified in a scanning spectrophotometer.13 A theoretical number of plasmid copies and reaction efficiency were calculated and serial 10-fold dilutions of the plasmid solution made. Duplicates of each serial dilution underwent real-time PCR. Results enabled the sensitivity and efficiency of the assay to be determined.
DNA was extracted from cell cultures of members of the order Rickettsiales (Anaplasma phagocytophilium, Bartonella bacilliformis, B. henselae, B. vinsonii, Ehrlichia chaffeenisis, Orientia tsutsugamushi, Rickettsia akari, R. australis, R. conorii, R. honei, "R. marmionii," R. prowazekii, R. rickettsii, R. siberica, R. typhi, and R. bellii) and other medically important bacteria (Escherichia coli, Enterococcus faecalis, Coxiella burnetii, Haemophilus influenzae, Klebsiella oxytoca, Klebsiella pneumoniae, Legionella pneumophilia, Pseudomonas aeruginosa, Proteus mirabilis, Proteus vulgaris, Staphylococcus aureus, Staphylococcus saprophyticus, and Streptococcus pneumoniae) with a DNA extraction kit (Gentra, Minneapolis, MN) using the manufacturer’s instructions. A real-time PCR was conducted on each specimen to determine the specificity of the assay. A conventional 16S rRNA PCR was used to determine the presence of bacterial DNA.14 All reactions that involved testing the specificity and sensitivity of this assay were also spiked with human DNA.
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RESULTS |
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The sensitivity of the assay (determined by performing real-time PCR reactions on plasmid DNA solutions ranging from 1 x 10° to 1 x 1011 copies per reaction) was shown to be 1 copy of the SFG gltA target. The Ct value of each reaction varied from 6 for 1 x 1011 SFG gltA copies per reaction, to a Ct value of 35 for 1 x 10° SFG gltA copies per reaction. The reaction efficiency, as calculated by the real-time PCR analysis software, was 128% due to the unproportional digestion of the probe compared with the amplicon produced.
All tested species of the SFG and TG including R. akari, R. australis, R. conorii, R. honei, "R. marmionii," R. prowazekii, R. rickettsii, R. siberica, and R. typhi produced a positive real-time PCR result with the exception of R. bellii, an ancestral group Rickettsia. All other members of the Rickettsiales and medically important bacteria tested were real-time negative. All bacteria tested produced a positive result for the conventional 16S rRNA PCR. All PCR reactions that were spiked with human DNA had no effect on the sensitivity or specificity of the assay.
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DISCUSSION |
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The citrate synthase gene, gltA, is a highly conserved gene among the genus Rickettsia.12 The highly conserved nature of the gltA gene makes it an ideal target for real-time PCR. The section of the genome that the probe and primers span is relatively homogeneous in the genus Rickettsia. The number of substitutions in this section of the gltA gene varies from 0 in most SFG rickettsiae to 4 in the TG. Nine substitutions in the ancestral group’s sequence (Table 1
) probably explains why the assay does not detect R. bellii. Although individual sensitivity assays were not performed for many of the rickettsial species tested, including the TG, it may be assumed that the assay will amplify, with similarly high sensitivities, SFG/TG rickettsiae (except R. bellii), due to the highly conserved nature of this region of the gene (Table 1). An exception may be R. canadensis, as it has three nucleotide substitutions in the primer CS-F. This new assay will detect all rickettsial species of the SFG and TG, but not the ancestral group, with very high sensitivities.
The ability to quantify rickettsiae is useful. Traditionally, enumeration of rickettsiae has been done via the plaque assay.16 These assays are time consuming, rely on the rickettsia being capable of infecting a cell monolayer, and are not that sensitive.17 More recent methodologies in enumerating rickettsia have been PCR based, with sensitivities of five copies per reaction.18 The recent development of a real-time assay to detect O. tsutsugamushi has demonstrated that high specificity and a sensitivity of one copy number is achievable.9 The development of a real-time PCR assay for R. prowazekii using the ompB gene has also been published,11 although it cannot detect other members of the genus Rickettsia and has a sensitivity of approximately 10 copies. Enumeration of living rickettsia inoculated into an animal or tissue culture is important for the study of rickettsial pathogenicity,19 host susceptibility,20 and vaccine efficacy.21 Due to its high sensitivity, the new real-time assay would be useful in the quantification of rickettsia within animal and human organs and tissues, including blood, where a very high sensitivity is needed.
The ability to quickly diagnose an acute rickettsial illness is important for the rapid administration of appropriate antibiotics. Serology is no longer considered an adequate marker of rickettsial illness even when used as a retrospective test. Cases of confirmed rickettsioses have now been described where no increase in rickettsial antibody titer was detected or where a positive serum titer was not detected at any stage during or after the illnesses.22 Traditional PCR techniques lack the sensitivity to diagnose infection when there are low numbers of rickettsiae in peripheral blood mononuclear cells.18 The development of a real-time PCR specific for SFG and TG rickettsiae is useful in overcoming the defects of serology and conventional PCR. The ability of this new real-time PCR to detect 1 copy number of the SFG citrate synthase gene target and the highly specific nature of the assay makes it a valuable tool for the diagnoses of acute rickettsial infection.
Received March 14, 2005. Accepted for publication August 19, 2005.
Acknowledgments: We would like to thank The Geelong Region Medical Research Foundation for financial assistance with this project and Lin Dillon from Westmead Hospital, Sydney, for supplying the Bartonella strains.
* Address correspondence to Nathan Unsworth, The Australian Rickettsial Reference Laboratory, The Geelong Hospital, P.O. Box 281, Geelong, Victoria, Australia 3220. E-mail: nathanu{at}barwonhealth.org.au 
Authors’ addresses: John Stenos, Stephen R. Graves, and Nathan B. Unsworth, The Australian Rickettsial Reference Laboratory, The Geelong Hospital, P.O. Box 281, Geelong, Victoria, Australia 3220, Telephone: 61 3 5226 7521, Fax: 61 3 5260 3183.
Reprint requests: Nathan Unsworth, The Australian Rickettsial Reference Laboratory, The Geelong Hospital, P.O. Box 281, Geelong, Victoria, Australia 3220, Telephone: 61-3-5226-7521, Fax: 61-3-5260-3183, E-mail: nathanu{at}barwonhealth.org.au.
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This article has been cited by other articles:
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  N. Unsworth, S. Graves, C. Nguyen, G. Kemp, J. Graham, and J. Stenos Markers of exposure to spotted fever rickettsiae in patients with chronic illness, including fatigue, in two Australian populations QJM, April 1, 2008; 101(4): 269 - 274. [Abstract] [Full Text] [PDF]
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