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Journal of Clinical Microbiology, November 2005, p. 5690-5695, Vol. 43, No. 11
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.11.5690-5695.2005
Malcolm Grow Medical Center, Andrews Air Force Base, Maryland 20762,1 Epidemic Outbreak Surveillance Advanced Diagnostics Laboratory, Lackland Air Force Base, San Antonio, Texas 78236,2 Texas A&M University Systems, San Antonio, Texas 78223,3 Nova Research Inc., Alexandria, VA 22308,4 DoD Center for Deployment Health Research, Naval Health Research Center, San Diego, California 92152,5 Center for Bio/Molecular Science & Engineering, Code 6900, Naval Research Laboratory, Washington, DC 203756
Received 25 February 2005/ Returned for modification 19 May 2005/ Accepted 15 August 2005
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The traditional microbial diagnosis of S. pyogenes relied on a throat culture that appeared as beta-hemolytic colonies on 5% sheep blood agar (6). The considerable lag time (24 to 48 h) for throat cultures could result in delayed diagnosis, deferred treatments, and decreased compliance (5). Rapid detection of group A streptococci (GAS) offers several advantages, such as providing earlier and appropriate treatment, decreasing transmission, and reducing symptom duration and suppurative complications, and is preferred by patients and physicians if the diagnosis can be made on the same day and antibiotic therapy can be provided immediately (5, 31). Rapid diagnostic tests, such as antigen tests, have evolved during the last 20 years to become more sensitive (in the range of 70 to 95% compared with the culture method). However, studies have shown that there still are substantial false-negative rates with rapid antigen testing, and a controversy exists regarding whether or not these rapid tests require a confirmatory assay(s) when the result is negative (5, 12, 30, 31). Further, Johnson et al. reported that false-positive rapid antigen detection caused by cross-reactive organisms may be more common than previously reported (17).
With the advancement of molecular techniques, the last 10 years have seen increasing emphasis on molecular diagnosis using PCR-based assays (2). PCR-based assays resulting in rapid amplification of the bacterial genome are the mainstay of new analysis methods, including Vir typing using restriction fragment length polymorphism (RFLP), fluorescent RFLP, PCR M typing, immuno-PCR, and real-time PCR (9, 14, 22, 24, 35, 36). Although PCR-based methods have clearly facilitated the detection of GAS, these detection techniques require multistep procedures and special laboratory setups and are subject to equivocal results.
With substantial progress in microarray technology, it is now possible to combine the sensitivity afforded by nucleic acid amplification with the specificity afforded by DNA-DNA hybridization for the detection of human pathogens (4, 16, 19-21, 28, 29, 37). Recently, high-density microarrays with resequencing capability have been used to identify pathogen species and drug resistance in a series of in vitro experiments using cultured microorganisms, including human immunodeficiency virus, mycobacteria, and rifampin-resistant Mycobacterium tuberculosis strains (13, 20, 34). More recently, sequence-specific identification of Francisella tularensis and Yersinia pestis was demonstrated in environmental samples with high-density microarrays with resequencing capability (39, 40).
We have previously described an Affymetrix resequencing respiratory pathogen microarray (RPM v.1) for simultaneous detection and characterization of the many different types of respiratory tract pathogens that cause common diseases with similar clinical symptoms (Lin et al., submitted for publication). RPM v.1 consists of prototype regions for the detection of human adenoviruses, influenza A virus (subtypes H1, H3, H5, N1, and N2), 12 other common respiratory pathogens (influenza B virus, parainfluenza virus, rhinovirus, respiratory syncytial virus, West Nile virus, coronavirus, Streptococcus pneumoniae, Streptococcus pyogenes, Mycobacterium pneumoniae, Neisseria meningitidis, Bordetella pertussis, and Chlamydia pneumoniae), and six Centers for Disease Control and Prevention category A bioterrorism pathogens (Bacillus anthracis, variola major virus, Ebola virus, Lassa virus, Francisella tularensis, and Yersinia pestis) known to cause febrile respiratory illness (i.e., "flu-like" symptoms). We demonstrated that we could unequivocally detect and type both DNA and RNA viruses from clinical samples as well as validate each tiled sequence on the RPM v.1 (with the exception of West Nile virus) using clinical and/or controlled laboratory samples (Lin et al., submitted for publication). Building on this foundation, a more comprehensive study was undertaken to evaluate the use of resequencing arrays for the detection of S. pyogenes and associated antibiotic resistance markers.
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Primer design and PCR amplification. The primers used for PCR are listed in Table 1. PCRs were performed in 25-µl volumes containing 20 mM Tris-HCl (pH 8.4), 50 mM KCl, 2 mM MgCl2, 200 µM each of the deoxynucleoside triphosphates (dNTPs), 200 to 300 nM primers, 1 U of Platinum Taq DNA polymerase (Invitrogen Life Technologies, Carlsbad, CA), and 1 µl of purified DNA template from clinical samples. The amplification reaction was carried out in Peltier PTC225 thermal cycler (MJ Research Inc., Reno, NV) with preliminary denaturation at 94°C for 3 min; followed by 40 cycles of 94°C for 30 s, 50°C for 30 s, 72°C for 40 s; and a final extension at 72°C for 10 min. The amplified products were electrophoresed on 2.5 to 3% Tris-acetate-EDTA (TAE) agarose gels and visualized by ethidium bromide staining. PCRs for GAS detection were carried out as previously described (18).
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TABLE 1. Oligonucleotide primers used for real-time PCR and multiplex PCR
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29 DNA polymerase with random hexamers that was performed according to the instructions of the GenomiPhi DNA amplification kit (Amersham Biosciences Corp., Sunnyvale, CA). The amplified products were ethanol precipitated according to the manufacturer's recommended protocol. Clinical samples. Throat swabs were collected by the Respiratory Disease Laboratory-Naval Health Research Center (NHRC) from patients with acute respiratory disease symptoms and immediately placed in 2-ml cryogenic vials containing 1.5 ml of transport medium. Nasal washes from the Epidemic Outbreak Surveillance (EOS) team at Lackland Air Force Base were collected from basic military trainees with acute respiratory disease symptoms. In both instances, samples were tested at the site of collection using conventional culture techniques and submitted for microarray-based detection in a masked fashion. GAS-positive samples were identified among culture plates by colony morphology (beta-hemolytic colonies) on 5% sheep blood agar plates after 24 h of incubation. Beta-hemolytic colonies were confirmed to be GAS by susceptibility to bacitracin and a positive reaction to the latex agglutination test (Hardy Diagnostics, Santa Maria, CA). The collection and transport of all clinical samples complied with the Wilford Hall Medical Center protocol for clinical investigation (FWH20020124H). Nucleic acid was extracted from clinical samples using the MasterPure DNA purification kit (Epicenter Technologies, Madison, WI) following the manufacturer's recommended protocol with slight modification. We omitted the RNase digestion step. Informed consent was obtained from all participants after the nature and possible consequences of the studies were explained.
Microarray hybridization and processing. Nucleic acids of clinical samples were subjected to either multiplex PCR or random amplification. The amplified products were fragmented and labeled according to the recommended protocol by Affymetrix (Santa Clara, CA) prior to hybridization. Microarray hybridization and processing were carried out according to the manufacturer's recommended protocol (Affymetrix, Santa Clara, CA). After scanning, the GCOS software is used to reduce the raw image (.DAT) file to a simplified file format (.CEL file) with intensities assigned to each of the corresponding probe positions. Finally, the GDAS software is used to apply an embedded version of the ABACUS algorithm (7) to produce an estimate of the correct base calls, comparing the respective intensities for the sense and antisense probe sets. To increase the percentage of base calls, we adjusted the parameters to allow highly permissive base calls (increased percentage) as described previously (supplemental material). The sequences from base calls made for each tiled region of the resequencing array then were exported from GDAS as the FASTA-formatted files. Resequencing pathogen identification (REPI) software was used to parse the output of the FASTA file into a format suitable for sequence similarity searches using the NCBI BLASTN algorithm (J. A. Thornton, 2004, U.S. patent application).
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Multiplex PCR versus random amplification. In developing microarray technology for identifying pathogens, front-end amplification is required to reach low levels of detection sensitivity without culturing the organisms (23). Two different amplification strategies, multiplex PCR and random amplification, were used as front-end amplification for GAS identification prior to hybridization to RPM v.1 chips. PCR or multiplex PCR is the most commonly employed front-end amplification method with microarrays for detection of pathogens. In this study, four sets of primers [speB, erm(B), erm(TR), and mef genes; Table 1], which we selected for detecting S. pyogenes as well as its antibiotic resistance markers on RPM v.1 chips, were combined in a single multiplex PCR assay and evaluated under various conditions to obtain amplification of all four target genes. The PCR products were verified by 2.5 to 3% TAE agarose gel electrophoresis (data not shown). The capability of the resequencing microarray to detect GAS was tested using RPM v.1 to interrogate PCR amplicons from prototype S. pyogenes (strain MGAS 8232). Primary sequence data generated by the hybridization revealed that RPM v.1 unequivocally identifies prototype S. pyogenes (strain MGAS 8232) without erythromycin resistance markers. In addition to multiplex PCR, we successfully demonstrated random amplification provides great utilities for the purpose of broad-spectrum respiratory pathogen surveillance. It combines the use of resequencing DNA microarrays with methods for microbial nucleic acid enrichment, random nucleic acid amplification, and automated sequence similarity searching for pathogen detection (Lin et al., submitted for publication). Similar data were observed when hybridizing randomly amplified nucleic acid from prototype S. pyogenes (strain MGAS 8232) to RPM v.1. Both PCR and random amplification achieved nearly identical levels of hybridization specificity (data not shown), with PCR showing higher sensitivity. Figure 1A shows a representative hybridization pattern of one of the S. pyogenes-positive clinical samples. The sequence information derived from GDAS base-calling algorithm (Fig. 1B and C) was used to query a database using a similarity search algorithm with REPI software (Fig. 1C), which was developed in our laboratory. Proof-of-concept experiments, utilizing clinical samples obtained from patients presenting GAS-induced illness, unequivocally demonstrated the ability of RPM v.1 chips for correct species and antibiotic marker identification.
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FIG. 1. Representative RPM v.1 chip image of GAS-positive clinical sample. (A) S. pyogenes-positive clinical sample hybridized to RPM v.1 chip following nucleic acid isolation and amplification using multiplex PCR primers. No significant cross-hybridization is observed. (B) Hybridization-based primary sequences generated through the chip image. GDAS software is used to apply an embedded version of the ABACUS (7) algorithm to produce an estimate of the correct base calls, comparing the respective intensities for the sense and antisense probe sets. (C) FASTA sequence output file and REPI analysis result of S. pyogenes showing the highest BLAST bit score across all respective tile regions where amplicons hybridized.
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TABLE 2. Comparison of resequencing microarray to culture results for S. pyogenes-positive clinical samples
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TABLE 3. Evaluation of RPM v.1 for S. pyogenes detection in clinical samples
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It is important that the new assay provide a cost-effective alternative to conventional approaches to the diagnosis, management, and surveillance of infectious diseases, most particularly respiratory infections. In the current format, the RPM v.1 assay is not yet optimized for speed (
12 to 24 h) or cost (400 U.S. dollars per chip, approximately 20 U.S. dollars per pathogen species) relative to those desired for rapid infectious disease diagnostics. However, the approach is highly amenable to improvement via reduction in array size (i.e., lower cost), process automation, and the availability of portable hardware for processing resequencing arrays. In the event of lower-cost or easily automated microarray alternatives, the resequencing array can be a higher-echelon component in a diagnostics/surveillance pipeline. With this objective in mind, we are currently working on developing automated technologies that will enable for point-of-care diagnostics applications using resequencing microarray technologies.
Since the mid-1980, incidents of S. pyogenes infection have been on the rise and have been associated with life-threatening diseases (6). Genotyping is essential for GAS outbreak investigation and surveillance. Traditional typing methods, such as serologic typing methods, rely on available antisera and often produce ambiguous results (2, 6). Recently, molecular typing methods, such as PCR M typing (PCR with rapid sequencing), PCR-enzyme-linked immunosorbent assay, and sequencing, Vir typing using RFLP, and fluorescent RFLP (2, 8, 9, 14, 21, 31, 32) have provided sensitive alternative methods. Despite the sensitivity and specificity associated with molecular typing methods, these methods are often limited to targeting one gene for genotyping purpose. With resequencing microarrays, we could target more than one gene for diagnostic and genotyping purposes, which will provide more accurate genotyping information for surveillance.
We choose the speB gene, coding for a cysteine proteinase produced by all group A streptococci, as the target gene for S. pyogenes detection on Affymetrix RPM v.1 due to its conserved and stable characteristics (24). Though useful in detecting S. pyogenes, the sequences obtained from microarrays do not provide enough genotype information for epidemiological assessment. S. pyogenes strains are commonly categorized by antigenic differences in M protein, a transmembrane surface protein that is also a crucial virulence factor (1). M protein is encoded by the emm gene of S. pyogenes. The 5' ends of the emm gene are hypervariable and code for serotype specificity for more than 80 distinct serotypes (10). Due to the size limitation of RPM v.1 "real estate," the RPM v.1 group A streptococcus genotyping capability was not fully explored in this study. Nonetheless, this study showed the potential of using a resequencing microarray for genotyping group A streptococci in future studies. Indeed, we included M protein (emm genes) in version 2 resequencing microarrays, which will enable us to detect and provide serotype information of GAS in future studies. Although not commonly considered a bioterror agent, outbreaks of streptococcal pharyngitis are common in military barracks. With the increasing prevalence of antibiotic resistance documented worldwide by the PROTEKT surveillance group (3) and with the increased deployment tempo of the U.S. military, the potential importance of rapid identification of emerging resistance becomes obvious. Three genes, erm(B), erm(TR), and mef, were chosen as targets for identification of erythromycin resistance markers. The primers used for amplifying these genes have been used successfully in surveying erythromycin resistance mechanisms with PCR-based assays (11, 26, 30). In this study, we successfully adapted the amplification strategy and expanded the direct sequencing capability of amplified PCR products with a resequencing microarray. The sequence information yielded can be used for comparison with available sequences in database. However, the present study did not generate sufficient sequence information (amplicons are too small) to reach a general conclusion about the specificity of the antibiotic resistance markers. Nonetheless, this method proved that the resequencing microarray-based assay can be expanded to include longer sequences for future study and provide more specific answers for antibiotic resistance markers.
In summary, we have demonstrated the potential of resequencing microarrays for efficient and accurate detection of GAS with the benefit of sequencing information from microarray analysis. With this method, we are able to show the capability of the resequencing microarray to provide unequivocal means for detecting S. pyogenes and its associated antibiotic-resistant markers. With further improvement of gene selection, we should be able to provide genotyping information of S. pyogenes and improve the specificity of antibiotic markers in future studies.
The opinions and assertions contained herein are those of the authors and are not to be construed as official or reflecting the views of the Department of Defense or the U.S. Government.
Supplemental material for this article may be found at http://jcm.asm.org/. ![]()
The Epidemic Outbreak Surveillance Consortium is an Air Force Medical Service initiative. The following are participating members of the EOS Consortium: (i) sponsorship, Peter F. Demitry (USAF/SGR) and Theresa Lynn Difato (USAF/SGR); (ii) executive board and principal investigators, Eric H. Hanson (The George Washington University [IPA]), Rosana R. Holliday (USAF/SGR [Ctr]), Robb K. Rowley (The George Washington University [IPA]), and Clark Tibbetts (The George Washington University [IPA]); (iii) operational board and senior scientists, Brian K. Agan (Wilford Hall Medical Center), Jerry Diao (USAF/SGR [Ctr]), Russell P. Kruzelock (Virginia Tech [IPA]), David A. Stenger (Naval Research Laboratory), and Elizabeth A. Walter (Texas A&M University Systems San Antonio [IPA]); (iv) technical advisors and collaborating investigators, Luke Daum (Air Force Institute for Operational Health), David Metzgar (Navy Health Research Center), Debra Niemeyer (USAF/SGR), and Kevin Russell (Navy Health Research Center); (v) research and clinical staff, Marie J. Archer (Naval Research Laboratory), Roger Bravo (Lackland AFB, Tex.), Nikki Freed (Naval Health Research Center), Julie Fuller (Naval Health Research Center), John Gomez (Lackland AFB, Tex.), Kevin Gratwick (Naval Health Research Center), Michael Jenkins (Wilford Hall Medical Center), Margaret Jesse (Lackland AFB, Tex.), Barry Johnson (Lackland AFB, Tex.), Erin Lawrence (Naval Health Research Center), Baochuan Lin (Naval Research Laboratory), Carolyn E. Meador (NOVA Research Incorporated), Hernan Melgarejo (Lackland AFB, Tex.), Kate M. Mueller (NOVA Research Incorporated), Chris Olsen (USAF/SGR [Ctr]), David Pearson (Lackland AFB, Tex.), Anjan Purkayastha (USAF/SGR [Ctr]), Jose J. Santiago (Lackland AFB, Tex.), Donald Seto (George Mason University [IPA]), Francine Fuentes Stotler (Lackland AFB, Tex.), Dzung Thach (Naval Research Laboratory), Jennifer A. Thornton (NOVA Research Incorporated), Zheng Wang (Naval Research Laboratory), Daisy Watson (Lackland AFB, Tex.), Sue A. Worthy (Lackland AFB, Tex.), and Gary J. Vora (Naval Research Laboratory); and (vi) operations support staff, Kenya Grant (USAF/SGR [Ctr]), Cheryl J. James (USAF/SGR [Ctr]), and Kathy Word (USAF/SGR [Ctr]). ![]()
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