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Antimicrobial Agents and Chemotherapy, March 2009, p. 1264-1267, Vol. 53, No. 3
0066-4804/09/$08.00+0     doi:10.1128/AAC.01104-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Quinolone Resistance in Neisseria gonorrhoeae: Rapid Genotyping of Quinolone Resistance-Determining Regions in gyrA and parC Genes by Melting Curve Analysis Predicts Susceptibility{triangledown}

Frédérique Vernel-Pauillac,1 Tiffany R. Hogan,2 John W. Tapsall,2 and Cyrille Goarant1*

Institut Pasteur de Nouvelle-Calédonie, Laboratoire de Recherche en Bactériologie, Nouméa Cedex, New Caledonia,1 WHO Collaborating Centre for STD, Department of Microbiology, The Prince of Wales Hospital, Sydney, Australia2

Received 18 August 2008/ Returned for modification 4 December 2008/ Accepted 26 December 2008


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ABSTRACT
 
We report a duplex real-time PCR assay for the simultaneous screening of mutations involved in fluoroquinolone resistance within gyrA and parC quninolone resistance-determining regions (QRDRs) in Neisseria gonorrhoeae. Our assay clearly detects all mutated QRDRs and allows the identification of common genotypes, whether the QRDRs contain single or double mutations, providing valuable epidemiological tools. When this method is used in conjunction with similar assays and in vitro analyses, essential antibiotic resistance surveillance can be performed for public health purposes.


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INTRODUCTION
 
Gonorrhea rates generally remain high in New Caledonia and Oceania, where penicillins are the recommended treatment (17, 19, 22). Increasing penicillin resistance means that fluoroquinolones are the next therapeutic option, but monitoring of relevant antibiotics for effectiveness for public health purposes (2, 7, 15, 16, 23) is required. However, resistance surveillance of gonococci is difficult because of culture-related and storage issues (3). Nonetheless, important epidemiological data may now be obtained if appropriate samples are conserved for genetic analysis (22).

The main determinant of fluoroquinolone resistance in gonococci is target site alteration by spontaneous mutations in the quinolone resistance-determining regions (QRDRs) of gyrA and parC (1, 4, 6, 20). The codons 91 and 95 of gyrA and 86 through 88 and 91 of parC are those most frequently associated with quinolone resistance in Neisseria gonorrhoeae (1, 6, 11, 13, 14, 20). The stepwise acquisition of mutations leading to progressive increases in MICs begins first with gyrA changes resulting in a less susceptible or resistant phenotype, followed by parC alterations that increase the resistance. We developed and validated a real-time PCR-based genotyping method using hybridization probes to detect QRDR mutations within gyrA and parC for the epidemiological surveillance of quinolone susceptibility.

The QRDR sequences and ciprofloxacin MICs for a panel of 21 selected N. gonorrhoeae WHO reference strains and other strains from the WHO Collaborating Centre in Sydney, Australia, were first determined (Table 1). Strains were categorized as less susceptible when MICs were 0.06 to 0.5 µg/ml, as resistant when MICs were >0.5 to <4 µg/ml, and as highly resistant when MICs were ≥4 µg/ml (18). A further 14 clinical strains isolated at the Institut Pasteur in Cambodia, for which MICs of ciprofloxacin ranged from 0.016 to >32 µg/ml (Table 1), and a 1-year collection of 100 isolates (only 1 of which was quinolone-resistant N. gonorrhoeae [QRNG]) from the bacteriology laboratory of the Institut Pasteur in New Caledonia were also examined. The ciprofloxacin MICs for these isolates were determined by the Etest (AB Biodisk, Solna, Sweden) diffusion method using WHO reference strains as controls. All strains and isolates were handled and DNA was extracted as described previously (22). Oligonucleotides were designed using the LightCycler probe design software version 2.0 (Roche Diagnostics) and synthesized by Sigma-Proligo (Singapore Pty. Ltd). The 761-bp amplified region of gyrA was probed for mutations in codons 91 through 95 by melting curve analysis with a LightCycler Red 705 probe. For parC analysis, an anchor probe was combined with a LightCycler Red 670-labeled sensor probe overlapping codons 86 through 91 inside a 607-bp amplified parC product. Primer and probe sequences and the reaction conditions are summarized in Table 2. Amplification and melting curve analysis of the two genes were conducted in a single multiplex run with a final volume of 20 µl. This protocol ensures multiplex genotyping using a compensation color program to correct emission spectrum overlaps of the dyes. Controls in every experiment included a blank capillary tube, wild-type DNA, and mutated DNA.


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  		TABLE 1. Reference and collection strains used in this study


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  		TABLE 2. Primers, probes, and amplification conditionsa used to genotype QRDRs of N. gonorrhoeae gyrA and parC genes

We obtained highly reproducible results both within and between successive runs, with distinguishable melting curves for both QRDRs (Fig. 1). The gyrA QRDR genotyping analysis used the 705-nm wavelength and identified eight different melting peaks, based on melting temperatures (Tms) and curve shapes (Fig. 1A). Wild-type gyrA QRDR sequences were identified by a mean Tm ± a standard deviation of 57.2 ± 0.1°C. Single mutants were easily evidenced by Tms in the range of 50.5 to 51.5°C. Among single mutants, D95N and D95G were distinguished by Tms of 50.7 ± 0.05°C and 51.0 ± 0.1°C, respectively, both with a sharp melting peak, whereas S91Y, S91F, and D95G were not visibly distinguished from one another due to the overlap of their melting peaks, with a Tm of 51.4 ± 0.1°C.


Figure 1
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  		FIG. 1. Representative melting peaks derived from negative derivatives of fluorescence ratios with respect to temperature and corresponding genotypes for gyrA (A) and parC (B). Both of these spectra were generated using color compensation as indicated in the text.

Four profiles specific to double mutants were clearly identified and confirmed by sequencing reactions (Table 1). In descending order of Tms, our assay discriminated the S91F/D95A genotype, displaying a Tm of 49.05 ± 0.1°C; the S91F/D95G genotype, with a Tm of 44.0 ± 0.2°C; then the S91F/D95N genotype, with a Tm of 43.35 ± 0.1°C; and lastly, the S91F/D95Y genotype (Tm of 42.35 ± 0.05°C). Furthermore, the S91F/D95N profile was characterized by a melting peak showing a reproducibly high level of fluorescence compared with the S91F/D95G profile, which always displayed a relatively low and oblate melting peak.

By using the 670-nm wavelength, the parC QRDR genotyping analysis identified 10 different melting peaks, based on Tms and curve shapes (Fig. 1B). Wild-type parC QRDR sequences displayed a Tm of 68.2 ± 0.1°C. Single mutants were evidenced by Tms in the range of 59.5 to 65.6°C. Some of the single mutants were clearly identified: those with the mutations E91G (Tm of 65.05 ± 0.05°C) and S87N (Tm of 59.7 ± 0.1°C), as well as those with the mutations S88P (Tm of 62.25± 0.05°C) and E91K (Tm of 60.8 ± 0.05°C). Discrimination between D86N (Tm of 61.6 ± 0.1°C), S87R (Tm of 61.25 ± 0.05°C), and E91Q (Tm of 60.75 ± 0.05°C) was possible thanks to the high level of reproducibility when amplifications of reference DNAs of the corresponding genotypes were included during the same run, allowing a precise comparison of the melting peaks. Double mutants were also identified, with S87N/91Q mutants showing a Tm of 49.15 ± 0.1°C and D86N/S88P mutants showing a Tm of 54.7 ± 0.1°C. A random selection of strains according to their specific melting peaks were sequenced, and all results confirmed the presumptive genotypes assigned based on melting curve analysis.

Accurate genotypic detection of QRNG strains was confirmed by the high correlation with the phenotypic classification of QRNG isolates from Cambodia (where QRNG is highly endemic) and New Caledonia. Phenotypes of decreased susceptibility or moderate resistance correlated with single changes at codon 95 or 91, respectively, in gyrA. Further, parC alterations were detected only in gyrA-mutated strains and corresponded to higher MIC levels than those for strains with gyrA changes only, as described previously. No QRNG strains without QRDR changes were detected, and all QRDR alterations were accompanied by phenotypic change. With the inclusion of control DNA representing common genotypes for gyrA and parC QRDRs, single and double mutations can be easily identified (Fig. 1).

The isolation, storage, and shipment of viable cultures are difficult, expensive, and time-consuming. Nucleic acid amplification techniques may offer complementary means that are useful for diagnosis but also relevant and accurate for the prediction of clinical resistance. Our technique thus allows the provision of important epidemiological data of public health relevance, even if cultures are nonviable (3), through the proper use of a relevant reference center with established facilities. The detection of resistant subtypes of gonococci by N. gonorrhoeae multiantigen sequence typing has been reported previously (8), but this correlative approach is limited by the extensive geographic and temporal variation in gonococcal subtypes. Other strategies to detect QRNG either are time-consuming and require complex equipment (10) or are limited in their applications (5, 12) because they are not able to detect all QRDR mutations. In our assay, all the gyrA and parC QRDR changes can be determined in a single run with a single capillary tube.

The sensitivity of the assay, in common with those of all sequence-specific techniques, is affected by mutations outside the sequence covered by the sensor probe. Resistance resulting from mutations elsewhere would also remain undetected. However, gyrB or parE changes do not confer significant fluoroquinolone resistance (6), and Qnr-like proteins have not been detected in gonococci (9).

The early detection of stepwise QRDR mutations in gonococci would not only improve surveillance, but also possibly postpone the emergence of multidrug resistance. If used judiciously in conjunction with the formerly described penicillin resistance determinant assays (21, 22) and existing in vitro methods, the assay described herein would contribute significantly to the essential resistance surveillance required in areas such as Oceania.


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ACKNOWLEDGMENTS
 
We are grateful to Bertrand Guillard for sharing quinolone-susceptible and quinolone-resistant N. gonorrhoeae isolates from Cambodia. Thanks are also due to the staff of the bacteriology lab of the Institut Pasteur in New Caledonia under the authority of R. Goursaud and C. Lethezer for providing the New Caledonian isolates for the study and MIC data.

This work was supported by grants from the International Network of Pasteur Institutes (General Delegation, Institut Pasteur, Paris, France).


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FOOTNOTES
 
* Corresponding author. Mailing address: Institut Pasteur de Nouvelle-Calédonie, Laboratoire de Recherche en Bactériologie, Nouméa Cedex, New Caledonia. Phone: (687) 252666. Fax: (687) 273390. E-mail: cgoarant{at}pasteur.nc Back

{triangledown} Published ahead of print on 5 January 2009. Back


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Antimicrobial Agents and Chemotherapy, March 2009, p. 1264-1267, Vol. 53, No. 3
0066-4804/09/$08.00+0     doi:10.1128/AAC.01104-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.




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