Neisseria gonorrhoeae susceptibility to ciprofloxacin

Neisseria gonorrhoeae has developed resistance to all antimicrobials currently available,^1,2 and there are only a few novel antibiotics in development.^3-5 New approaches for combating that resistance are urgently needed. One new approach calls for targeted therapy with antibiotics previously thought to be ineffective^6-7; it has been made possible by the development of molecular assays that predict in vitro antimicrobial susceptibility.⁸ One of those molecular assays for determination of N. gonorrhoeae susceptibility to ciprofloxacin has been studied extensively.^9-18

Previous reports have shown that mutation at codon 91 of the gyrase A (gyrA) gene of N. gonorrhoeae can reliably predict in vitro susceptibility to ciprofloxacin.^8,19 While there are other mutations conferring ciprofloxacin resistance, it has been shown that those mutations usually occur in parallel with the gyrA mutation.^10,11,20,21 Additionally, in the United States, it is estimated that approximately 80 percent of N. gonorrhoeae infections are susceptible to ciprofloxacin.²² The ability to predict susceptibility by mutation at a single locus, and the high prevalence of susceptible infections, make ciprofloxacin an appealing option for targeted therapy. What is needed, however is the ability to rapidly determine genotype results.

In 2007, we developed a real-time polymerase chain reaction (RT-PCR)-based assay for the determination of mutation at codon 91 of N. gonorrhoeae.²¹ Since then, there have been 11 studies (N=4777) with samples from ten countries using RT-PCR techniques to compare gyrA genotype results with traditional susceptibility testing methods.^9-18,21 Those studies found between 93.8 percent and 100 percent sensitivity and 93.2 percent and 100 percent specificity of wild-type gyrA N. gonorrhoeae for the prediction of ciprofloxacin susceptibility. Notably, one study reported improvement of assay sensitivity among urine samples after restricting the assay to amplification of only the Ser91 codon.²¹ Furthermore, four studies compared mutation at codon 91 of the gyrA gene in N. gonorrhoeae to other Neisseria species and found 100 percent specificity for N. gonorrhoeae.^9,12,15,16

Therefore, there is strong evidence that codon 91gyrA gene determination can reliably predict, with sufficient sensitivity and specificity, N. gonorrhoeae susceptibility to ciprofloxacin; and since RT-PCR returns results within 24 hours, those rapid assays can enable targeted ciprofloxacin therapy of N. gonorrhoeae infections. Other rapid molecular assays for prediction of antimicrobial susceptibility are currently in clinical use, such as the FDA-approved molecular assay for Mycobacterium tuberculosis, which uses RT-PCR for amplification and molecular probes for mutation within the rifampin-resistance determining region23; or the RT-PCR amplification of known resistance genes in screening for methicillin-resistant Staphylococcus aureus.²⁴

Targeted therapy will have many benefits, among which may be the reduction in the emergence of ceftriaxone, as a previous study demonstrated that treatment may be the major driver of resistance.²⁵ The assay we developed in 2007 was verified in accordance with Clinical Laboratory Improvement Amendments9 and implemented into routine clinical practice for all N. gonorrhoeae positive specimens at UCLA health in November 2015. Further studies are underway to characterize the impact of that implementation.

REFERENCES

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Lao-Tzu Allan-Blitz studies at the David Geffen School of Medicine, University California Los Angeles. He is currently living in Peru researching syphilis and gonococcal resistance among high-risk populations. He received his bachelors degree in interdisciplinary studies from New York University.

Jeffrey D. Klausner, MD, MPH, a member of the MLO Editorial Advisory Board, is a professor of medicine in the Division of Infectious Diseases and the Program in Global Health at the David Geffen School of Medicine, University of California Los Angeles, and at the Fielding School of Public Health, University of California, Los Angeles.