Gyrase Mutations Are Associated with Variable Levels of Fluoroquinolone Resistance in Mycobacterium tuberculosis

Abstract

g Molecular diagnostics that rapidly and accurately predict resistance to fluoroquinolone drugs and especially later-generation agents promise to improve treatment outcomes for patients with multidrug-resistant tuberculosis and prevent the spread of disease. Mutations in the gyr genes are known to confer most fluoroquinolone resistance, but knowledge about the effects of gyr mutations on susceptibility to early-versus later-generation fluoroquinolones and about the role of mutation-mutation interactions is limited. Here, we sequenced the full gyrA and gyrB open reading frames in 240 multidrug-resistant and extensively drugresistant tuberculosis strains and quantified their ofloxacin and moxifloxacin MIC by testing growth at six concentrations for each drug. We constructed a multivariate regression model to assess both the individual mutation effects and interactions on the drug MICs. We found that gyrB mutations contribute to fluoroquinolone resistance both individually and through interactions with gyrA mutations. These effects were statistically significant. In these clinical isolates, several gyrA and gyrB mutations conferred different levels of resistance to ofloxacin and moxifloxacin. Consideration of gyr mutation combinations during the interpretation of molecular test results may improve the accuracy of predicting the fluoroquinolone resistance phenotype. Further, the differential effects of gyr mutations on the activity of early-and later-generation fluoroquinolones requires further investigation and could inform the selection of a fluoroquinolone for treatment. Global surveillance for drug-resistant tuberculosis (TB) suggests that at least 3.5% of the 9 million incident TB cases are multidrug resistant (MDR), i.e., resistant to isoniazid and rifampin. Fluoroquinolones are among the most effective drugs available for the treatment of MDR TB (1, 2). The importance of the fluoroquinolone drug class is implied in how extensively drugresistant (XDR) TB is defined, i.e., as MDR TB with additional in vitro resistance to second-line injectables and any member of the fluoroquinolone drug class. The different fluoroquinolone agents vary in their potency against Mycobacterium tuberculosis, with third-and fourth-generation agents (e.g., levofloxacin, gatifloxacin, and moxifloxacin) having higher activity than second-generation agents (e.g., ciprofloxacin or ofloxacin) (3, 4). These differences have led the World Health Organization (WHO) to recommend against extrapolating in vitro culture-based resistance results from second-to later-generation fluoroquinolones and to revise the in vitro testing drug concentration upward to 2 mg/liter for moxifloxacin (4, 5). Revisions of drug testing concentrations also highlight the need for quantitative resistance testing with MIC measurements when new diagnostic technologies are being compared to culture-based drug susceptibility tests.Fluoroquinolones exert their antibacterial activity by inhibiting DNA gyrase and topoisomerase IV, limiting the cell's capacity for DNA replicat...

Genome wide association with quantitative resistance phenotypes in Mycobacterium tuberculosis reveals novel resistance genes and regulatory regions

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Pharmacokinetics of Levofloxacin in Multidrug- and Extensively Drug-Resistant Tuberculosis Patients

“…Hopefully, WGS will relate all resistance mutations to phenotypic susceptibility, enabling rapid dose selection (32). According to a TBNET consensus statement, adoption of a standard WHO MDR-TB treatment regimen according to the local drug resistance pattern is the first step to a more individualized TB treatment in MDR-TB high-prevalence countries, especially when the prevalence of FQ resistance is high.…”

Drug resistance mechanisms and drug susceptibility testing for tuberculosis

“…In the presence of strains bearing low-level resistance mutations, the use of higher doses of the drugs is associated with a positive outcome. [104][105][106][107][108][109][110][111] Low-level resistance should be identified by testing at higher concentrations than CC.…”