目标：
为了确定对大肠杆菌中的硝酸硝唑啉耐药的机理。

方法：
在不同浓度的硝基氧line中选择自发的耐硝基氧line的突变体。 WGS和应变重建被用来定义抗性的遗传基础。抗性的机制基础是通过定量PCR（qPCR）和靶基因的过表达来确定的。还确定了耐药性突变和与其他抗生素的交叉耐药性的适应性成本。

结果：
转录阻遏物emrR的突变通过增加EmrAB-TolC外排泵的emrA和emrB基因的表达，赋予了对硝唑啉的低水平抗性[nitroxoline MIC（MICNOX）=16μmg/ L]。这些抗性突变体没有显示出适应性降低，并且显示出对萘啶酸的交叉抗性。具有较高抗性的第二步突变体（MICNOX = 32-64 mg / L）在emrR基因中具有突变，同时具有50 kb的扩增，marA基因突变或lon基因上游的IS。后者的突变由于tolC基因表达的增加而导致较高水平的硝氧嘧啶抗性，这可通过在低水平抗性突变体中从可诱导质粒中过表达tolC来证实。此外，仅当与nfsAB双敲除突变结合使用时，emrR突变才赋予对呋喃妥因抗药性的小幅提高。但是，耐呋喃妥因的nfsAB突变体对硝氧嘧啶没有交叉抗性。

结论：
导致EmrAB-TolC泵表达增加的不同基因中的突变导致对硝唑啉的抗性增加。结构相似的抗生素硝氧氮和呋喃妥因似乎具有不同的作用方式和耐药机制。

OBJECTIVES:
To determine the mechanism of resistance to the antibiotic nitroxoline in Escherichia coli.

METHODS:
Spontaneous nitroxoline-resistant mutants were selected at different concentrations of nitroxoline. WGS and strain reconstruction were used to define the genetic basis for the resistance. The mechanistic basis of resistance was determined by quantitative PCR (qPCR) and by overexpression of target genes. Fitness costs of the resistance mutations and cross-resistance to other antibiotics were also determined.

RESULTS:
Mutations in the transcriptional repressor emrR conferred low-level resistance to nitroxoline [nitroxoline MIC (MICNOX) = 16 mg/L] by increasing the expression of the emrA and emrB genes of the EmrAB-TolC efflux pump. These resistant mutants showed no fitness reduction and displayed cross-resistance to nalidixic acid. Second-step mutants with higher-level resistance (MICNOX = 32-64 mg/L) had mutations in the emrR gene, together with either a 50 kb amplification, a mutation in the gene marA, or an IS upstream of the lon gene. The latter mutations resulted in higher-level nitroxoline resistance due to increased expression of the tolC gene, which was confirmed by overexpressing tolC from an inducible plasmid in a low-level resistance mutant. Furthermore, the emrR mutations conferred a small increase in resistance to nitrofurantoin only when combined with an nfsAB double-knockout mutation. However, nitrofurantoin-resistant nfsAB mutants showed no cross-resistance to nitroxoline.

CONCLUSIONS:
Mutations in different genes causing increased expression of the EmrAB-TolC pump lead to an increased resistance to nitroxoline. The structurally similar antibiotics nitroxoline and nitrofurantoin appear to have different modes of action and resistance mechanisms.

https://www.ncbi.nlm.nih.gov/pubmed/31633764