Zheng Jie, Cui Shenghui, Meng Jianghong
Department of Nutrition and Food Science, University of Maryland, College Park, MD 20742, USA.
J Antimicrob Chemother. 2009 Jan;63(1):95-102. doi: 10.1093/jac/dkn448. Epub 2008 Nov 4.
Multidrug resistance (MDR) including fluoroquinolone resistance in Salmonella Typhimurium can result from overexpression of efflux pumps. We examined the mechanisms of fluoroquinolone resistance among in vitro-induced ciprofloxacin-resistant Salmonella Typhimurium mutants, LTL and LTH, derived from laboratory strain LT2.
Deletion mutation and RT-PCR techniques were employed to study the role of efflux pumps in fluoroquinolone resistance and their regulation cascades.
In addition to point mutations in DNA gyrase (gyrA, gyrB) and topoisomerase IV (parC, parE) genes, increased expression of efflux pump genes, such as acrAB and acrEF, was observed in fluoroquinolone-resistant Salmonella strains. Constitutive expression of ramA containing a 9 bp deletion in the promoter region was directly associated with the overexpression of acrAB and acrEF and conferred an MDR phenotype in LTL. Inactivation of ramA increased the antimicrobial susceptibility of LTL, whereas complementation with the mutant allele induced an MDR phenotype in drug-susceptible Salmonella Typhimurium LT2, as demonstrated by 2- to 64-fold increases in resistance to fluoroquinolones, tetracycline and chloramphenicol. On the other hand, inactivation of mutant soxRS resulted in a slight increase in the susceptibility of LTH to several fluoroquinolone drugs, and the introduction of the mutant allele had no effect on antimicrobial susceptibility of LT2, indicating that constitutive expression of soxRS played a minimum role in fluoroquinolone resistance.
Mutations in the promoter region of ramA appear to play a role in the up-regulation of RamA and AcrAB, and RamA is an activator of the MDR regulation cascade in Salmonella Typhimurium.
鼠伤寒沙门氏菌中的多药耐药性（MDR）包括对氟喹诺酮类药物的耐药性，可能是由外排泵的过表达引起的。我们研究了源自实验室菌株LT2的体外诱导的环丙沙星耐药鼠伤寒沙门氏菌突变体LTL和LTH中氟喹诺酮耐药性的机制。
采用缺失突变和RT-PCR技术研究外排泵在氟喹诺酮耐药性中的作用及其调控级联反应。
除了DNA促旋酶（gyrA、gyrB）和拓扑异构酶IV（parC、parE）基因中的点突变外，在氟喹诺酮耐药沙门氏菌菌株中还观察到外排泵基因（如acrAB和acrEF）的表达增加。启动子区域含有9 bp缺失的ramA的组成型表达与acrAB和acrEF的过表达直接相关，并在LTL中赋予多药耐药表型。ramA的失活增加了LTL的抗菌敏感性，而用突变等位基因互补则在药敏鼠伤寒沙门氏菌LT2中诱导了多药耐药表型，对氟喹诺酮类、四环素和氯霉素的耐药性增加了2至64倍。另一方面，突变型soxRS的失活导致LTH对几种氟喹诺酮类药物的敏感性略有增加，而引入突变等位基因对LT2的抗菌敏感性没有影响，表明soxRS的组成型表达在氟喹诺酮耐药性中起最小作用。
ramA启动子区域的突变似乎在RamA和AcrAB的上调中起作用，并且RamA是鼠伤寒沙门氏菌多药耐药调控级联反应的激活剂。
