Masecar B L, Robillard N J
Pharmaceutical Division, Miles Inc., West Haven, Connecticut 06516.
Antimicrob Agents Chemother. 1991 May;35(5):898-902. doi: 10.1128/AAC.35.5.898.
Spontaneous quinolone-resistant mutants of MP050, a quinolone-susceptible clinical strain of Serratia marcescens, were isolated on nutrient agar containing 0.5 microgram of ciprofloxacin per ml. One mutant, designated MP051, was selected for further study. Quinolone MICs for MP051 were 4- to 16-fold higher than those for MP050; nonquinolone MICs were unchanged. The DNA gyrase isolated from MP051 was 24-fold less sensitive to inhibition of supercoiling by ciprofloxacin than the DNA gyrase isolated from MP050 was. Inhibition studies on reconstituted combinations of heterologous gyrase subunits showed that the decreased inhibition was dependent on the A subunit of DNA gyrase from MP051. Further evidence that this decreased inhibition was due to a gyrA mutation was provided by analysis of Escherichia coli gyrA gene expression in S. marcescens heterodiploids containing pNJR3-2, a broad-host-range gyrA gene probe. Quinolone susceptibilities of MP051 heterodiploids containing the wild-type E. coli gyrA gene decreased to those of MP050, while quinolone susceptibilities of MP050 containing the same plasmid were unchanged. These results indicate that spontaneous quinolone resistance in MP051 was due to a mutation in gyrA.
从一株对喹诺酮敏感的粘质沙雷氏菌临床菌株MP050中，在每毫升含0.5微克环丙沙星的营养琼脂上分离出了自发的喹诺酮抗性突变体。选择了一个名为MP051的突变体进行进一步研究。MP051的喹诺酮最低抑菌浓度（MIC）比MP050高4至16倍；非喹诺酮类药物的MIC没有变化。从MP051中分离出的DNA促旋酶对环丙沙星抑制超螺旋的敏感性比从MP050中分离出的DNA促旋酶低24倍。对异源促旋酶亚基重组组合的抑制研究表明，抑制作用的降低取决于MP051的DNA促旋酶A亚基。通过分析含有广泛宿主范围的gyrA基因探针pNJR3-2的粘质沙雷氏菌异源二倍体中大肠杆菌gyrA基因的表达，进一步证明了这种抑制作用的降低是由于gyrA突变所致。含有野生型大肠杆菌gyrA基因的MP051异源二倍体的喹诺酮敏感性降至MP050的水平，而含有相同质粒的MP050的喹诺酮敏感性没有变化。这些结果表明，MP051中的自发喹诺酮抗性是由于gyrA突变引起的。
