K. Maneewannakul et Sb. Levy, IDENTIFICATION OF MAR MUTANTS AMONG QUINOLONE-RESISTANT CLINICAL ISOLATES OF ESCHERICHIA-COLI, Antimicrobial agents and chemotherapy, 40(7), 1996, pp. 1695-1698
Quinolone-resistant clinical Escherichia coil isolates were examined f
or mutations in the marRAB operon of the multiple antibiotic resistanc
e (mar.) locus. Among 23 strains evaluated, 8 were chosen for further
study: 3 that showed relatively high levels of uninduced, i.e., consti
tutive, expression of the operon and 5 with variable responses to indu
ction by salicylate or tetracyclines. The marR genes, specifying the r
epressor of the operon, cloned from the three strains constitutively e
xpressing the operon did not reduce the level of expression of beta-ga
lactosidase from a marO::lacZ transcriptional fusion and were therefor
e mutant; however, marR genes cloned from the five other clinical stra
ins repressed LacZ expression and were wild type. All three mutant mar
R genes contained more than one mutation: a deletion and a point mutat
ion. Inactivation of the mar locus in the three known marR mutant stra
ins with a kanamycin resistance cassette introduced by homologous reco
mbination reduced resistance to quinolones and multiple antibiotics. T
hese findings indicate that mar operon mutations exist in quinolone-re
sistant clinical E. coli isolates and contribute to quinolone and mult
idrug resistance.