Morganella morganii SE10MM harboring quinolone resistance determinant qnrD was investigated in our study. An entirely sequenced novel 2,662 bp qnrD-plasmid pSE10MM was identified and deposited at GenBank under accession number KU160530. Nucleic acid sequence of pSE10MM showed 94-97% similarity to previously detected qnrD-plasmids of Proteus mirabilis strains. Phylogenetic analysis by Geneious 9.0.5 showed clusters of plasmids with possible common origin. Initial expression of qnrD gene was found 12.5 normalized to rpoB housekeeping gene. Subsequently, a sub-minimum inhibitory concentration (1 mg/L) ciprofloxacin exposure resulted in a fold change of 30.06 at 24 hours. In contrast, qnrD-plasmid pSE10MM copy number increased in time from 1.1 to 6.63. Chromosomal mutations of gyrA with S83I, gyrB with S463A, and parC with S80I amino acid substitutions were detected, but no other mutations have occurred as a consequence of ciprofloxacin exposure. Elevated expression of qnrD correlated with that of recA in M. morganii during ciprofloxacin exposure, which indicates SOS-dependent regulation of qnrD. Protective effect of QnrD plays a role in fluoroquinolone-resistant strain even in the presence of chromosomal mutations in gyrase and topoisomerase IV.
Keywords: ciprofloxacin exposure; plasmid; quinolone resistance.