Characterization of qnr-plasmids from M. morganii isolates of various sources

Background: Fluoro-/Quinolones are important antimicrobial substances for treatment of animal and human infections. The most common cause for quinolone resistances are point mutations in the coding sequences of the bacterial DNA gyrase and topoisomerase gene. However, several plasmid-mediated resistance factors, i.e. a modified aminoglycoside acetyl transferase gene, a quinolone efflux pump, a multidrug resistance efflux pump and pentapeptide repeat protein-encoding genes were reported to be involved in the occurrence of quinolone resistant bacteria. In this study, qnr-carrying plasmids from various Morganella morganii isolates were characterized to determine their genetic background and their impact on the development and distribution of the resistance. Material/methods: Morganella morganii isolates of various sources were characterized according to their susceptibility to fluoroquinolones, nalidixic acid and other antimicrobial substances by antimicrobial susceptibility testing. Molecular analysis of Fluoro-/Quinolone resistant isolates indicated the presence of different qnr-plasmids. By genome sequencing the genetic background of the plasmids was determined and further bioinformatically analyzed. Furthermore, for some of the plasmids stability experiments and transfer studies were performed with different bacteria of the Enterobacterales. Results: Plasmid profiling revealed the presence of a previously described predominant 2.7 kb qnrD-plasmid type in three and a novel 1.9 kb plasmid type in one of the investigated isolates. Sequence analysis showed, that the smaller plasmid encodes a qnrD2 gene variant. However, all analyzed qnrD/qnrD2-plasmids are genetically closely related. Stability examinations revealed that the replication stability of the qnrD2-plasmid seems to be limited over long-time periods under nonselective conditions among various genera of the Enterobacteriaceae (i.e. E. coli, S. Typhimurium). Up to now, the detected plasmids were classified as non-transmissible via conjugation or mobilization. Conclusions: This study disclosed the existence of a predominant qnrD-prototype plasmid in M. morganii isolates. However, up to now their impact for the distribution of Fluoro-/Quinolone resistances is unknown