Open AccessPersistent, Toxin-Antitoxin System-Independent, Tetracycline Resistance-Encoding Plasmid from a Dairy Enterococcus faecium Isolate
Author(s) -
Xinhui Li,
Valente B. Álvarez,
W. J. Harper,
Hua H. Wang
Publication year - 2011
Publication title -
applied and environmental microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.552
H-Index - 324
eISSN - 1070-6291
pISSN - 0099-2240
DOI - 10.1128/aem.05168-11
Subject(s) - plasmid , biology , tetracycline , enterococcus faecalis , microbiology and biotechnology , replicon , enterococcus faecium , gene , genetics , escherichia coli , antibiotics
A tetracycline-resistant (Tetr ) dairyEnterococcus faecium isolate designated M7M2 was found to carry bothtet (M) andtet (L) genes on a 19.6-kb plasmid. After consecutive transfer in the absence of tetracycline, the resistance-encoding plasmid persisted in 99% of the progenies. DNA sequence analysis revealed that the 19.6-kb plasmid contained 28 open reading frames (ORFs), including atet (M)-tet (L)-mob gene cluster, as well as a 10.6-kb backbone highly homologous (99.9%) to the reported plasmid pRE25, but without an identified toxin-antitoxin (TA) plasmid stabilization system. The derived backbone plasmid without the Tetr determinants exhibited a 100% retention rate in the presence of acridine orange, suggesting the presence of a TA-independent plasmid stabilization mechanism, with its impact on the persistence of a broad spectrum of resistance-encoding traits still to be elucidated. Thetet (M)-tet (L) gene cluster from M7M2 was functional and transmissible and led to acquired resistance inEnterococcus faecalis OG1RF by electroporation and inStreptococcus mutans UA159 by natural transformation. Southern hybridization showed that both thetet (M) andtet (L) genes were integrated into the chromosome ofS. mutans UA159, while the whole plasmid was transferred to and retained inE. faecalis OG1RF. Quantitative real-time reverse transcription-PCR (RT-PCR) indicated tetracycline-induced transcription of both thetet (M) andtet (L) genes of pM7M2. The results indicated that multiple mechanisms might have contributed to the persistence of antibiotic resistance-encoding genes and that the plasmids pM7M2, pIP816, and pRE25 are likely correlated evolutionarily.