Heterogeneity of vat(E)-carrying plasmids in Enterococcus faecium recovered from human and animal sources

Persistent, toxin-antitoxin system-independent, tetracycline resistance-encoding plasmid from a dairy Enterococcus faecium isolate

A tetracycline-resistant (Tet(r)) dairy Enterococcus faecium isolate designated M7M2 was found to carry both tet(M) and tet(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 a tet(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 Tet(r) 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. The tet(M)-tet(L) gene cluster from M7M2 was functional and transmissible and led to acquired resistance in Enterococcus faecalis OG1RF by electroporation and in Streptococcus mutans UA159 by natural transformation. Southern hybridization showed that both the tet(M) and tet(L) genes were integrated into the chromosome of S. mutans UA159, while the whole plasmid was transferred to and retained in E. faecalis OG1RF. Quantitative real-time reverse transcription-PCR (RT-PCR) indicated tetracycline-induced transcription of both the tet(M) and tet(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.

Mobile genetic elements and their contribution to the emergence of antimicrobial resistant Enterococcus faecalis and Enterococcus faecium

Mobile genetic elements (MGEs) including plasmids and transposons are pivotal in the dissemination and persistence of antimicrobial resistance in Enterococcus faecalis and Enterococcus faecium. Enterococcal MGEs have also been shown to be able to transfer resistance determinants to more pathogenic bacteria such as Staphylococcus aureus. Despite their importance, we have a limited knowledge about the prevalence, distribution and genetic content of specific MGEs in enterococcal populations. Molecular epidemiological studies of enterococcal MGEs have been hampered by the lack of standardized molecular typing methods and relevant genome information. This review focuses on recent developments in the detection of MGEs and their contribution to the spread of antimicrobial resistance in clinically relevant enterococci.

Distribution of streptogramin resistance genes and genetic relatedness among quinupristin/dalfopristin-resistant Enterococcus faecium recovered from pigs and chickens in Korea

Fifty-four quinupristin/dalfopristin-resistant Enterococcus faecium (QDREF) isolated from chickens and pigs during 2002-2003 in Korea were screened by PCR for the presence of streptogramin resistance genes vatD, vatE, and vgbA, and macrolide resistance gene ermB. None of the QDREF isolates carried vgbA and vatD genes, while vatE and ermB were detected in 9.2% and 74% of the isolates, respectively. Twenty-six percent (14/54) of the QDREF isolates contained none of the resistance determinants tested. Pulsed-field gel electrophoresis (PFGE) patterns revealed high heterogeneity: 47 different patterns for 54 QDREF evaluated. Identical PFGE types were observed in two pairs of chicken isolates and a pair of pig isolates, respectively, but chicken isolates did not share PFGE pattern with pig isolates, suggesting clonal spread of QDREF strain between the same species of animals but not between different species of animals. This is the first report, to our knowledge, of vatE-positive E. faecium isolates and also the first evidence of clonal spread of QDREF strain between animals in Korea.