Clonal expansion within clonal complex 2 and spread of vancomycin-resistant plasmids among different genetic lineages of Enterococcus faecalis from Portugal

Abstract

Objectives: The aim of this study was to assess the diversity of Enterococcus faecalis populations recovered in different regions of Portugal during the last decade (1996-2007) and to analyse their genetic elements associated with vancomycin resistance.

Methods: Forty E. faecalis isolates (22 vancomycin-susceptible and 18 vancomycin-resistant) representing disseminated and/or multiresistant strains from different sources (humans, animals and the environment) were characterized by PFGE and multilocus sequence typing. Genes encoding putative virulence markers and the backbone of Tn1546 were investigated by PCR. Plasmid analysis included determination of size, content (S1 hybridization) and comparison of restriction fragment length polymorphism patterns.

Results: The 40 E. faecalis isolates (22 PFGE types) mostly clustered within the worldwide-spread clonal complexes (CCs) CC2 (13 ST6 mostly corresponding to an epidemic strain, where ST stands for sequence type), CC21 (3 ST21, 1 ST22 and 1 ST224) and ST16 (n = 7), but also comprised ST159, ST35, ST19, ST26, ST30, ST41, ST55, ST59, ST117, ST160 and ST200. CC2 and CC21 were isolated from both hospital and community settings. Similar Tn1546-like elements encoding VanA were found on related plasmids within strains belonging to different clonal lineages and recovered in distinct hospitals over several years.

Conclusions: The predominance of E. faecalis CC2 is mainly due to the dissemination of a particular clone persistently recovered for 11 years. The presence in the community of specific strains belonging to major clonal lineages highlights the role of community-associated hosts as possible reservoirs of putative human pathogenic enterococci. Both clonal expansion and dissemination of epidemic conjugative VanA plasmids seem to join forces in the establishment of pathogenic E. faecalis strains.