The Enterococcus: a Model of Adaptability to Its Environment

Abstract

The genus Enterococcus comprises a ubiquitous group of Gram-positive bacteria that are of great relevance to human health for their role as major causative agents of health care-associated infections. The enterococci are resilient and versatile species able to survive under harsh conditions, making them well adapted to the health care environment. Two species cause the majority of enterococcal infections: Enterococcus faecalis and Enterococcus faecium Both species demonstrate intrinsic resistance to common antibiotics, such as virtually all cephalosporins, aminoglycosides, clindamycin, and trimethoprim-sulfamethoxazole. Additionally, a remarkably plastic genome allows these two species to readily acquire resistance to further antibiotics, such as high-level aminoglycoside resistance, high-level ampicillin resistance, and vancomycin resistance, either through mutation or by horizontal transfer of genetic elements conferring resistance determinants.

Keywords: Enterococcus; antibiotic resistance; horizontal gene transfer.

FIG 1

Timeline of relevant events in the history of enterococci as human pathogens (blue rectangles), appearance of antibiotic resistance (green rectangles), and antibiotic clinical debut (red rectangles). The timeline begins in 1899 with the first formal description of putative enterococci, as round enteric bacteria. The timeline then jumps to 1964 to the first description of the transfer of chloramphenicol resistance, only 15 years after its clinical introduction. Similar stories occurred for aminoglycosides and glycopeptides. Since the late 1980s, the prevalence of vancomycin-resistant (VR) E. faecium has been increasing, as has the overall percentage of enterococcal HAIs. Resistance to the newest introduced antibiotics, linezolid and daptomycin, emerged very rapidly after their clinical introduction, but the majority of enterococci remain susceptible. MDR, multidrug resistance.

FIG 2

Depictions of known glycopeptide resistance operons. (A) The four glycopeptide resistance operons that yield peptidoglycan precursors terminating in d-Ala-d-Lac. Arrows reflect the directions of transcription and relative sizes of the open reading frames. (B) The five glycopeptide resistance operons that yield peptidoglycan precursors terminating in d-Ala-d-Ser. See the text for descriptions of the open reading frame roles.

FIG 3

Transposable elements in enterococci. The cartoon depicts the three kinds of transposable elements found in enterococci: conjugative transposons, such as Tn5382 carrying the vanB2 operon; insertion sequence elements (IS) in the Tn3 family, such as Tn1546 carrying vanA resistance; and composite transposons, such as Tn4001, for high-level aminoglycoside resistance. These three types of transposable elements can exist either in the chromosome or in plasmids as part of larger mobilizable elements.