Inducible expression eliminates the fitness cost of vancomycin resistance in enterococci

Abstract

Inducible vancomycin resistance in enterococci is due to a sophisticated mechanism that combines synthesis of cell wall peptidoglycan precursors with low affinity for glycopeptides and elimination of the normal target precursors. Although this dual mechanism, which involves seven genes organized in two operons, is predicted to have a high fitness cost, resistant enterococci have disseminated worldwide. We have evaluated the biological cost of VanB-type resistance due to acquisition of conjugative transposon Tn1549 in Enterococcus faecium and Enterococcus faecalis. Because fitness was dependent on the integration site of Tn1549, an isogenic set of E. faecalis was constructed to determine the cost of inducible or constitutive expression of resistance or of carriage of Tn1549. A luciferase gene was inserted in the integrase gene of the transposon to allow differential quantification of the strains in cocultures and in the digestive tract of gnotobiotic mice. Both in vitro and in vivo, carriage of inactivated or inducible Tn1549 had no cost for the host in the absence of induction by vancomycin. In contrast, induced or constitutively resistant strains not only had reduced fitness but were severely impaired in colonization ability and dissemination among mice. These data indicate that tight regulation of resistance expression drastically reduces the biological cost associated with vancomycin resistance in Enterococcus spp. and accounts for the widespread dissemination of these strains. Our findings are in agreement with the observation that regulation of expression is common in horizontally acquired resistance and represents an efficient evolutionary pathway for resistance determinants to become selectively neutral.O ne of the key parameters influencing emergence and stability of antibiotic resistance is the biological cost that resistance determinants impose on cell growth (1). Many studies have shown that resistant bacteria are less fit than their susceptible counterpart, although they can acquire compensatory mutations that restore fitness (2). However, most reports have dealt with resistance due to chromosomal mutations, and fewer have evaluated the fitness change due to acquisition of mobile genetic elements conferring clinically relevant resistance (3).Enterococci are commensals of the intestine of humans and animals but can also be opportunistic pathogens. This bacterial genus has acquired resistance to many antibiotics, and glycopeptides are often the ultimate treatment of infections due to these The vanA operon is part of transposon Tn1546, which is often carried by self-transferable plasmids, accounting for its spread (9). Dissemination of VanB-type resistance results from the spread of conjugative transposon Tn1549 (10) located on plasmids or in the chromosome. Despite the sophisticated dual mechanism of resistance, glycopeptide-resistant enterococci have disseminated worldwide.Two studies have investigated the burden of glycopeptide resistance imposed on enterococci, with conflicting results. A fitness di...

In Vivo Studies Suggest that Induction of VanS-Dependent Vancomycin Resistance Requires Binding of the Drug to d -Ala- d -Ala Termini in the Peptidoglycan Cell Wall

“…As a consequence, PG synthesis never takes place entirely via D-Ala-D-Lac-containing precursors, a possibility that has evidently been selected against during the process of evolution. Indeed, even the predominant production of D-Ala-D-Lac-containing cell wall precursors through constitutive expression of vanHAX comes at a severe fitness cost in bacteria, and this is believed to be why the majority of vancomycin resistance systems are inducible systems (42,43). D-Lac is synthesized directly from the key glycolytic intermediate pyruvate, and production of the D-Ala-D-Lac depsipeptide is therefore likely to be energetically significantly more costly than D-Ala-D-Ala synthesis that is derived via isomerization of L-Ala (44), which can be salvaged from proteolytic pathways.…”

Current Trends of Emergence and Spread of Vancomycin-Resistant Enterococci

“…Expression of vancomycin resistance comes along with a fitness burden under nonselective conditions (in the absence of glycopeptides). Accordingly VRE with a constitutive phenotype are less competitive under non-selective conditions (Foucault et al, 2010).…”

Construction and Application of a luxABCDE Reporter System for Real-Time Monitoring of Enterococcus faecalis Gene Expression and Growth

“…Green fluorescent protein (GFP)-based fluorescence has been used for assessing intraand interspecies plasmid mobilization in E. faecalis (2,22) and to determine the roles of extracellular enterococcal proteases during biofilm development (53). On the other hand, Foucault and coworkers (14) have employed for the first time the luciferase gene lucR inserted in the integrase gene of the transposon Tn1549 to bioluminescently tag E. faecalis and to quantify the enterococcal intestinal colonization of gnotobiotic mice by light emission levels. However, a lucR-encoded firefly luciferase variant emits red light at 612 nm after exogenous addition of D-luciferin (4), thus requiring laborious sample preparation procedures.…”