Inducible expression eliminates the fitness cost of vancomycin resistance in enterococci.
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.
Foucault
Marie-Laure
ML
Unité des Agents Antibactériens, Institut Pasteur, 75724 Paris Cedex 15, France.
Depardieu
Florence
F
Courvalin
Patrice
P
Grillot-Courvalin
Catherine
C
D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics*; D004251:DNA Transposable Elements; D013293:Enterococcus faecalis / Q000187:drug effects / Q000235:genetics*; D016984:Enterococcus faecium / Q000187:drug effects / Q000235:genetics*; D051379:Mice; D008809:Mice, Inbred C3H; D014158:Transcription, Genetic; D014640:Vancomycin / Q000494:pharmacology*; D020713:Vancomycin Resistance / Q000235:genetics*
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D004251:DNA Transposable Elements; C079913:VanB protein, Enterococcus; D014640:Vancomycin