Analysis of the tolerance of pathogenic enterococci and staphylococcus aureus to cell wall active antibiotics

Abstract

Objectives: Tolerance refers to thephenomenonthat bacteria do not significantly diewhenexposed to bactericidal antibiotics. Enterococci areknownfor their high tolerance to these drugs, but the molecular reasons why they resist killing are not understood. In a previous study we showed that the superoxide dismutase (SOD) is implicated in this tolerance. This conclusion was based on the results obtained with one particular strain of Enterococcus faecalis and therefore the objective of the present communicationwas to analyse whether dependence of tolerance on active SOD is a general phenomenon for enterococci and another Gram-positive pathogen, Staphylococcus aureus. Methods: Mutants deficient in SOD activity were constructed in pathogenic enterococci. The wild-type sodA gene wascloned intoanexpression vectorandtransformed into SOD-deficient strains forcomplementation with varying levels of SOD activity. Previously constructed SOD-deficient strains of S. aureus were also included in this study. Tolerance to vancomycin and penicillin was then tested. Results: We demonstrated that the dependence on SOD of tolerance to vancomycin and penicillin is a common trait of antibiotic-susceptible pathogenic enterococci. By varying the levels of expression we could also show that tolerance to vancomycin is directly correlated to SOD activity. Interestingly, deletion of the sodA gene in a nontolerant Enterococcus faecium strain did not further sensitize the mutant to bactericidal antibiotics. Finally, we showed that the SOD enzymes of S. aureus are also implicated in tolerance to vancomycin. Conclusion: High tolerance of enterococci to cell wall active antibiotics can be reversed by SOD deficiency. © The Author 2013. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved.