P1763 Should we use to predict enterococci susceptibility to piperacillin? Anne-Gaëlle Leroy*1, Antoine Rambaud1, Lise Crémet1, Jocelyne Caillon1

1 University Hospital of Nantes, Nantes, France Background: Enterococci are frequently involved in , endocarditis and urinary tract infections. The management of these infections is sometimes complicated because Enterococci are intrinsically resistant to some (such as , and ). Enterococci indeed produce a PBP5 showing a poor affinity for ß-lactams leading to natural resistance to cephalosporins. Ampicillin stays effective against wild strains but near 70% of E. faecium have however acquired resistance to ampicillin by producing an increased amount of the low-affinity PBP5 or producing PBP5 that has further decreased affinity to ampicillin compared to susceptible strains. According to EUCAST 2018, Enterococci susceptibility to piperacillin should be deducted from their susceptibility to ampicillin. do not belong to the molecules traditionally tested and their activities are also usually deducted from ampicillin. The aim of this work was to evaluate the in vitro activities of piperacillin and carbapenems against ampicillin susceptible Enterococci and to confirm that ampicillin can be used to predict the activity of piperacillin and carbapenems. Materials/methods: 20 strains of E. faecalis and 33 strains of E. faecium susceptible to ampicillin and isolated in the bacteriology laboratory from January 2016 to December 2017 were studied. The MICs of piperacillin, and were assessed for all these strains, using E-test strips according to CASFM/EUCAST 2018 recommendations. Results: The 20 strains of E. faecalis were susceptible to the three molecules tested. However, the MICs of meropenem were higher than the other molecules: 84% of the strains were classified as intermediate to this molecule. As far as E. faecium is concerned: 51% of the strains were intermediate to piperacillin and 27% resistant, 22% were intermediate to imipenem dans 32% resistant, 15% were intermediate to meropenem dans 73% resistant. Conclusions: In the light of these results, it seems important to be careful as far as ampicillin susceptibility is used to predict the activity of piperacillin and carbapenems. MICs of piperacillin and carbapenems should be assessed before considering their use in E. faecium infections. Underlying mechanisms of resistance still need to be explored.

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