615. Daptomycin (DAP) Synergy with β-Lactams in DAP-Resistant (DAP-R) E. faecium (Efm) Is Dependent On PBP5 Sequence and β-Lactam-binding Affinity

BACKGROUND: DAP in combination with β-lactams is a viable option to treat recalcitrant DAP-R/tolerant strains of Efm. Ampicillin (AMP), ceftaroline (CPT), and ertapenem (ERT) have the best synergism. Using a DAP tolerant strain (503; DAP MIC 2 µg/mL) of Efm, we previously showed that AMP, CPT, and ERT combined with DAP were effective in reducing bacterial loads and prevented emergence of resistance in a simulated endocardial vegetation model. However, against a DAP-R Efm strain (R497, DAP MIC of 16 µg/mL), CPT, ERT failed to synergize with DAP. Here, we dissect the mechanistic basis of the differing DAP plus β-lactam synergistic effect. METHODS: We performed comparative transcriptional profiling of pbp genes in Efm 503 vs. R497 using qRT–PCR. PBP5 protein levels were assessed by immunoblotting. The β-lactam-binding affinity of PBPs was quantified with bocillin-FL staining and SDS–PAGE. PBP5 sequences of Efm Com15 (AMP and DAP-susceptible strain) and clinical strains S447, 503 and R497 (all with AMP MIC > 256 µg/mL) were compared in silico to identify amino acid (AA) differences in key protein sites which were verified with sequencing RESULTS: Pbp gene transcripts and PBP5 amounts were similar between 503 vs. R497. Interestingly, bocillin SDS–PAGE showed increased β-lactam binding affinity in PBP5 of 503 compared with that of R497 and S447. PBP5 sequences of S447 and R497 were identical. All three clinical strains had classic mutations (M485A and 466’S) important for high-level AMP-R. However, 503 had additional substitutions in the transpeptidase domain (H408Q, A462V, T546N, T558A, S582G, V586L) and penicillin-binding domain (Q632K, L642P) compared with R497 and S447. The latter AA sequences in 503 are common to AMP-susceptible Efm strains CONCLUSION: We uncovered that a “hybrid” pbp5 allele of 503 (DAP-tolerant) correlated with synergism of DAP plus AMP, CPT or ERT and was associated with increased PBP5 β-lactam binding affinity. Lack of synergism of DAP plus CPT or ERT is associated with specific PBP amino acids in the transpeptidase and penicillin-binding domains. Thus, pbp5 alleles are major determinants of the DAP plus β-lactam synergistic effect and could be used as a diagnostic tool to guide therapy in recalcitrant Efm infections DISCLOSURES: All authors: No reported disclosures.