P11 Evaluation of in vitro activity of double β-lactam therapy and relationship with PBP activity in Escherichia coli isolates

BACKGROUND: PBPs are involved in the construction of peptidoglycan, which is the major constituent of bacterial cell walls and the target of β-lactam antibiotics. There is little published research analysing the relationship between β-lactams with differing bacterial PBP targets and how they can be manipulated in combinations with respect to clinical or microbiological (e.g. resistance) outcomes (i.e. does expanded PBP activity via a combination lead to better in vitro/in vivo outcomes). OBJECTIVES: To systematically explore the relationship between double β-lactam therapy (with and without at least one partner being a β-lactamase inhibitor antibiotic such as co-amoxiclav) and in vitro activity against susceptible Escherichia coli strains. METHODS: We systematically explored the relationship between double β-lactam therapy combinations against seven E. coli strains of variable resistance in vitro. This included fully susceptible isolates, ESBL producers and carbapenemase producers (CPEs). For each of 10 antibiotics, the MIC was determined individually, and subsequently in combination with 9 further antibiotics, using the MTS(™) ‘cross’ synergy method (Liofilchem, 2012). RESULTS: Overall, 86/630 (13.6%) of all combinations tested showed synergy and 408/630 (64.8%) were additive; 136/630 (21.6%) combinations showed indifference. Of the 86 ‘bug–drug’ combinations that showed synergy, 42/86 (49%) included ceftazidime/avibactam, representing 42/126 (33%) of all ceftazidime/avibactam-based combinations tested, and 56/86 (65%) of synergistic combinations covered PBP2. Synergy was most commonly detected in ESBL producers (58/86; 67% of combinations) and less frequently seen in CPEs (2/86; 2% of combinations) and fully susceptible isolates (8/86; 9% of combinations). Additive effects were seen in 92/180 (51%) combinations versus ESBLs, compared with 18/90 (20%) in CPEs, versus 154/180 (86%) in fully susceptible isolates. No antagonism was identified with any antibiotic combination. CONCLUSIONS: In the combinations tested, synergy or additive effects were common (78%); similar to our previous work with fosfomycin/β-lactam combinations (89%), but higher than we found with fosfomycin/non-β-lactam combinations (28%). Many of the synergistic bug–drug combinations identified contained a β-lactam inhibitor as a partner and/or provided PBP2 activity. This provisionally suggests a role for PBP2 (also targeted by avibactam) in synergy, although the presence of a β-lactamase inhibitor may also be important. Confirmation using an alternative method and mechanistic elucidation is required. The clinical/microbiological importance of such effects remains unclear.