Impact of the inoculum size on the in vivo activity of the aztreonam-avibactam combination in a murine model of peritonitis due to Escherichia coli expressing CTX-M-15 and NDM-1

Background: The combination of aztreonam (ATM) and avibactam (AVI) is an attractive option to treat infections caused by extended spectrum 𝛽-lactamase plus NDM-1-producing Enterobacteriaceae. Since ATM activity was shown to be severely impacted by an increase in the inoculum size in vitro, we wondered whether ATM-AVI activity could be impaired in high inoculum infections. Methods: We analyzed the impact of the inoculum size on ATM-AVI activity in vitro and in a murine model of peritonitis due to susceptible E. coli CFT073-pTOPO and its isogenic derivatives producing NDM-1 (E. coli CFT073-NDM1) and CTX-M-15 plus NDM-1 (E. coli CFT073-CTXM15-NDM1). The impact of the inoculum size on bacterial morphology was studied by microscopic examination. Results: In vitro, at standard (10⁵) inoculum, E. coli CFT073-CTXM15-NDM1 was resistant to ATM but susceptible to the ATM-AVI combination. At high (10⁷) inoculum, MICs of ATM alone and of the ATM-AVI combination reached > 512 and 64 mg/L respectively, against all tested strains. ATM led to bacterial filamentation when active against the bacteria, i.e., in monotherapy or in combination with AVI against susceptible E. coli CFT073-pTOPO, and only in combination with AVI against E. coli CFT073-CTXM15-NDM1. In vivo, increase in the inoculum led to a drastic decrease in the activity of ATM alone against E. coli CFT073-pTOPO, and of ATM-AVI against E. coli CFT073-CTXM15-NDM1. Conclusion: Our results suggest a high in vivo impact of the inoculum increase on the activity of ATM alone against ATM-susceptible E. coli, and of ATM-AVI against CTX-M-15 plus NDM-1 producing E. coli. Clinicians must be aware of the risk of failures when using AZT-AVI in high inoculum infections.