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

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 <i>Enterobacteriaceae</i>. Since ATM activity was shown to be severely impacted by an increase in the inoculum size <i>in vitro</i>, we wondered whether ATM-AVI activity could be impaired in high-inoculum infections. We analyzed the impact of the inoculum size on ATM-AVI activity <i>in vitro</i> and in a murine model of peritonitis due to susceptible <i>Escherichia coli</i> CFT073-pTOPO and its isogenic derivatives producing NDM-1 (<i>E. coli</i> CFT073-NDM1) and CTX-M-15 plus NDM-1 (<i>E. coli</i> CFT073-CTXM15-NDM1). The impact of the inoculum size on bacterial morphology was studied by microscopic examination. <i>In vitro</i>, at standard (10⁵) inoculum, <i>E. coli</i> 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 <i>E. coli</i> CFT073-pTOPO and only in combination with AVI against <i>E. coli</i> CFT073-CTXM15-NDM1. <i>In vivo</i>, increase in the inoculum led to a drastic decrease in the activity of ATM alone against <i>E. coli</i> CFT073-pTOPO and ATM-AVI against <i>E. coli</i> CFT073-CTXM15-NDM1. Our results suggest a high <i>in vivo</i> impact of the inoculum increase on the activity of ATM alone against ATM-susceptible <i>E. coli</i> and of ATM-AVI against CTX-M-15 plus NDM-1 producing <i>E. coli</i>. Clinicians must be aware of the risk of failures when using ATM-AVI in high-inoculum infections.