Temocillin efficacy and emergence of resistance in Enterobacter cloacae Complex in a murine peritonitis model

Background Temocillin (TEM) is a seducing alternative to treat multidrug-resistant Enterobacterales thanks to a narrow spectrum, resistance to hydrolysis by ESBLs and some carbapenemases and limited impact on the gut microbiota. However, its efficacy on Enterobacter cloacae complex (ECC) is unclear. We evaluated TEM efficacy against ECC, in vitro and in a high-inoculum murine peritonitis model. Methods Five TEM-susceptible E. xiangfangensis UTI strains and an isogenic pair of E. asburiae clinical isolates—E. asburiae TEM_S and E. asburiae TEM_R—were studied. MICs, bactericidal kinetics and emergence of resistance were assessed in vitro and in a murine peritonitis model using cefepime (FEP) as reference. Both in vitro and in vivo resistant mutants were sequenced. Results Among the six TEM-S isolates tested, resistant mutants emerged in vitro in all but one after 24 h exposure to 2× MIC TEM. In vivo, when infected with E. asburiae (both TEM-S and TEM-R), mice mortality was significantly higher when treated with TEM compared with FEP, and resistant mutants emerged in 5/12 mice infected with E. asburiae TEM_S (0/12 with FEP). When infected with TEM_S E. xiangfangensis isolates, no significant difference was evidenced between mice treated with TEM or with FEP. Conclusions Rapid and frequent emergence of resistant mutants to TEM was evidenced in vitro for distinct TEM-susceptible ECC strains. This led to TEM inefficacy in a high-inoculum peritonitis model with TEM_S E. asburiae. TEM should be used with caution for the treatment of severe ECC infections, even against susceptible strains.