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Background: Conflicting evidence has been reported on the impact of ertapenem use on the susceptibility of Pseudomonas spp. to group 2 carbapenems. No extensive data for Acinetobacter baumannii are currently available., Methods: A retrospective time-series segmented regression analysis was conducted in a tertiary centre from January 2001 to December 2011. Ertapenem was introduced in January 2005. Antimicrobial drug use was defined as the number of defined daily doses/100 patient-days (DDDs/100 PDs). Susceptibility (CLSI) was measured in terms of proportion and incidence density., Results: Mean monthly use of imipenem was 2.9 ± 0.9 DDDs/100 PDs, as compared with 1.2 ± 0.7 DDDs/100 PDs for meropenem and 1.0 ± 0.7 DDDs/100 PDs for ertapenem (after its introduction). After ertapenem adoption, a downward trend was seen in the use of imipenem (P = 0.016) and ciprofloxacin (P = 0.004). A total of 6272 Pseudomonas aeruginosa and 1093 A. baumannii isolates were evaluated. Susceptibility of P. aeruginosa to imipenem improved after ertapenem introduction, both according to the proportion of susceptible isolates (P = 0.002) and to the incidence density of resistance (P ≤ 0.001). No significant change was seen in A. baumannii susceptibility to imipenem (P = 0.772). By multiple linear regression analysis, the incidence density of imipenem-resistant P. aeruginosa increased with the use of imipenem (P = 0.003) and ciprofloxacin (P = 0.008). Occurrence of outbreaks (P ≤ 0.001) and use of gentamicin (P = 0.007) were associated with A. baumannii resistance to imipenem., Conclusions: Use of ertapenem was directly associated with a downward trend in the use of imipenem and ciprofloxacin, which may have contributed to improve the susceptibility of P. aeruginosa to imipenem. Ertapenem use had no impact on the susceptibility of A. baumannii to imipenem.