The emergent issue of carbapenem-resistant Acinetobacter baumannii (A. baumannii) and Pseudomonas aeruginosa (P. aeruginosa) is a major problem in Thailand. The wide use of carbapenems can increase selective pressure of bacterial resistance. The objective of this study was to determine the relationship between carbapenem consumption and the susceptibility rates of A. baumannii and P. aeruginosa, including multi-drug resistance (MDR) strains. This was a retrospective study. Carbapenem consumption and susceptibility profiles were collected from 2007 to 2013 at the Her Royal Highness Princess Maha Chakri Sirindhorn Medical Center, Thailand. We found that the susceptibility rate of A. baumannii to imipenem and meropenem from the sputum and the bronchoalveolar lavage (BAL) specimens was significantly decreased during the study period, but no significant change was found in the P. aeruginosa data. The relationship between carbapenem consumption and the susceptibility rate of A. baumannii had a clear association with the use of ertapenem. We found a statistically significant negative correlation between ertapenem consumption and the susceptibility rate of A. baumannii to imipenem (r = −0.91; p = 0.004) and meropenem (r = −0.97; p = 0.000) in the data from the non-ICU wards. In addition, imipenem use had a moderate negative correlation with the MDR P. aeruginosa data but no statistical significance (r = −0.714; p > 0.05). In conclusion, our study suggested there is an association between carbapenem use and the susceptibility of A. baumannii and P. aeruginosa. Notwithstanding this, information on ecological factors should be considered for further study. These findings showed the need to optimize the carbapenem prescription policy. Avoiding carbapenem overuse and rethinking the appropriate initial therapy might decrease the rate of resistant organisms.

(1) Background: Resistant Pseudomonas aeruginosa (PA) infections have limited treatment options. Data on the activity of ceftolozane-tazobactam (C-T) against PA in Thailand are limited. Objectives: The objective of this study was to identify the in vitro activity of C-T against general and resistant PA isolates from patients with real clinical infections from the HRH Princess Maha Chakri Sirindhorn Medical Center (MSMC) compared to other antibiotics and to study the resistant molecular patterns of those PA strains which were resistant to C-T. (2) Materials and Methods: This was an in vitro susceptibility study of 100 PA isolates plus an additional seven resistant PA isolates collected from MSMC patients. All PA isolates were tested with susceptibility broth (Sensititre™) and C-T minimal inhibitory concentration (MIC) test strips (Liofilchem, Roseto degli, Abruzzi, Italy). The C-T-resistant PA isolates were analyzed for six β-lactamase genes (bla_CTX-M, bla_NDM, bla_IMP, bla_VIM, bla_OXA-23 and bla_OXA-48) and the mcr-1 gene. (3) Results: A total of 100 PA isolates were collected between January 2020 and January 2021 and between February 2021 and September 2021 for the additional 7 resistant isolates. There were 18 resistant PA isolates (6 MDR, 11 XDR and 1 pan-drug resistant isolate). The overall susceptibility of the initial 100 PA isolates and the 18 resistant PA isolates was 94% and 44.5%, respectively, for C-T. The C-T susceptibility rates for isolates non-susceptible to ceftazidime, piperacillin-tazobactam, carbapenems and antipseudomonal β-lactams were 65.5%, 69.7%, 50% and 44.5%, respectively. Among the 10 isolates which were resistant to C-T, there were only 3 isolates found to have the resistant gene, which included 1 for bla_IMP, 1 for bla_VIM and 1 for bla_NDM. (4) Conclusions: Although C-T was the best susceptibility antibiotic overall for PA isolates and MDR PA isolates at the MSMC, most of the XDR PA isolates and the PDR PA isolate were not susceptible to C-T. The mechanisms for C-T resistance involved multiple factors including the presence of bla_IMP, bla_VIM and bla_NDM.