Impact of dissimilar susceptibility breakpoints for doripenem on susceptibility and carbapenem discordance for Pseudomonas aeruginosa

Evaluation of clinical outcomes in patients with bloodstream infections due to Gram-negative bacteria according to carbapenem MIC stratification

Predictive modeling suggests that actual carbapenem MIC results are more predictive of clinical patient outcomes than categorical classification of the MIC as susceptible, intermediate, or resistant. Some have speculated that current CLSI guidelines' suggested thresholds are too high and that clinical success is more likely if the MIC value is ≤1 mg/liter for certain organisms. Patients treated with carbapenems and with positive blood cultures for Pseudomonas aeruginosa, Acinetobacter baumannii, or extended-spectrum beta-lactamase (ESBL)-producing Gram-negative bacteria were considered for evaluation in this clinical retrospective cohort study. Relevant patient demographics and microbiologic variables were collected, including carbapenem MIC. The primary objective was to define a risk-adjusted all-cause hospital mortality breakpoint for carbapenem MICs. Secondarily, we sought to determine if a similar breakpoint existed for indirect outcomes (e.g., time to mortality and length of stay [LOS] postinfection for survivors). Seventy-one patients met the criteria for study inclusion. Overall, 52 patients survived, and 19 died. Classification and regression tree (CART) analysis determined a split of organism MIC between 2 and 4 mg/liter and predicted differences in mortality (16.1% versus 76.9%; P < 0.01). Logistic regression controlling for confounders identified each imipenem MIC doubling dilution as increasing the probability of death 2-fold (adjusted odds ratio [aOR] 2.0; 95% confidence interval [CI], 1.3 to 3.2). Secondary outcomes were similar between groups. This study revealed that patients with organisms that had a MIC of ≥4 mg/liter had worse outcomes than patients whose isolates had a MIC of ≤2 mg/liter, even after adjustment for confounding variables. We recommend additional clinical studies to better understand the susceptibility breakpoint for carbapenems.

In vitro antimicrobial activity of "last-resort" antibiotics against unusual nonfermenting Gram-negative bacilli clinical isolates

In this prospective multicentric study, we assessed the in vitro antimicrobial activity of carbapenems (imipenem, meropenem, and doripenem), tigecycline, and colistin against 166 unusual nonfermenting Gram-negative bacilli (NF-GNB) clinical isolates collected from nine French hospitals during a 6-month period (from December 1, 2008, to May 31, 2009). All NF-GNB isolates were included, except those phenotypically identified as Pseudomonas aeruginosa or Acinetobacter baumannii. Minimal inhibitory concentrations (MICs) of antimicrobial agents were determined by using the E-test technique. The following microorganisms were identified: Stenotrophomonas maltophilia (n=72), Pseudomonas spp. (n=30), Achromobacter xylosoxidans (n=25), Acinetobacter spp. (n=18), Burkholderia cepacia complex (n=9), Alcaligenes faecalis (n=7), and Delftia spp. (n=5). All isolates of Acinetobacter spp., A. faecalis, and Delftia spp. were susceptible to the three carbapenems. Imipenem exhibited the lowest MICs against Pseudomonas spp., and meropenem, as compared with imipenem and doripenem, displayed an interesting antimicrobial activity against A. xylosoxidans and B. cepacia complex isolates. Conversely, no carbapenem exhibited any activity against S. maltophilia. Except for S. maltophilia isolates, tigecycline and colistin exhibited higher MICs than carbapenems, but covered most of the microorganisms tested in this study. To our knowledge, no prior study has compared antimicrobial activity of these five antibiotics, often considered as "last-resort" treatment options for resistant Gram-negative infections, against unusual NF-GNB clinical isolates. Further studies should be carried out to assess the potential clinical use of these antibiotics for the treatment of infections due to these microorganisms.

In vitro activities of doripenem and other carbapenems against clinically important bacteria isolated in intensive care units: nationwide data from the SMART Programme

This nationwide surveillance of clinically important bacteria from the intensive care units (ICUs) of major teaching hospitals throughout Taiwan investigated the susceptibilities to doripenem and other comparator carbapenems from September through November 2005. Minimum inhibitory concentrations (MICs) were determined for 1,311 clinical isolates using the broth microdilution method according to Clinical and Laboratory Standards Institute (CLSI) 2005 guidelines. Doripenem showed similar (within four-fold difference of MICs) in vitro activity to meropenem for Enterobacteriaceae and probably comparable activity to meropenem against important nosocomial non-fermentative Gram-negative bacilli (NFGNBs), including Pseudomonas aeruginosa, Acinetobacter baumannii and Burkholderia cepacia. Among the four carbapenems analysed, doripenem and meropenem exhibited better in vitro activity than imipenem or ertapenem against extended-spectrum beta-lactamase (ESBL)-producing Klebsiella pneumoniae and Escherichia coli isolates. However, the meropenem MIC(90) against ESBL-producing K. pneumoniae isolates was 2 microg/ml. Besides, doripenem with the MIC(90) of 0.5 microg/ml to Streptococcus pneumoniae possibly suggested its potential therapeutic effect regarding community-acquired pneumonia. Because of the heavy resistance burden in Taiwan, closely monitoring the evolutionary trend of carbapenem susceptibilities against clinically important pathogens is crucial in the future.