In vitro activities of 'new' and 'conventional' antibiotics against multi-drug resistant Gram negative bacteria from patients in the intensive care unit.

OBJECTIVES: To identify and collect clinical isolates of multi-drug resistant Gram negative bacteria and to perform antimicrobial susceptibility testing using an extended panel of antibiotics including tigecycline, colistin, aztreonam, piperacillin/tazobactam and ampicillin/sulbactam.

METHODS: Minimum inhibitory concentrations (MICs) using the Epsilometer test (E-test) methodology were determined for 28 distinct multi-drug resistant Gram negative isolates from patients in the intensive care unit (ICU).

RESULTS: Tigecycline had good in vitro activity against Acinetobacter species and Enterobacter cloacae, and colistin had potent in vitro activity against Acinetobacter, E. cloacae and Pseudomonas aeruginosa. Enterobacter cloacae and Serratia marcescens but not P. aeruginosa or Acinetobacter species were susceptible to piperacillin/tazobactam. Ampicillin/sulbactam had poor in vitro activity for most isolates tested. The activity of tigecycline and colistin did not appear to be affected by the presence of extended spectrum beta-lactamases (ESBLs) and metallo-beta-lactamases (MBLs) and aztreonam maintained its in vitro activity against the Enterobacteriaceae tested despite the presence of MBLs.

CONCLUSIONS: Tigecycline and colistin have potent in vitro activity and might have useful therapeutic activity in patients with infections due to multi-drug resistant Acinetobacter species and E. cloacae, including those harbouring ESBLs and MBLs. In addition, colistin demonstrated potent in vitro activity against multi-drug resistant P. aeruginosa.