In vitro susceptibility pattern of cephalosporin-resistant Gram-negative bacteria

To determine the in vitro activity of various antimicrobial agents including ertapenem, imipenem, meropenem, fosfomycin, netilmicin, colistin, and piperacillin/tazobactam against clinical isolates of cephalosporin-resistant gram-negative bacteria.

All clinical isolates of gram-negative bacteria obtained from patients receiving care at Queen Sirikit National Institute of Child Health (QSNICH), Bangkok, Thailand, from 2006-2007 were evaluated for antimicrobial susceptibility. Those resistant to all cephalosporins were further assessed for additional disc susceptibility and MIC test using E-tests.

Each of the fifty-five strains of extended spectrum beta-lactamase (ESBL) producing K. pneumoniae and E. coli were tested. The results showed excellent in vitro activity of the studied drugs against ESBL-producing K. pneumoniae with percent susceptibility of 100, 100, 100, 89.8, and 92.7 for ertapenem, imipenem, meropenem, fosfomycin, and colistin, respectively. MIC90 of ertapenem, imipenem, meropenem, fosfomycin, and colistin against K. pneumoniae were 0.23, 0.09, 0.38, 59.2, and 0.75 microg/ml, respectively. Piperacillin/tazobactam inhibited 68.2% of the tested isolates of K. pneumoniae. All studied drugs, except netilmicin, exhibited good activity against ESBL-producing Escherichia coli with 100% sensitivity for carbapenem, fosfomycin, colistin and 95.8% for piperacillin/tazobactam. MIC90 of ertapenem, imipenem, meropenem, fosfomycin, and colistin against Escherichia coli were 0.177, 0.25, 0.064, 2.85, and 0.58 microg/ml, respectively. Six strains of cephalosporin-resistant P. aeruginosa were isolated and tested for MIC. The results showed percent susceptibility of 66.7 and 33.3 for piperacillin/tazobactam and colistin, respectively. MIC90 of piperacillin/tazobactam and colistin against P. aeruginosa were 256 and 8 microg/ml, respectively. Twenty-four strains of cephalosporin-resistant Acinetobacter spp. were isolated with percent susceptibility of 17.6 and 95.5 for piperacillin/tazobactam and colistin, respectively. MIC90 of piperacillin/tazobactam and colistin against Acinetobacter spp. were 256 and 1.4 microg/ml, respectively.

Carbapenems, fosfomycin, and colistin exhibited excellent in vitro activity against both ESBL-producing K. pneumoniae and E. coli. Piperacillin/tazobactam exhibited good in vitro susceptibility against ESBL- producing E. coli, but not K. pneumoniae. Colistin was the most potent in vitro activity of antibiotics against cephalosporins-resistant Acinetobacter spp. However, cephalosporin-resistant Pseudomonas aeruginosa remained problematic, we recommend performing in vitro susceptibility test to determine appropriate antibiotic uses. E-test methods have been shown to be more accurate than disc diffusion test for evaluating colistin susceptibility.