Evolution of antimicrobial susceptibility patterns of aerobic and facultative gram-negative bacilli causing intra-abdominal infections: results from the SMART studies 2003-2007.

Study for Monitoring Antimicrobial Resistance Trends (SMART) is an ongoing global antimicrobial surveillance program focused on clinical isolates from intra-abdominal infections (IAI). The objective of this subanalysis was to assess the evolution of the antimicrobial susceptibility patterns among aerobic and facultative gram-negative bacilli (GNB) recovered over a 5-year period at our institution. We tested the in vitro activity of the antimicrobials, commonly used to treat IAI, against consecutive unique isolates from IAI using microdilution techniques according to the CLSI guidelines for MIC testing. All isolates were screened phenotypically for extended-spectrum beta-lactamase (ESBL) production. Isolates recovered within 48 h of hospitalization were considered community-acquired (CA). Over the study period a total of 572 aerobic and facultative gram-negative bacilli were recovered from 510 patients, of which 258 (45%) were CA. Enterobacteriaceae composed 91% of the total isolates. Escherichia coli was the most common isolated species (52%). Susceptibility rates of Enterobacteriaceae ranged from 96.5 %-100 % to ertapenem, 96.5 %-100 % to imipenem, 87.7%-94.3% to piperacillin-tazobactam, 85.1%-94.3% to cefotaxime, 89.5%-100% to cefepime, 76.3%-84.8% to ciprofloxacin, and 93.8%-100% to amikacin. ESBL were detected in 6.3% of E. coli, 5.7% of Klebsiella spp. and 2.7% of Enterobacter spp. ESBL producers generally had a more antibiotic- resistant profile than non-ESBL producers and 16% of them were CA. Susceptibility rates to ertapenem, imipenem, piperacillin-tazobactam, ceftazidime, cefepime, ciprofloxacin and amikacin were, respectively, for P. aeruginosa: 28.2 %, 58.9%, 82%, 84.6 %, 76.9 %, 71.8% and 82%; for Acinetobacter baumannii: 33.3 %, 100 %, 66.6 %, 66.6 %, 66.6%, 66.6% y 66.6%, and for Stenotrophomonas maltophilia: 0%, 0%, 0%, 28.6%, 0%, 42.9% and 14.3%. Over the 5 year-study period we have not observed significant increases in resistance of aerobic and facultative GNB causing IAI to commonly used beta-lactam antimicrobial drugs. A minority of ESBL-producing Enterobacteriaceae were CA. Carbapenems, including group I agents like ertapenem, were the most reliably active drugs in vitro against isolates producing IAI.