In vitro Activity of Tigecycline and Comparators against Carbapenem-Susceptible and Resistant Acinetobacter baumannii Isolated from Severe Infections in Italy

Background: In a recent multi-centre Italian survey (2003–2004), conducted in 45 laboratoriesthroughout Italy with the aim of monitoring microorganisms responsible for severe infections andtheir antibiotic resistance, Acinetobacter baumannii was isolated from various wards of 9 hospitalsas one of the most frequent pathogens. One hundred and seven clinically significant strains of A.baumannii isolates were included in this study to determine the in vitro activity of tigecycline andcomparator agents.Methods: Tests for the susceptibility to antibiotics were performed by the broth microdilutionmethod as recommended by CLSI guidelines. The following antibiotics were tested: aztreonam,piperacillin/tazobactam, ampicillin/sulbactam, ceftazidime, cefepime, imipenem, meropenemtetracycline, doxycycline, tigecycline, gentamicin, amikacin, ciprofloxacin, colistin, andtrimethoprim/sulphametoxazole. The PCR assay was used to determine the presence of OXA,VIM, or IMP genes in the carbapenem resistant strains.Results: A. baumannii showed widespread resistance to ceftazidime, ciprofloxacin and aztreonamin more than 90% of the strains; resistance to imipenem and meropenem was 50 and 59%respectively, amikacin and gentamicin were both active against about 30% of the strains and colistinabout 99%, with only one strain resistant. By comparison with tetracyclines, tigecycline anddoxycycline showed a higher activity. In particular, tigecycline showed a MIC90 value of 2 mg/L andour strains displayed a unimodal distribution of susceptibility being indistinctly active againstcarbapenem-susceptible and resistant strains, these latter possessed OXA-type variant enzymes.Conclusion: In conclusion, tigecycline had a good activity against the MDR A. baumannii strainswhile maintaining the same MIC90 of 2 mg/L against the carbapenem-resistant strains.