Acinetobacter spp. susceptibility to tigecycline: a worldwide perspective

Sir, In the April 2007 issue of the Journal of Antimicrobial Chemotherapy, Navon-Venezia et al. reported high resistance rates to tigecycline in multiple clones of multidrug-resistant (MDR) Acinetobacter baumannii (n 1⁄4 82). The authors found that 66% (54/82) were resistant to tigecycline (MIC 8 mg/L), 12% (10/82) were intermediate (MIC 4–6 mg/L) and 22% (18/ 82) were susceptible (MIC 2 mg/L). They used Etest to determine MICs and all the values correlated 100% with inhibition zone diameters using the disc diffusion method with tigecycline discs. We agree with the authors that MDR in Acinetobacter spp. represents a global challenge to physicians; for this reason we will try to offer a wider point of view. The Tigecycline Evaluation and Surveillance Trial (TEST) is a worldwide programme that includes, at the moment, 4247 isolates of Acinetobacter spp. of which only 2% have a tigecycline MIC 2 mg/L. Based on this data, we compare the results of Navon-Venezia et al. with those of TEST, using the global data and the Argentinean sub-set data. We selected from the TEST database the isolates of MDR A. baumannii with the same resistance profile as those analysed by Navon-Venezia et al. (i.e. A. baumannii resistant to aminoglycosides, cephalosporins and fluoroquinolones) (Table 1). In contrast with the 78% published by Navon-Venezia et al., only 5.3% of the global isolates and 2% of the Argentinean sub-set isolates of MDR A. baumannii in TEST had tigecycline MICs 2 mg/L. We asked ourselves what would be the probability of encountering such a difference if all the isolates belong to the same population. We performed a proportion test to do so. The probability was very low (P ,, 0.0001). Regarding the MDR A. baumannii isolates as previously defined, and additionally resistant or intermediate to imipenem, the resistance ratio to tigecycline showed important differences (95%, 5.6% and 0% for Navon-Venezia et al., TEST global and Argentinean sub-set, respectively). Tigecycline has been approved for the treatment of complicated intra-abdominal infections and complicated skin and skin