Synergy Testing by E-Test and Microdilution Checkerboard for Fosfomycin Combined with Tigecycline against KPC-Producing Klebsiella pneumoniae.

Background: KPC-producing Klebsiella pneumoniae (KPC-Kp) has become a serious threat to patients worldwide, as the treatment options are limited. The combination of fosfomycin with other antibiotics has been reported but with inconsistent results. Thus, we performed synergy testing of fosfomycin combined with tigecycline by E-test, which was easy to perform and to determine results, compared with microdilution checkerboard, which is considered to be the "gold standard", to evaluate the agreement between the two methods.

Methods: Thirty non-repetitive KPC-Kp isolates from different patients were included in this study. Bacterial identification and routine antibiotic susceptibility testing were performed by a VITEK 2 Compact automated system. The KPC producing isolates were identified by modified Carbapenem Inhibitory Method (mCIM) and PCR amplification of carbapenemase genes. Synergy testing of fosfomycin combined with tigecycline was performed by E-test (E-test stripes were placed at 90° angle), with microdilution chequerboard performed in parallel. Fractional inhibitory concentration index (FICI) was calculated. Statistical analyses were performed by SPSS 18.0 software.

Results: All 30 KPC-Kp were mCIM test positive and KPC-2 producing. The susceptibility rates of fosfomycin and tigecycline were 36.7% (11/30) and 63.3% (19/30), respectively. Both checkerboard and E-test results showed that most MICs of fosfomycin and tigecycline decreased in the combination group. FICI showed 13.3% - 16.7% isolates were synergistic, 30.0% - 36.7% were additive, and 50.0% - 53.3% were indifferent. No antagonism was found. There was no significant difference between the two groups (p > 0.05), and the overall agreement (with FICI difference ≤ 0.25 in each isolate) between the two methods was 76.7% (23/30).

Conclusions: The synergy testing results determined by E-test correlated well with microdilution checkerboard. Thus E-test synergy testing has the potential to be used in routine clinical laboratory.