Direct Identification, Antimicrobial Susceptibility Testing, and Extended-Spectrum β-Lactamase and Carbapenemase Detection in Gram-Negative Bacteria Isolated from Blood Cultures

Abstract

Purpose

To shorten the turnaround time for blood culture (BC) analyses, a rapid method was developed for the direct identification, antimicrobial susceptibility testing (AST), and multidrug resistance testing of bacteria-positive BCs.

Materials and Methods

The mixtures in BC bottles were treated with the multistep centrifugation method developed here and the conventional culture-based method. The bacterial sediment obtained after centrifugation was analyzed directly with MALDI-TOF MS and Vitek 2 Compact, and AST was performed directly with the Kirby–Bauer (K–B) disk diffusion, VITEK 2 Compact, and E-test methods. Extended spectrum lactamases (ESBLs) were detected with discs containing cefotaxime, cefotaxime/clavulanate, ceftazidime, and ceftazidime/clavulanate, and carbapenemase was detected with the modified carbapenem inactivation method (mCIM) and EDTA-mCIM (eCIM).

Results

All the results of direct testing were compared to those of the conventional methods, to evaluate the accuracy of the direct methods. The accuracies of the direct Vitek 2 Compact and MALDI-TOF MS methods were 95.5% (214/224) and 90.2% (202/224), respectively. Direct AST with K–B, Vitek 2, and E-test showed category agreement of 96.0% (2611/2721), 96.1% (2614/2721), and 97.4% (2650/2721), respectively, and the major errors and very major errors were < 2% for all three methods. In the direct determination of ESBLs, the results for cefotaxime combined with cefotaxime/clavulanate were completely consistent with those after the standard isolation method. The carbapenemase detection rate with direct mCIM and eCIM was exactly the same as that with the standard method.

Conclusion

These direct procedures based on multistep centrifugation are not only highly accurate but are appropriate for clinical laboratory use because the turnaround time is shorter.

Keywords:

• blood culture
• direct identification
• direct antimicrobial susceptibility testing
• extended-spectrum β-lactamase
• carbapenemase
• multistep centrifugation

Acknowledgments

We thank International Science Editing (http://www.internationalscienceediting.com) for editing this manuscript. We would like to express our sincere gratitude to Professor Yongzhong Ning (Department of Laboratory Medicine of Beijing Chuiyangliu Hospital) for his valuable comments, suggestions, and revisions of this manuscript.

Disclosure

The authors report no conflicts of interest associated with this work.

Additional information

Funding

This study was financially supported by the Special Foundation for National Science and Technology Basic Research Program of China (grant nos. 2019FY101200 and 2019FY101204), the Foundation of Hebei Provincial Department of Finance (grant no. 361004), and the International Scientific and Technology Corporation Program of Hebei Provincial Department of Science and Technology of China (grant no. 183977118D).