Evaluation of modified saponin preparation method for the direct identification and antimicrobial susceptibility testing from positive blood culture

Background: We evaluated the two in-house sample preparation methods (saponin method (SAP) and [saponin + Sputazyme] method (SSPZ)) for direct identification of microorganisms using MALDI-TOF MS from positive blood culture bottles. Also, we evaluated the [saponin + Sputazyme] method for direct antimicrobial susceptibility testing (AST) using Vitek 2 system. Methods: For direct identification, 163 prospective, monomicrobial positive blood culture bottles and 25 contrived blood culture bottles spiked with 25 infrequently isolated bacterial strains were included. For direct AST, pellets obtained by SSPZ method from 102 prospective blood culture bottles were tested. The results from the direct identification and AST were compared with those from the routine diagnostic method performed with colonies sub cultured on solid media. Results: In 163 prospective specimens, SAP method correctly identified 132/163 (81.0%) isolates and SSPZ method correctly identified 148/163 (90.8%) isolates (P =.018). Among the 92 Gram-positive isolates, the correct identification rate was significantly higher with the SSPZ method than the SAP method (92.4% vs. 81.5%), respectively (P =.041). However, the SSPZ method failed to identify Streptococcus pneumoniae. Among the 64 Gram-negative isolates, the correct identification rate was 82.8% (53/64) and 87.5% (56/64) for the former and the latter method, respectively (P =.491). Compared with standard methods direct AST showed 98.5% (1523/1547) agreement. Conclusion: The addition of Sputazyme improved the identification of commonly isolated bacteria, especially for Gram-positive isolates and yeasts and can be applied for direct antimicrobial susceptibility testing of bacteria. Although SAP method showed better results for Campylobacter spp. and anaerobic bacteria, considering their very low incidence, routine use of SSPZ will be more practical.