Evaluation of the Merlin MICRONAUT System for Rapid Direct Susceptibility Testing of Gram-Positive Cocci and Gram-Negative Bacilli from Positive Blood Cultures

Abstract

Bloodstream infections are life-threatening conditions which require the timely initiation of antimicrobial therapy. Inappropriate initial antimicrobial therapy of septic patients is associated with adverse outcomes (13,15,20). Automated blood culture systems that monitor blood culture bottles continuously for bacterial growth minimize the time necessary to detect positive blood cultures. Once bacterial growth is detected in blood cultures, rapid identification and susceptibility testing of the isolate are important tasks for the clinical microbiology laboratory. Reducing the turnaround time of microbiological analysis by using automated systems can lead to significant reductions in patient morbidity, mortality, and costs (3, 9, 27).While standard antimicrobial susceptibility testing (AST) of bacteria commonly involves pure overnight subcultures, preparation of the inoculum for automated susceptibility testing directly from the positive blood culture appears extremely attractive with respect to the time to results. Thus, direct antimicrobial susceptibility testing of isolates from positive blood cultures with many automated testing systems, like the Phoenix (BD, Heidelberg, Germany), the VITEK and VITEK 2 (BioMérieux, Nürtingen, Germany), the Sensititre (Trek Diagnostics, West Lake, OH), and the MicroScan (Dade Behring, Eschborn, Germany) systems, has been evaluated previously (4-6, 12, 14, 18, 21, 24, 26, 28). In general, good agreement between direct and standard susceptibility testing results was observed when gram-negative bacilli were tested, including both members of the Enterobacteriaceae and Pseudomonas species (4-6, 12, 14, 18, 21, 24, 26, 28). For direct testing of gram-positive cocci from blood cultures, only limited data from small studies are available for the VITEK, the VITEK 2, the Sensititre, and the MicroScan systems (5,6,8,18,26,29). A significantly higher rate of disagreement between direct and standard testing results for gram-positive cocci than for gramnegative bacilli was found. Reporting of false susceptibility of staphylococci to oxacillin and of enterococci to various antibiotics (18, 26) is a major problem with enormous clinical relevance. Since gram-positive cocci cause the majority of bloodstream infections (23, 29), rapid and reliable automated susceptibility testing of gram-positive bacteria is highly desirable.We evaluated the automated MICRONAUT system (Merlin, Bornheim-Hesel, Germany) for rapid direct microtiter broth susceptibility testing of gram-positive cocci and gram-negative bacilli from BACTEC bottles with positive blood cultures. The study was conducted under routine working conditions in the clinical microbiological laboratory of the University Hospital of Ulm, Ulm, Germany, during a 9-month period and included 850 positive blood cultures.

Evaluation of direct inoculation of the BD PHOENIX system from positive BACTEC blood cultures for both Gram-positive cocci and Gram-negative rods

“…These errors thus would not have clinical consequences. Funke et al [18] also described a categorical agreement of 99.0%, which is comparable with or higher than results from studies on other direct methods of AST [7,13-16,26]. Therefore, we conclude that also for GNR, results of the direct Phoenix method for AST can be used to guide antibiotic therapy in bloodstream infections.…”

Antibiotic Susceptibility Testing of Grown Blood Cultures by Combining Culture and Real-Time Polymerase Chain Reaction Is Rapid and Effective

“…Many of these direct methods however showed disappointing results for Gram-positive cocci (GPCs) [28], [31] or they were not tested at all [26], [32]. Our method can be applied to the majority of clinically relevant GPCs and (facultative) aerobic GNRs.…”

A Comprehensive Review of the Present and Future Antibiotic Susceptibility Testing (AST) Systems

“…Here, dried antibiotics at different concentrations are placed in the microtiter wells, which get dissolved in the bacterial suspension when it is added. These plates are incubated for period of 6-24 hours and read using a photometer [13]. The results are interpreted and reported as breakpoints, MICs or combination of both.…”