Bloodstream infections are life-threatening conditions which require timely initiation of appropriate antimicrobial therapy. The accuracy of direct disk diffusion susceptibility testing of positive blood cultures was investigated, including for the first time b-lactam/b-lactam-inhibitor combination antibiotics. Results of direct testing, following the guidelines of the Clinical and Laboratory Standards Institute, were compared to standard microtitre broth dilution susceptibility testing of the subcultured isolate on the Merlin MICRONAUT system. Altogether, 758 isolates and 4930 organism/antibiotic combinations from 590 patients were evaluated. With regard to Gram-positive cocci (n=532), agreement between both methods was found in 93.9 % of cases, with 1.6 % very major, 1.1 % major and 2.6 % minor errors. For Gram-negative rods (n=226), agreement was found in 91.9 % of cases, with 1.2 % very major, 0.7 % major and 6.3 % minor errors. When applying the breakpoints of the Deutsches Institut fü r Normung for interpretation of MICRONAUT tests, agreement of direct disk diffusion with standard testing decreased to 82.4 % in Gram-negative rods, with 3.6 % very major, 0.5 % major and 13.4 % minor errors. A high rate of disagreement was observed with oxacillin and gentamicin in Gram-positive cocci, and with cefuroxime, amoxycillin/ clavulanate and piperacillin/tazobactam in Gram-negative rods. In conclusion, the limitations of direct disk diffusion testing of positive blood cultures must be kept in mind by the clinical microbiologist and should, where necessary, be communicated to the clinician to ensure adequate treatment of severely ill patients.

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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.

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