Comparison of four rapid methods for identification of Enterobacteriaceae from blood cultures

Positive blood cultures containing gram-negative bacilli were utilized for direct identification by two automated systems, the AutoMicrobic system (Vitek Systems, Inc., Hazelwood, Mo.) and the MS-2 (Abbott Diagnostics, Dallas, Tex.), and two commercial kits, the Micro-ID system (General Diagnostics, Warner-Lambert Co., Morris Plains, N.J.) and the same-day API 20E (Analytab Products, Plainview, N.Y.). Samples of 10 to 15 ml were aseptically removed from radiometrically positive BACTEC bottles (Johnston Laboratories, Cockeysville, Md.) and divided among four sterile tubes. The tubes were centrifuged at 107 X g for 10 min. The supernatants were centrifuged at 1,510 X g for 10 min, and pellets were tested for cytochrome oxidase by means of Pathotec strips. Oxidase-negative pellets were suspended in appropriate media as suggested by the manufacturers. All systems were inoculated, incubated, and interpreted according to the instructions of the manufacturers. The Micro-ID system was read after 4 h of incubation; the three remaining systems were read after 5 h. Results were compared with those of the 18-h API 20E inoculated from pure subcultures of the organisms. Correlation of 90% or more with the API 20E was achieved by the AutoMicrobic and Micro-ID systems. The same-day API 20E and the MS-2 demonstrated 60 and 44% correlation, respectively, with the 18-h API 20E.