Rapid identification and antimicrobial susceptibility testing of gram-negative bacilli from blood cultures by the AutoMicrobic system

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Abstract
A procedure was developed which allows direct identification and antimicrobial susceptibility testing of fermentative and nonfermentative gram-negative bacilli from positive blood cultures. A 10-ml sample was removed from turbid blood culture bottles, and the bacteria were washed and concentrated by centrifugation. The bacterial pellet was used to inoculate an Enterobacteriaceae Plus Identification Card and a Gram-Negative General Susceptibility Card of the Auto-Microbic system. Results with these cards were compared with results obtained with standard techniques for 196 blood cultures seeded with recent clinical isolates. Identification of most cultures was available in 8 h, whereas the antimicrobial susceptibility results were available in an average of 4.7 h for all organisms. Direct identification was correct for 95% of the cultures, whereas the antimicrobial susceptibility data had an average agreement of 87% with 3.8% very major and 1.4% major errors. In using this procedure, it was possible to provide accurate preliminary identification and results of antimicrobial susceptibility tests for gram-negative bacilli on the same day that a blood culture was determined to be positive.