Four-hour presumptive identification of Enterobacteriaceae from blood cultures

A modification of the Minitek identification system for the four-hour identification of gram-negative bacilli recovered from blood cultures was developed. This method enabled our laboratory to identify correctly 99 of 100 consecutive isolates of all members of Enterobacteriaceae representing seven genera and nine species. All rapid identifications were confirmed using conventional biochemical methods. The procedure employs 11 Minitek biochemical discs inoculated with 0.1 ml of a dense organism suspension. Two-tenths milliliter of this suspension was added to a blank well for motility testing. In most instances, identification reactions were identical to those routinely used for the Minitek system. Only with the biochemical citrate disc does the interpretation vary from the Minitek standard. In addition to the blood culture isolates, 22 unknown organisms were also tested. All were correctly identified within four hours of the detection of visible turbidity in broth. This method offers a convenient, economic, and rapid means of identifying gram-negative bacilli isolated from blood cultures.