DIRECT ANTIMICROBIAL SUSCEPTIBILITY TESTING OF GRAM-NEGATIVE BACILLI IN BLOOD CULTURES BY AN ELECTROCHEMICAL METHOD

Nonfastidious aerobic gram-negative bacilli (GNB) are commonly isolate d from blood cultures. The feasibility of using an electrochemical met hod for direct antimicrobial susceptibility testing of GNB in positive blood cultures was evaluated. An aliquot (10 mu l) of l:10-diluted po sitive blood cultures containing GNB was inoculated into the Bactomete r module well (bioMerieux Vitek Hazelwood, Mo.) containing 1 mi of Mue ller-Hinton broth supplemented with an antibiotic. Susceptibility test s were performed in a breakpoint broth dilution format, with the resul ts being categorized as resistant, intermediate, or susceptible. Seven antibiotics (ampicillin, cephalothin, gentamicin, amikacin, cefamando le, cefotaxime, and ciprofloxacin) were used in this study, with each agent being tested at the two interpretive breakpoint concentrations. The inoculated modules were incubated at 35 degrees C, and the change in impedance in each well was continuously monitored for 24 h by the B actometer. The MICs of the seven antibiotics for each blood isolate we re also determined by the standardized broth microdilution method. Of 146 positive blood cultures (1,022 microorganism-antibiotic combinatio ns) containing GNB tested by the direct method, the rates of very majo r, major, and minor errors were 0, 1.1, and 2.5%, respectively, The im pedance method was simple; no centrifugation, preincubation, or standa rdization of the inocula was required, and the susceptibility results were normally available within 3 to 6 h after inoculation. The rapid m ethod may allow proper antimicrobial treatment almost 30 to 40 h befor e the results of the standard methods are available.