Rapid differentiation of fermentative from nonfermentative gram-negative bacilli in positive blood cultures by an impedance method

Abstract

Rapid differentiation of fermentative gram-negative bacilli (fermenters) from nonfermentative gram-negative bacilli (nonfermenters) in positive blood cultures may help physicians to narrow the choice of appropriate antibiotics for empiric treatment. An impedance method for direct differentiation of fermenters from nonfermenters was investigated. The bacterial suspensions (or positive culture broths containing gram-negative bacteria) were inoculated into the module wells of a Bactometer (bioMérieux, Inc., Hazelwood, Mo.) containing 1 ml of Muller-Hinton broth. The inoculated modules were incubated at 35 degrees C, and the change in impedance in each well was continuously monitored. The amount of time required to cause a series of significant deviations from baseline impedance values was defined as the detection time (DT). The percent change of impedance was defined as the change of impedance at the time interval from DT to DT plus 1 h. After testing 857 strains of pure cultures (586 strains of fermenters and 271 strains of nonfermenters), a breakpoint (2.98%) of impedance change was obtained by discriminant analysis. Strains displaying impedance changes of greater than 2.98% were classified as fermenters; the others were classified as nonfermenters. By using this breakpoint, 98.6% (340 of 345) of positive blood cultures containing fermenters and 98% (98 of 100) of positive blood cultures containing nonfermenters were correctly classified. The impedance method was simple, and the results were normally available within 2 to 4 h after direct inoculation of positive blood culture broths.

FIG. 1

Growth curves of E. coli CCRC 15481 (A), K. pneumoniae CCRC 11546 (B), P. aeruginosa CCRC 10944 (C), and A. baumannii CCRC 15885 (D). Bacterial growth was monitored by the signals of capacitance (curves a), impedance (curves b), and conductance (curves c), respectively.

FIG. 2

Impedance growth curves of E. coli CCRC 11509 (A) and P. aeruginosa 516 (a clinical isolate) (B) in TSB (curves a), MHB (curves b), BHI (curves c), and OF glucose medium (curves d).

FIG. 3

Discriminant analysis of pure cultures of fermenters and nonfermenters. Each closed circle (fermenters) or open circle (nonfermenters) represents the mean of the percent change of impedance in the interval from DT to DT+1 h. A breakpoint (2.98%) (dashed line) was obtained by discriminant analysis. The breakpoint divided strains into two groups: the fermenters and nonfermenters.