Evaluation of the RapID CB Plus System for Identification of Corynebacterium Species and Other Gram-Positive Rods

Abstract

Due to the difficulty of identifying Corynebacteriumspp. with standard methods, we compared them with the RapID CB Plus system (Remel, Lenexa, Kans. [formerly Innovative Diagnostic Systems, Norcross, Ga.]), which consists of 4 carbohydrate and 14 preformed enzyme tests, for the identification of 98 clinical isolates of Corynebacterium sp., other coryneforms,Listeria monocytogenes, and 17 ATCC strains. Forty (95%) of 42 strains of Corynebacterium spp. were accurately identified to the species level by the RapID CB Plus system, and two additional strains of C. striatum were identified with one additional conventional test for lipid requirement. Twenty-seven (75%) of the 36 coryneform strains tested were identified correctly to the species level. However, three of four strains of Brevibacterium sp. and all seven of theL. monocytogenes strains were identified to the genus level only. Actinomyces strains had variable results, and the one strain of Arcanobacterium haemolyticum tested was not identified. Overall, the RapID CB Plus system compared favorably with the conventional methods, was easy to inoculate and interpret, and is promising as a new method for identification of gram-positive bacilli.

Evaluation of the RapID CB Plus System for Identification of Coryneform Bacteria andListeriaspp

“…Difficulties with the identification of Listeria spp. on the species level only with commercial identification systems but not by other conventional tests have been reported before (4,6).…”

“…RapID CB Plus system. The RapID CB Plus system is composed of 4 rapid carbohydrate utilization tests and 14 single-substrate enzymatic tests (6). A catalase test and the observation of the production of yellow pigment are supplementary tests used in the system.…”

“…It was inoculated with suspensions of test organisms corresponding to a turbidity equal to that of a no. 4 McFarland standard as described previously by Hudspeth et al (6). The suspensions were used within 10 min of preparation, and special attention was given to an even distribution of the inoculation fluid in each reaction well.…”

“…The RapID CB Plus panels were incubated for 4 h at 37°C in ambient air. After incubation, the needed reagents were added (6) and the panels were independently read by two different persons. In case of ambiguous results (which occurred for fewer than 5% of the strains examined), a third person was asked to read the panels.…”

“…We therefore considered it appropriate to evaluate the RapID CB Plus panel by a comprehensive stress test rather than by a weighted laboratory profile (7) in order to challenge the depth of the system's database. This evaluation is the first one from Europe on this new identification system for coryneform bacteria and includes a more comprehensive spectrum of challenge organisms than a previous study (6).…”