Evaluation of CHROMagar Orientation for differentiation and presumptive identification of gram-negative bacilli and Enterococcus species.

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A new chromogenic plate medium, CHROMagar Orientation, was evaluated for use in the differentiation and presumptive identification of gram-negative bacilli and Enterococcus species by a multipoint inoculation (replicator) technique. In this study, 1,404 gram-negative bacilli and 74 enterococcal isolates were tested on CHROMagar Orientation. Six control American Type Culture Collection strains were also included with the testing to ensure quality control of the media. Of the Escherichia coli isolates (n = 588) tested, 99.3% produced a pink-to-red color. Only in four isolates that were O-nitrophenyl-beta-D-galactopyranoside (ONPG) negative did this result differ. Proteus mirabilis and P. vulgaris were well differentiated on this medium. P. mirabilis (n = 184) produced a clear colony with diffusible brown pigment around the periphery. By contrast, 15 of 16 P. vulgaris isolates produced bluish-green colonies with a slight brown background. All Aeromonas hydrophila isolates (n = 26) tested produced clear to pink colonies at 35 to 37 degrees C. This colony color changed to blue after 2 to 3 h of incubation at room temperature. A. hydrophila exhibited stronger color and better growth at 30 degrees C. Serratia marcescens (n = 29) demonstrated an aqua blue color that deepened to a darker blue when exposed to room temperature. All enterococcal isolates (n = 74) resulted in a blue color and gave pinpoint colonies on purity subcultures at 35 to 37 degrees C after 18 h of incubation. Similarity in color resulted in failure to discriminate accurately between Klebsiella, Enterobacter, and Citrobacter species. However, these species could be readily differentiated from other members of the family Enterobacteriaceae. Pseudomonas aeruginosa (n = 151) was easily differentiated from members of the Enterobacteriaceae but was less easily distinguishable from other gram-negative nonmembers of the Enterobacteriaceae. The medium was found to facilitate easy visual detection of mixed bacterial isolates in culture. When used in a replicator system, it easily detected mixed growths of organisms which may have otherwise led to false antibiotic susceptibility results. These mixed growths were not obvious on the routine susceptibility testing medium (Isosensitest).

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