EVALUATION OF CHROMAGAR ORIENTATION FOR DIFFERENTIATION AND PRESUMPTIVE IDENTIFICATION OF GRAM-NEGATIVE BACILLI AND ENTEROCOCCUS SPECIES

A new chromogenic plate medium, CHROMagar Orientation, was evaluated f or use in the differentiation and presumptive identification of gram-n egative 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 con trol 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, O nly in four isolates that were O-nitrophenyl-beta-D-galactopyranoside (ONPG) negative did this result differ, Proteus mirabilis and P, vulga ris were well differentiated on this medium, P, mirabilis (n = 184) pr oduced a clear colony with diffusible brown pigment around the periphe ry, By contrast, 15 of 16 P, vulgaris isolates produced bluish-green c olonies with a slight brown background, AIL Aeromonas hydrophila isola tes (n = 26) tested produced clear to pink colonies at 35 to 37 degree s C. This colony color changed to blue after 2 to 3 h of incubation at room temperature, A, hydrophila exhibited stronger color and better g rowth at 30 degrees C, Serratia marcescens (n = 29) demonstrated an aq ua blue color that deepened to a darker blue when exposed to room temp erature. All enterococcal isolates (n = 74) resulted in a blue color a nd gave pinpoint colonies on purity subcultures at 35 to 37 degrees C after 18 h of incubation. Similarity in color resulted in failure to d iscriminate accurately between Klebsiella, Enterobacter., and Citrobac ter species. However, these species could be readily differentiated fr om other members of the family Enterobacteriaceae. Pseudomonas aerugin osa (n = 151) was easily differentiated from members of the Enterobact eriaceae but was less easily distinguishable from other gram-negative nonmembers of the Enterobacteriaceae. The medium was found to facilita te easy visual detection of mixed bacterial isolates in culture. When used in a replicator system, it easily detected mixed growths of organ isms which may have otherwise led to false antibiotic susceptibility r esults. These mixed growths were not obvious on the routine susceptibi lity testing medium (Isosensitest).