Nonselective nature of Coomassie Brilliant Blue agar for the presumptive identification of Aeromonas salmonicida in clinical specimens

Abstract

To demonstrate the nonselective and nondifferential nature of Coomassie Brilliant Blue (CBB) agar for the presumptive diagnosis of furunculosis, bacteria isolated from kidney and mucus samples of lake trout Salvelinus namaycush and Atlantic salmon Salmo salar were studied. Three populations of salmonids with enzootic furunculosis were screened for Aeromonas salmonicida . All blue "A. salmonicida -like" colonies appearing on CBB agar were subcultured and identified by biochemical characterization using standard microbiological methods. Bacteria belonging to one of 4 groups were cultured as blue colonies on CBB agar, including non-Enterobacteriaceae glucose fermenters (3 species), pseudomonads (9 species), nonpseudomonad glucose nonfermenters (6 species), and Gram-positive organisms (3 species). Results indicated that the screening of colonies as potential A. salmonicida can be accelerated using CBB agar, but the medium alone cannot differentiated nor select for A. salmonicida . It is recommended that when CBB agar is used as a primary plating medium, all blue colonies should be subcultured and characterized minimally with triple sugar iron (TSI) agar, cytochrome oxidase, ornithine decarboxylase, motility, and gelatin before an identification is made. Presumptive identification of A. salmonicida using colony color on CBB agar alone was not validated by this study.