Long-wave UV light fluorescence for identification of black-pigmented Bacteroides spp.

Black-pigmented Bacteroides strains were grown on blood agar, and the colonies were evaluated for fluorescence from long-wave UV light. Most test strains of Bacteroides melaninogenicus subsp. intermedius exhibited a brilliant red fluorescence. B. melaninogenicus subsp. melaninogenicus fluoresced mostly red-orange. Bacteroides asaccharolyticus showed a yellow or red fluorescence. The intensity of the Bacteroides fluorescence weakened when the black pigment of the colonies developed. In contrast, neither young nor old colonies of the oral species Bacteroides gingivalis displayed fluorescence. Since B. gingivalis can produce severe oral infections and also can seed to nonoral sites, awareness of the inability of this organism to fluoresce is important for microbiologists utilizing UV light fluorescence to screen for black-pigmented Bacteroides spp. The present data also indicate that UV light fluorescence may be a rapid method of distinguishing some black-pigmented Bacteroides spp.