Lack of constitutive beta-glucosidase (esculinase) in the genus Fusobacterium

Esculin has been incorporated into both a medium and test with 20% bile for many years to differentiate Bacteroides from Fusobacterium organisms. After 24 to 48 h, all members of the Bacteroides fragilis group grow in 20% bile and hydrolyze esculin. Fusobacterium mortiferum can both grow in bile and hydrolyze esculin, thus limiting the use of the bile-esculin medium and test. The hypothesis that constitutive esculinase (beta-glucosidase) could differentiate Bacteroides from Fusobacterium organisms was investigated. Clinical isolates and American Type Culture Collection clones of the B. fragilis group and other species of Bacteroides and Fusobacterium were tested. All B. fragilis were positive within 30 min. In no case was a Fusobacterium organism positive for constitutive enzyme in a hydrolyzable substrate-based test. The percentage of positive results for other species of Bacteroides agreed with those published in the literature for the esculin test. The genus Fusobacterium can be separated from Bacteroides organisms based on a lack of constitutive beta-glucosidase in the former in a 30-min one-tube test.