Taxonomic Investigations on Expressed and Cryptic Phospho-β-glucosidases in Enterobacteriaceae

In the Enterobacteriaceae , β-glucosides are catabolized by a complex system formed of three permeases, with partly overlapping substrate specificities, and two hydrolytic enzymes, phospho-β-glucosidase A and B, which hydrolyze only phosphorylated β-glucosides. Some Enterobacteriaceae such as Klebsiella-Aerobacter ( Enterobacter ) possess the complete system; others possess only parts of it or may have a cryptic phospho-β-glucosidase activity without permease activity. A screening test applied to strains belonging to several genera of Enterobacteriaceae showed that strains of Citrobacter, Hafnia , and Serratia exhibit a degree of similarity in phospho-β-glucosidase activity and inducibility which could be useful in their taxonomic characterization; others, such as Aerobacter aerogenes, Erwinia , and Proteus vulgaris , are more heterologous. Owing to the presence of inducible phospho-β-glucosidases A and B in Citrobacter , the fermentation of β-methyl glucoside and the fermentation of arbutin in mixture with cellobiose could be of diagnostic value in the differentiation of Citrobacter from Salmonella . Wild-type strains of Escherichia coli, Shigella , and Salmonella are phenotypically similar in their inability to catabolize β-glucosides, the presence of constitutive P-β-glucosidase A, and the lack of β-glucoside permeases I and II. Their β-glucoside-fermenting mutants show, however, a phospho-β-glucosidase and β-glucoside permease activity which is characteristic for mutants from each genus. The differences in the phenotype of the mutants reflect probable differences in the presence of cryptic genes in the wild-type strains and could be of evolutionary significance.