Cloning and expression of the Bacteroides fragilis YCH46 neuraminidase gene in Escherichia coli and Bacteroides uniformis

Physical and genetic map of the Bacteroides fragilis YCH46 chromosome

The chromosome of Bacteroides fragilis strain YCH46 was shown to be a single circular DNA molecule of about 5.3 Mb having 16 NotI, seven AscI, and six I-CeuI sites. A physical map of the chromosome was constructed by four independent experimental approaches: linking clone analysis, cross-Southern hybridization, partial restriction digestion, and two-dimensional pulsed-field gel electrophoresis. Six rRNA operons and 10 known genes were localized on the physical map.

Detection of Bacteroides fragilis by PCR assay targeting the neuraminidase-encoding gene

Oligonucleotide primers were designed on the basis of the sequence of the neuraminidase-encoding gene (nanH) of Bacteroides fragilis and used for the specific detection of this anaerobe by the nested PCR assay. Fifty-nine of 60 representative strains of Bact. fragilis were detected, while none of 45 strains of other species generated visible PCR products. The detection limits of Bact. fragilis cells and DNA by the nested PCR were 10 colony-forming units and 10 fg of chromosomal DNA, respectively. The PCR assay targeting the nanH gene has the potential for the detection of Bact. fragilis.

Purification and characterization of a fibrinogen-degrading protease in Bacteroides fragilis strain YCH46

A novel fibrinogenolytic protease was purified from Bacteroides fragilis strain YCH46. The protease was extracted from cells by ultrasonic treatment and was purified 425-fold with a recovery of 2.1% by sequential procedures using azocasein as a substrate. The purified protease showed a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an estimated molecular weight of 100 kDa, which was consistent with the value obtained by gel filtration, indicating a monomeric native structure. Its optimal pH, Km, and Vmax for azocasein were 7.5, 0.2%, and 286 U/min/mg, respectively. The protease activity was completely inhibited by addition of 1 mM Hg2+, Cu2+, Zn2+, diisopropyl fluorophosphate, N-ethylmaleimide or p-chloromercuribenzoate but not by the inhibitors of metalloprotease or aspartic protease, suggesting that the enzyme is a serine-thiol-like protease. The protease hydrolyzed azocasein, casein, fibrinogen, gelatin, and azocoll, but not bovine serum albumin, ovalbumin, fibrin, fibronectin, immunoglobulins, transferrin, hemoglobin or types I, III, and IV collagen. The enzyme also hydrolyzed the chromogenic substrates alanyl-alanine p-nitroanilide, L-valyl-alanine p-nitroanilide, alanyl-alanyl-valyl-alanine p-nitroanilide, and glycyl-proline p-nitroanilide, but was inert toward L-alanine p-nitroanilide, alanyl-alanyl-alanine p-nitroanilide, and N-alpha-benzoyl-DL-arginine p-nitroanilide. The protease completely hydrolyzed the alpha-chain of fibrinogen at 37 C within 10 hr and at the same time the time required for clotting of protease-treated fibrinogen by thrombin was prolonged. The fibrinogenolytic activity of a crude extract of B. fragilis was stronger than that of other species of the Bacteroides fragilis group tested: B. ovatus, B. distasonis, B. eggerthii, B. uniformis, and B. thetaiotaomicron. These results suggest that the fibrinogenolytic protease is an important biological factor in Bacteroides infection.

Molecular cloning of the leuB gene from Bacteroides fragilis by functional complementation in Escherichia coli

Clones containing the Bacteroides fragilis leuB-complementing gene were isolated by screening of a B. fragilis genomic library constructed in Escherichia coli. One recombinant clone, designated pOT865, with the smallest DNA insert (4.5 kb) could complement three independent leuB mutations in E. coli and the leuB-complementing determinant in pOT865 was localized to a region of 1.5-kb DNA. The results of Southern blot analysis suggested that a single copy of the cloned gene was present in the B. fragilis genome. The cloned fragment appeared to contain a sequence that could function as promoter in E. coli and direct the synthesis of a 42-kDa protein. These results suggest that the cloned segment contains the structural gene for beta-isopropylmalate dehydrogenase (leuB).