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Environmental regulation of glycosidase and peptidase production by Streptococcus gordonii FSS2

Institute of Dental Research, United Dental Hospital, Surry Hills, NSW 2010, Australia¹
Department of Microbiology, Immunology and Parasitology, Louisiana State University Medical Center, New Orleans, LA, USA²

Author for correspondence: Derek W. S. Harty. Tel: +61 2 9293 3348. Fax: +61 2 9293 3368. e-mail: derekh{at}dentistry.usyd.edu.au

The synthesis of cell-associated and secreted proteins by Streptococcus gordonii FSS2, an infective endocarditis (IE) isolate, was influenced by both environmental pH and carbon source. Controlling the pH at 7·5 in stirred batch cultures showed that cell-associated and secreted protein concentrations were increased during late exponential and stationary phase by 68% and 125%, respectively, compared with similar cultures without pH control. The expression of five glycosidase and eight peptidase activities were examined using fluorogen-labelled synthetic substrates. Enzyme activities were significantly down-regulated during exponential growth, increasing during stationary phase (P<0·01) whether the culture pH was controlled at pH 7·5 or allowed to fall naturally to pH 4·4. Culture-supernatant activities were significantly increased (P<0·05) when the pH was maintained at 6·0 or 7·5, indicating modulation of enzyme activity by pH. Growth under nitrogen-limitation/glucose-excess conditions resulted in a significant repression of cell-associated glycosidase activities (P<0·01), whilst in the supernatant, activities were generally reduced. The expression of peptidase activities in the culture supernatant did not significantly change. The results suggest a possible role for catabolite repression by glucose in regulating enzyme expression. When S. gordonii FSS2 was cultured with 50% (v/v) added heat-inactivated foetal bovine serum, several cell-associated enzyme activities increased initially but were then reduced as the culture time was extended to 116 h. Culture-supernatant enzyme activities (N-acetyl-ß-D-glucosaminidase, N-acetyl-ß-D-galactosaminidase, thrombin, Hageman factor, collagenase and chymotrypsin), however, were significantly increased (P<0·01) over the same time period. The findings indicated that most of the important glycosidases synthesized by S. gordonii FSS2 were down-regulated by acid growth conditions and may also be subject to catabolite repression by glucose but conversely may be up-regulated by growth in serum. These results may have implications for streptococcal growth in an IE vegetation and in the mouth between meals or during sleep.

Keywords: Infective endocarditis, Streptococcus gordonii, pH, enzymes

Abbreviations: 7-AMC, 7-amido-4-methylcoumarin; Ca, activated protein C; CDM, chemically defined medium; FBS, foetal bovine serum; IE, infective endocarditis; 4-MU, 4-methyl-umbelliferyl; PAS, periodic acid/Schiff; Xa, activated protein X

^a Present address: Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA, USA.

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