Regulation of urease expression of Actinomyces naeslundii in biofilms in response to pH and carbohydrate

Abstract

The hydrolysis of urea by the urease enzymes of oral bacteria is believed to have a major impact on oral microbial ecology and to be intimately involved in oral health and diseases. Actinomyces naeslundii is a ureolytic bacterium that is adapted to tolerate the rapid and dramatic fluctuations in nutrient availability, carbohydrate source, and pH in dental biofilms. Our research objectives were to better understand the regulation of the expression of urease under environmental conditions that closely mimic those in dental biofilms. A. naeslundii ATCC12104 were grown in a chemostat biofilm reactor with carbohydrate-limited medium for 3 days followed by a carbohydrate pulse, at pH 7.0 and at pH 5.5. Urease activities and ureC gene messenger RNA levels of cells in the biofilm were measured before and after the carbohydrate pulse. We found that the neutral pH environments and excess carbohydrate availability could both result in enhancement of urease activity in biofilm cells. The ureC messenger RNA level of A. naeslundii biofilm cells cultivated at pH 7.0 was approximately 10-fold higher than that of cells grown at pH 5.5, but no changes in ureC gene expression were detected after the carbohydrate pulse. Neutral pH environments and excess carbohydrate availability could promote urease expression of A. naeslundii in biofilms, but only neutral pH environments could up-regulate the ureC gene expression and the pH regulates ureC gene expression at a transcriptional level.