Urease production by Streptococcus thermophilus

In order to identify potential alternative sources of urease for the removal of urea from alcoholic beverages, 205 strains of lactic acid bacteria belonging to 27 different species were screened for urease production. Only Streptococcus thermophilus produced urease. Cell permeabilization with toluene allowed to increase activity significantly. Optimal pH for urease activity in whole and permeabilized cells and of cell free extracts differed slightly, but was in the range 6.0–7.0. Significant activity was retained at pH 3.0 and 8.0, and, for cell free extracts, at pH 4.0 in the presence of ethanol. Urease production was evaluated in fermentations with pH control (5.25–6.5) and without pH control. Very little urease was produced in absence of urea, which at 5 g/l slowed growth significantly in fermentations without pH control, but prevented a decrease in pH below 5.1 and resulted in higher final biomass. Optimal pH for growth was between 6.0 and 6.5 but specific urease activity was higher for fermentations at low pH at the beginning of the exponential phase. However, a higher total urease activity was obtained at pH 6.0 and 6.5 because of higher biomass. Potential technological applications of urease production by S. thermophilus are discussed.