STREPTOCOCCUS THERMOPHILUS UREASE ACTIVITY: PHYSIOLOGICAL ROLE AND TECHNOLOGICAL RELEVANCE IN DAIRY AND NON-DAIRY APPLICATIONS

The aim of this Ph.D. work was to investigate the physiological role and the technological relevance of the urease activity of the dairy bacterium Streptococcus thermophilus. It has been achieved a deeper comprehension of this peculiar enzymatic activity following different approaches. A milk-based medium that allows the discrimination between urease-positive S. thermophilus strains and urease-negative ones based on the colonies morphology had been developed; it was used as screening method looking for urease-defective mutants after UV mutagenesis of urease-positive strains of industrial interest. Moreover, a cytofluorimetric protocol for the evaluation of urease activity of various samples containing S. thermophilus was developed: the proposed applications are related to the evaluation of the urease activity of starter culture biomasses of S. thermophilus and to the enumeration of the S. thermophilus population in probiotic products containing this species, among others. We propose that the cytofluorimetric method should be seen as an innovative tool to put besides the standard ones to evaluate the quality of the products previously mentioned. The investigation of the physiological role of urease of S. thermophilus cells growing in milk highlighted that in presence of this enzymatic activity the overall metabolism of the species is boosted. Moreover, the cooperative role of urease, previously described for the yogurt consortium, has been extended also to the cooperation between a urease-positive S. thermophilus strain and a urease-negative one, supporting the proposal of the urease activity as an altruistic cooperative trait, which is costly for urease-positive species but provides a local benefit to the urease-negative species sharing the same environment, which can take advantage of the release of ammonia. At industrial level, urease activity is still considered more for its detrimental effects than for the positive effects exerted on S. thermophilus: we proposed different strategies to overcome this industrial problem. Firstly, we produced mutants of urease-positive strains of industrial interest, but they showed lower acidification rate compared to their wild types. So, we proposed to modify the production process of the biomasses with the aim of obtaining cells which carry a lower urease activity compared to those currently produced. In conclusion, the present work gives new insight in the comprehension of the urease activity of the dairy bacterium S. thermophilus, on how it can be exploited to improve the acidification performances of the strains and how it cannot, so far, be controlled during the milk acidification processes.