A Tale of Two Cell Factories: Heterologous protein secretion in Bacillus subtilis and Lactococcus lactis

Abstract

Bacteria, like the soil Bacillus subtilis, are often used as “cell factories” for the production of proteins with applications in food processing, detergents and the chemical industry. The application of such proteins is sustainable and cheap. A possible alternative host for protein production is the cheese bacterium Lactococcus lactis. The present thesis describes pro's and con's of both cell factories with focus on systems that facilitate the secretion of proteins into the growth medium. The secretion of proteins is of interest, because purification of secreted proteins is relatively easy. In addition, this thesis describes research on the improvement of both cell factories. To this end, novel protein expression systems were developed for L. lactis, and these were tested for production of proteins from the human pathogen Staphylococcus aureus, which may find future application in vaccines. To optimize protein secretion in B. subtilis a completely new approach was developed, which keeps the envelope of the clean during the production process. This involved overproduction of a protein that degrades other proteins and their remnants, thereby 'unclogging' the bacterium. In this manner, the production of a starch-degrading enzyme was no less than 10-fold increased. Further, three other bottlenecks for protein secretion were identified. Altogether, the documented studies show that it is possible to achieve considerable improvements in bacterial protein production by the analysis and subsequent elimination of bottlenecks. It is thus feasible to expand the spectrum of proteins that can be produced for applications in our daily lives, healthcare and industry.