Cell-to-cell molecular transfer and environmental stress adaptation in Bacillus subtilis

In this thesis, we approached two different subjects, both focusing on a popular Gram-positive model organism - Bacillus subtilis. In the first experimental part of this thesis (Chapters 2 and 3), we addressed the intra- and interspecies transfer of DNA and protein transfer between B. subtilis and members of lactic acid bacteria (LAB). Here, we showed the ubiquity of horizontal gene transfer (HGT) in mixed co-cultures and proposed novel applications for natural strain improvement which are highly desired in producing novel starter cultures for food fermentation. The second part of this dissertation explores the physiological and transcriptional responses of B. subtilis to osmotic and antibiotic stress on a population-wide and single-cell level (Chapters 4, 5 and 6). This work sheds new light on how preadaptation to adverse conditions can modulate metabolic activity, affect cell membrane polarization, and finally increase tolerance to certain antibiotics in bacteria. We showed that B. subtilis, after being pretreated with osmotic stress, enters the viable but non-culturable state (VBNC) and becomes resistant to aminoglycoside antibiotics. Besides, the observations made in this work draw attention to the typical methods of cell enumeration and their error-prone outcomes and highlight the importance of VBNC bacteria when testing new antibiotic compounds for clinical applications