In this work we designed a competition model system for bacterium Bacillus subtilis where the wild type is the producer of extracellular enzyme α-amylase, while the non-producing amyE mutant strain exploits products of enzyme when grown in coculture with the wild type. By growing the two strains in the growth medium with starch as the sole carbon source we created the conditions where the production of enzyme α-amylase was essential for the microbial growth. In order to research the microbial social interactions we used two methods: growth in shaken liquid culture and on a solid growth medium. By using both shaken culture and solid medium we wanted to explore whether the metabolic cost of the production of α-amylase is sufficient to be observed as a lower cell concentration or whether the collapse of the population occurs when wild type and mutant strain are simultaneously inoculated. The results of our experiments suggest that the metabolic price of the production of α-amylase is low, which explains why we did not observe the difference in strain ratios of co-cultures. We also did not observe the collapse of co-cultures when grown for 100 generations in any of the experiments. As shaken conditions in nature are uncommon, we decided to extend the research on a solid growth medium. The results suggest that wild type gains advantage when we increased the distance between wild type and mutant strain, likely due to the limited diffusion and preferential nutrient acquisition. To further explore this phenomenon we incoulated fresh solid growth medium with the incolum of the of the old biofilm co-culture. Our results show the advantage of wild type to the mutant strain, as the distances between the cells at the time of the inculation on the fresh medium were now large enough to make α-amylase partially private.
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