Changing growth behavior of heavy-metal tolerant bacteria: Media optimization using droplet-based microfluidics

Overproduction of secondary metabolites requires know-how for media optimization and design of effective screening procedures. Micro-segmented flow was applied to test the growth characteristics of heavy-metal tolerant soil bacteria (Bacillus sporothermodurans and Streptomyces tendae) toward changing media composition. The applied microfluidic technique was shown to be suitable for media optimization, with effects of media solutes like phosphorus and nitrogen, and a trace metal solution. For inorganic phosphate and trace metals, the synthesis of secondary metabolites had a much narrower tolerance for concentration ranges than vegetative cells. As we worked with metal-tolerant strains, some tests were done in the presence of copper-II ions in the bacteria-specific critical concentration of 0.25 mM. In particular, for the Bacillus strain, the macronutrient phosphorus resulted in a threefold increase in growth with rising concentration. Copper led to a prolonged lag phase. For Streptomyces tendae, no similarly enhancing effect was observed. The trace metal solution promoted bacterial growth only in the case of Streptomyces tendae, for higher concentration ranges. For the nitrogen source, especially Streptomyces tendae responded with changes between the light scattering and the fluorescence signal. This could be a hint for the production of fluorescent secondary metabolites.