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Institute of Bio- and Geosciences

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Phenotypic heterogeneity of bacterial populations

It is a well-known fact that even isogenic microbial populations grown under well-defined conditions exhibit significant variation with respect to phenotypic traits. This can be due to mutations or genetic rearrangements, but can also arise due to variation in specific factors, such as metabolic state, cell age, or noise in gene expression. To investigate phenotypic variations of bacterial populations, we monitor gene expression using reporter gene constructs in combination with flow cytometry and live cell imaging (Figure 1). One phenomenon, currently under study in our lab, is the spontaneous induction of the prophage CGP3 in Corynebacterium glutamicum, which occurs even in the absence of a specific trigger.

Furthermore, we apply microscopy and flow cytometry to monitor phenotypic patterns of bacterial populations in bioprocesses. Such analyses provide valuable information with regard to growth and metabolic activity of a particular population. Protocols have been established to monitor cell size, DNA content, membrane potential and integrity (viability), and metabolite production at single cell resolution.

Graphic Phenotypic heterogeneity of bacterial populationsFigure 1: Phenotypic heterogeneity in microbial populations. (A) Bulk measurements may obscure critical information with respect to variation at the single cell level. (B) Isogenic microcolony of a C. glutamicum L-valine producing strain containing an amino acid sensor. Fluorescent cells represent the producing subpopulation.

Genetically encoded biosensors

Genetically encoded biosensors, which transform the intracellular metabolite production into an optical readout (e.g. fluorescence), represent a convenient tool to monitor small molecules at the single cell level. In the field of industrial microbiology, however, their potential for the detection of metabolites in single cells as well as their implementation in strain development and high-throughput (HT) screenings has rarely been exploited so far. We are developing transcription factor-based biosensors for the intracellular detection of metabolite production or stress stimuli in industrial microorganisms. Several recent studies highlight the great potential of biosensor-driven strain development and single cell analysis and promise the revolution of traditional approaches towards a "bright" future of industrial microbiology (Figure 2).

Graphic genetically encoded biosensorsFigure 2: Versatile application of genetically-encoded biosensors in strain development and single cell analysis.

Control of metal ion homeostasis

Microorganisms living in a complex and varying environment such as soil must be able to take up all essential metal ions, but at the same time limit their intracellular levels to prevent toxicity. We have described transcriptional regulators which constitute a complex hierarchical regulatory network controlling iron homeostasis in the Gram-positive soil bacterium Corynebacterium glutamicum (Figure 3). A current focus of our work is the interplay of two homologous two-component systems (HrrSA and ChrSA) in heme-dependent gene expression. Interaction on multiple levels is required to optimally balance the use of heme as an alternative iron source, but at the same time avoid toxic intracellular levels thereof.

Control of iron homeostasisFigure 3: A glimpse into the control of iron homeostasis in C. glutamicum.