PD Dr. Christian Riedel

General Research Interests

Our group is interested in the biology of commensal and pathogenic bacteria of the human gastrointestinal tract and their interaction with the host. We are particularly interested in phagocytic uptake of microorganisms by different populations of human macrophages and the subsequent effects on the immune system. Another focus of interest is the adaptation and response of these organisms to various habitats and stress conditions. Moreover, we are interested the identification, characterization and production of antimicrobial peptides (AMPs) of bacterial and human origin that are active against pathogenic bacteria.

For all these projects we are continuously developing new molecular tools for the microorganisms we study. This includes the development of CRISPRi and CRISPR/Cas systems for L. monocytogenes and Bifidobacterium bifidum, the generation of sensor strains to analyze AMPs, and the establishment of heterologous AMP production.

Listeria monocytogenes and AMPs

L. monocytogenes is a ubiquitous, Gram-positive soil bacterium and a frequent contaminant of processed food products. Its abilities to withstand and grow under a wide range of environmental conditions (temperature, pH, salt, etc.) and to form biofilms on different surfaces make L. monocytogenes a serious concern in the food processing industries. Moreover, L. monocytogenes is able to cause life-threatening infections in immuno-compromised persons and a model organism for intracellular pathogenesis.


Within a European research consortium that aims at investigating the effect of high pressure processing on integrity and viability of L. monocytogenes, we develop novel molecular tools that allow conditional silencing or deletion of genes that can be targeted to increase sensitivity of L. monocytogenes to high pressure processing. As a “by-product” of these activities, we have recently constructed sensor strains of L. monocytogenes the express ratiometric pH-dependent fluorescent proteins that allow measurement of intracellular pH using changes in the fluorescent properties (Figure 1 and Crauwels et al., 2018).


Figure 1: (A) Relative fluorescence units (RFU; excitation 350–490 nm; emission at 510 nm) of L. monocytogenes EGD-e/pNZ-Phelp-pHluorin2 in buffer at the indicated pH with 0.005% CTAB for permeabilization. (B) The ratio of fluorescence intensity (emission at 510 nm) after excitation at 400 or 470 nm can be used to determine membrane damaging activity of AMPs (here nisin as an example) in different strains of L. monocytogenes. (C) The strain L. monocytogenes EGD-e/pNZ-Phelp-pHluorin2 can be used to identify isolates from bovine raw milk that produce compounds with membrane-damaging activity (presumably AMPs).

Using these strains, we are able to analyze the activity of membrane damaging compounds such as bacteriocins and other AMPs. The developed tools are employed in studies aiming at the identification of novel AMP-producing bacteria or AMPs from other sources, the characterization of their activity and the identification of their receptors or mechanisms of resistance.

In recent years, we have been investigating interactions between L. monocytogenes and different populations of primary human macrophages (Figure 2 and Neu et al., 2014). The abovementioned sensor strains can also be used to study the subcellular localization and the genes that are required to gain access to these sites or provide resistance against the conditions encountered.

Figure 2: Scanning electron microscopy of an ex vivo differentiated human macrophage taking up L. monocytogenes bacteria. Inset: zoom-in on the part of the image marked with a white box depicting one bacterium captured during the process of phagocytosis. Image courtesy of C. Neu (University of Ulm), all rights reserved.



Bifidobacteria are anaerobic Gram-positives and an important group of bacteria in the lower intestinal tract of humans. Particularly high numbers are observed in breast-fed infants and various health-promoting properties of bifidobacteria are attributed to their presence in the gut. Based on these health-promoting properties, bifidobacteria are widely used as so-called probiotic supplements in pharmaceutical and dairy products. For some strains anti-inflammatory effects have been reported in vitro and in preclinical studies. Thus, one potential field of application is their use as alternative or supplementary treatment in inflammatory conditions of the gastrointestinal tract.

Previous studies of our group have identified strains of the bifidobacteria that show high adhesion to intestinal epithelial cells and display potent anti-inflammatory activity in vitro and in murine models of colitis (Preising et al. 2010; Philippe et al. 2011, Grimm et al. 2015). In order to unravel the molecular mechanisms of host colonization and anti-inflammatory activity, we sequences the genomes of several strains of bifidobacteria (Zhurina et al. 2011; Zhurina et al., 2013; Bottacini et al., 2014) and identified a number of candidate genes potentially involved in these properties (Westermann et al. 2013). Past and current projects aim at investigating the role of these candidates to adhesion, colonization and inhibition of inflammation (e.g. Gleinser et al., 2012; Wei et al., 2014; Grimm et al., 2015, Wei et al., 2015). A further aspect is the potential use of bifidobacteria as vectors to for delivery of therapeutic genes into solid tumors (Osswald et al., 2015a)


Figure 3: Overlay of brightfield and fluorescence microscopy images of primary human macrophages containing different B. bifidum strains labeled by plasmid-based expression of different fluorescent proteins inside. Pictures were acquired on a Zeiss Axio Observer.Z1 microscope with a 63x objective. Image courtesy of V. Grimm (University of Ulm), all rights reserved.