Comparative Metagenomic Analysis of Bacteriophages and Prophages in Gnotobiotic Mouse Models.

Gnotobiotic murine models are important to understand microbiota-host interactions. Despite the role of bacteriophages as drivers for microbiome structure and function, there is no information about the structure and function of the gut virome in gnotobiotic models and the link between bacterial and bacteriophage/prophage diversity. We studied the virome of gnotobiotic murine Oligo-MM12 (12 bacterial species) and reduced Altered Schaedler Flora (ASF, three bacterial species). As reference, the virome of Specific Pathogen-Free (SPF) mice was investigated. A metagenomic approach was used to assess prophages and bacteriophages in the guts of 6-week-old female mice. We identified a positive correlation between bacteria diversity, and bacteriophages and prophages. Caudoviricetes (82.4%) were the most prominent class of phages in all samples with differing relative abundance. However, the host specificity of bacteriophages belonging to class Caudoviricetes differed depending on model bacterial diversity. We further studied the role of bacteriophages in horizontal gene transfer and microbial adaptation to the host's environment. Analysis of mobile genetic elements showed the contribution of bacteriophages to the adaptation of bacterial amino acid metabolism. Overall, our results implicate virome "dark matter" and interactions with the host system as factors for microbial community structure and function which determine host health. Taking the importance of the virome in the microbiome diversity and horizontal gene transfer, reductions in the virome might be an important factor driving losses of microbial biodiversity and the subsequent dysbiosis of the gut microbiome.

In vitro interaction network of a synthetic gut bacterial community.

A key challenge in microbiome research is to predict the functionality of microbial communities based on community membership and (meta)-genomic data. As central microbiota functions are determined by bacterial community networks, it is important to gain insight into the principles that govern bacteria-bacteria interactions. Here, we focused on the growth and metabolic interactions of the Oligo-Mouse-Microbiota (OMM12) synthetic bacterial community, which is increasingly used as a model system in gut microbiome research. Using a bottom-up approach, we uncovered the directionality of strain-strain interactions in mono- and pairwise co-culture experiments as well as in community batch culture. Metabolic network reconstruction in combination with metabolomics analysis of bacterial culture supernatants provided insights into the metabolic potential and activity of the individual community members. Thereby, we could show that the OMM12 interaction network is shaped by both exploitative and interference competition in vitro in nutrient-rich culture media and demonstrate how community structure can be shifted by changing the nutritional environment. In particular, Enterococcus faecalis KB1 was identified as an important driver of community composition by affecting the abundance of several other consortium members in vitro. As a result, this study gives fundamental insight into key drivers and mechanistic basis of the OMM12 interaction network in vitro, which serves as a knowledge base for future mechanistic in vivo studies.

E. coli enhance colonization resistance against Salmonella Typhimurium by competing for galactitol, a context-dependent limiting carbon source.

The composition of intrinsic microbial communities determines if invading pathogens will find a suitable niche for colonization and cause infection or be eliminated. Here, we investigate how commensal E. coli mediate colonization resistance (CR) against Salmonella Typhimurium (S. Tm). Using synthetic bacterial communities, we show that the capacity of E. coli Mt1B1 to block S. Tm colonization depends on the microbial context. In an infection-permissive context, E. coli utilized a high diversity of carbon sources and was unable to block S. Tm invasion. In mice that were stably colonized by twelve phylogenetically diverse murine gut bacteria (OMM12), establishing a protective context, E. coli depleted galactitol, a substrate otherwise fueling S. Tm colonization. Here, Lachnospiraceae, capable of consuming C5 and C6 sugars, critically contributed to CR. We propose that E. coli provides CR by depleting a limited carbon source when in a microbial community adept at removing simple sugars from the intestine.

The human symbiont Mucispirillum schaedleri: causality in health and disease.

Trillions of bacteria inhabit the mammalian gastrointestinal tract. In the majority of hosts, these symbionts contribute largely to beneficial functions promoting microbe-host homeostasis. However, an increasing number of human diseases is associated with altered microbiota composition and enrichment of certain bacterial species. A well-known example of this is Mucispirillum schaedleri, which has been associated with inflammatory conditions in the intestine. Mucispirillum spp. belong to the phylum Deferribacteres and are prevalent but low abundant members of the rodent, pig and human microbiota. Recently, M. schaedleri was causally linked to the development of Crohn's disease-like colitis in immunodeficient mice. While this study certifies a considerable pathogenic potential, the same organism can also promote health in the immunocompetent host: M. schaedleri protects from Salmonella enterica serovar Typhimurium (S. Tm)-induced colitis by interfering with the expression of the pathogen´s invasion machinery. In this review, we summarize the current knowledge on the mammalian gut symbiont M. schaedleri and its role in intestinal homeostasis and discuss open questions and perspectives for future research.