Univ.-Prof. Dr. Matthias Horn

Matthias Horn
Head of the Department of Microbiology and Ecosystem Science
Head of the Division of Microbial Ecology
University of Vienna
Centre for Microbiology and Environmental Systems Science
Division of Microbial Ecology
Djerassiplatz 1
A-1030 Vienna
Phone: +43 1 4277 91208

Microbial symbioses

Symbiosis, the living together of two dissimilar organisms (De Bary, 1898), is a key principle in nature which contributed to the origin of eukaryotes around two billion years ago and continues to have a profound impact on the ecology and evolution of extant organisms. My lab is interested in microbial symbioses in which eukaryotes serve as hosts for intracellular bacteria. By focusing on selected model systems, we are studying molecular and evolutionary mechanisms underlying and driving these associations.

Chlamydiae are important pathogens of humans but also occur as symbionts of environmental, free-living amoebae. Analysis of only recently discovered chlamydial symbionts such as Protochlamydia amoebophila, Parachlamydia acanthamoebae, and Simkania negevensis, and comparison with their pathogenic counterparts allows us to investigate the evolution of the intracellular life style of chlamydiae and their adaptation to different eukaryotic hosts. This is complemented by the analysis of other, phylogenetically distant bacterial symbionts of amoebae, including Amoebophilus asiaticus.

Another model for symbiosis and the interaction between microbes and eukaryotes studied in the lab are insects and their bacterial symbionts. We are particularly interested in the less well studied microbial symbionts such as the reproductive manipulator Cardinium hertigii, and those of mealybugs and adelgids.

Common to all our studies is the lack of methods to cultivate the microbial symbiosis partners in the lab. We thus use a spectrum of cultivation-independent, molecular biology techniques including the 16S rRNA approach and fluorescence in situ hybridization, comparative genomics, metagenomics, and transcriptomics using next generation sequencing, proteomics, Raman microspectroscopy, phylogenetic and molecular evolution analyses.

Research interests

  •     Microbial symbioses, with emphasis on bacterial symbionts of protozoa and insects
  •     Evolution of intracellular bacteria
  •     Microbial genome evolution
  •     Molecular biology of intracellular bacteria and their interaction with eukaryotic host cells

Recent projects

Chlamydial peptidoglycan. The majority of bacteria possess a peptidoglycan sacculus consisting of a disaccharide backbone crosslinked by peptide chains, which is crucial for cell division, maintaining cell shape and resisting osmotic stress. Whether chlamydiae contain this structure has long been debated, but in an international collaboration we have shown that the amoeba symbiont Protochlamydia amoebophila does synthesize peptidoglycan (Pilhofer, Aistleitner, et al., 2013)

Massive expansion of gene families in the Chlamydiae. Analysing the gene family landscape of members of the phylum Chlamydiae, we discovered massive and lineage-specific expansions of eukaryotic-like ubiquitination-related genes, unmatched among bacteria. Gene birth-and-death evolution in concert with genomic drift might be responsible for the evolution of these gene families, which represents a previously undescribed mechanism by which isolated bacterial populations diversify (Domman et al. 2014)

Nucleicultrix - a bacterial symbiont in the control center of its eukaryotic host. We recently discovered a bacterial symbiont with an unusual intracellular niche. Within few hours after infection the bacteria named Nucleicultrix amoebiphila invade the nucleus of amoebae, where they mutliply with surprisingly little effect on host fitness. This microbial association is an ideal model system to further investigate evolution and molecular mechanisms of the enigmatic intranuclear lifestyle (Schulz et al., 2014; Schulz and Horn, 2015).

Current Projects

Selected Publications

  • Domman D, Horn M, Embley TM, Williams TA. 2015. Plastid establishment did not require a chlamydial partner. Nat. Commun. 6: 6421.
  • Schulz F, Horn M. 2015. Intranuclear bacteria: inside the cellular control center of eukaryotes. Trends Cell Biol. doi: 10.1016/j.tcb.2015.01.002.
  • Domman D, Collingro A, Lagkouvardos I, Gehre L, Weinmaier T, Rattei T, Subtil A, Horn M. 2014. Massive expansion of ubiquitination-related gene families within the Chlamydiae. Mol. Biol. Evol. 31: 2890-2904.
  • Schulz F, Lagkouvardos I, Wascher F, Aistleitner K, Kostanjsek R, Horn M. 2014. Life in an unusual intracellular niche: a bacterial symbiont infecting the nucleus of amoebae. ISME J. 8: 1634-1644.
  • Pilhofer M, Aistleitner K, Biboy J, Gray J, Kuru E, Hall E, Brun YV, VanNieuwenhze MS, Vollmer W, Horn M, Jensen GJ. 2013. Discovery of chlamydial peptidoglycan reveals bacteria with murein sacculi but without FtsZ. Nat. Commun. 4: 2856.
  • Sixt BS, Siegl A, Müller C, Watzka M, Wultsch A, Tziotis D, Montanaro J, Richter A, Schmitt-Kopplin P, Horn M. 2013. Metabolic features of Protochlamydia amoebophila elementary bodies - a link between activity and infectivity in Chlamydiae. PLoS Pathogens 9: e1003553.
  • Penz T, Schmitz-Esser S, Kelly SE, Cass BN, Müller A, Woyke T, Malfatti SE, Hunter MS, Horn M. 2012. Comparative genomics suggests an independent origin of cytoplasmic incompatibility in Cardinium hertigii. PLoS Genetics 8: e1003012.
  • Toenshoff ER, Penz T, Narzt T, Collingro A, Schmitz-Esser S, Pfeiffer S, Klepal W, Wagner M, Weinmaier T, Rattei T, Horn M. 2012. Bacteriocyte-associated gammaproteobacterial symbionts of the Adelges nordmannianae/piceae complex (Hemiptera: Adelgidae) ISME J. 6: 384-396.
  • Collingro A, Tischler P, Weinmaier T, Penz T, Heinz E, Brunham RC, Read TD, Bavoil PM, Sachse K, Kahane S, Friedman MG, Rattei T, Myers GSA, Horn M. 2011. Unity in variety - the pan-genome of the Chlamydiae. Mol. Biol. Evol. 28: 3253-3270.
  • Horn M. 2008. Chlamydiae as symbionts in eukaryotes. Ann. Rev. Microbiol. 62: 113-131.
  • Horn M, Collingro A, Schmitz-Esser S, Beier CL, Purkhold U, Fartmann B, Brandt P, Nyakatura GJ, Droege M, Frishman D, Rattei T, Mewes HW, Wagner M. 2004. Illuminating the evolutionary history of chlamydiae. Science 304: 728-730.

The Group

Joining the team

Information on open research positions can be found here. If you are interested in joining our team with your own fellowship, please check out our PhD & postdoc program and get in touch with Matthias for details.