ARTICLE Stress-induced formation of cell wall-deficient cells in filamentous actinomycetes Karina Ramijan 1 , Eveline Ultee 1 , Joost Willemse 1 , Zheren Zhang 1 , Joeri A. J. Wondergem 2 , Anne van der Meij 1 , Doris Heinrich 2,3 , Ariane Briegel 1 , Gilles P. van Wezel 1 & Dennis Claessen 1 The cell wall is a shape-defining structure that envelopes almost all bacteria and protects them from environmental stresses. Bacteria can be forced to grow without a cell wall under certain conditions that interfere with cell wall synthesis, but the relevance of these wall-less cells (known as L-forms) is unclear. Here, we show that several species of filamentous actinomycetes have a natural ability to generate wall-deficient cells in response to hyper- osmotic stress, which we call S-cells. This wall-deficient state is transient, as S-cells are able to switch to the normal mycelial mode of growth. However, prolonged exposure of S-cells to hyperosmotic stress yields variants that are able to proliferate indefinitely without their cell wall, similarly to L-forms. We propose that formation of wall-deficient cells in actinomycetes may serve as an adaptation to osmotic stress. DOI: 10.1038/s41467-018-07560-9 OPEN 1 Molecular Biotechnology, Institute of Biology, Leiden University, P.O. Box 9505, 2300 RA Leiden, The Netherlands. 2 Biological and Soft Matter Physics, Huygens-Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands. 3 Fraunhofer Institute for Silicate Research ISC, Neunerplatz 2, 97082 Würzburg, Germany. Correspondence and requests for materials should be addressed to D.C. (email: D.Claessen@biology.leidenuniv.nl) NATURE COMMUNICATIONS | (2018)9:5164 | DOI: 10.1038/s41467-018-07560-9 | www.nature.com/naturecommunications 1 1234567890():,;