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, yielding wall-less cells known as L-forms. We have shown that several species of filamentous actinomycetes have a natural ability to generate wall-deficient cells in response to hyperosmotic stress. This wall-deficient state is transient and these cells are able to switch to the normal mycelial mode of growth. Interestingly, prolonged exposure to hyperosmotic stress yields variants that are able to proliferate indefinitely without their cell wall, similarly to L-forms. This indicates that the formation of wall-deficient cells in actinomycetes serves as an adaptation to osmotic stress. We are currently studying this remarkable response in more detail, and exploit the unique properties of these cells to obtain fundamental new insight into the plasticity of bacterial life.


  • D. Claessen & J. Errington (2019). Cell wall-deficiency as a coping strategy for stress. Invited review for Trends in Microbiology.
  • E. Ultee, K. Ramijan, R.T. Dame, A. Briegel & D. Claessen (2019). Stress-induced adaptive morphogenesis in bacteria. Adv Microb Phys 74 (in press).
  • K. Ramijan, E. Ultee, J. Willemse, Z. Zhang, J.A.J. Wondergem, A. van der Meij, D. Heinrich, A. Briegel, G.P. van Wezel & D. Claessen (2018). Stress-induced formation of cell wall-deficient cells in filamentous actinomycetes. Nat Commun 9: 5164.