The cell wall is an essential structure in bacteria that provides protection to the cell and determines cell shape. Expansion of bacteria relies on inserting new cell wall material at designated sites in the cell. In a subset of bacteria, particularly within the phylum actinobacteria, this process occurs at the cell poles. In the well-defined models of polar growth, the cytoskeletal protein DivlVA plays a core role by providing a scaffold that recruits the biosynthetic machinery for peptidoglycan at growing tips (1). In addition to coordinating peptidoglycan synthesis, DivIVA also interacts with proteins responsible for the synthesis of a cellulose-like glycan (2), which is crucial for providing protection during cell wall remodeling that occurs at hyphal tips (3). Both processes are essential for maintaining cell wall integrity during tip extension. How both processed are properly coordinated in space and time was not clear. In our latest paper in Nature Communications, we identified a stomatin-like protein, called StlP, in _Streptomyces coelicolor_, which plays a crucial role in maintaining normal polar growth under hyperosmotic stress conditions by forming a membrane microdomain.
This membrane microdomain contributes to establishing a region with high membrane fluidity and localization of the machinery required for synthesis of peptidoglycan (PG) and the cellulose-like glycan. Deletion of StlP resulted in a remarkable thinning of the cell wall and the extrusion of cells at hyphal tips (see Figure below). We had observed the ability to extrude such cells before in another filamentous actinobacterium called _Kitasatospora viridifaciens_, that naturally lacks an StlP homologue. Excitingly, expression of StlP in _K. viridifaciens_ led to the appearance of a membrane microdomain under conditions of hyperosmotic stress and prevented the extrusion of cell wall-deficient cells. Given that StlP is widespread across actinobacteria, our data suggest a broader role for such membrane microdomains in stress adaption of these organisms.
From our study, we now understand how cell wall-deficient (CWD) cells are formed in these actinobacteria—namely, through an imbalance between cell wall and membrane synthesis. This mechanism may be widespread among bacteria capable of forming such cells. The ability to transition into a CWD state can offer several advantages to bacteria, such as enhanced uptake of genetic material and increased resistance to external stresses like osmotic pressure and antibiotics, which is particularly relevant in clinical contexts (4). Notably, CWD cells have been observed in environments such as mammalian macrophages (5) and the human urinary tract (6). Understanding the mechanism behind CWD cell emergence not only sheds light on their biological significance but may also provide valuable leads for developing strategies to combat bacteria that naturally produce these cells.
References
1. Klas Flärdh "Essential role of DivIVA in polar growth and morphogenesis in _Streptomyces coelicolor_ A3 (2)." _Molecular Microbiology_ 6 (2003): 1523-1536.
2. Hongbin Xu et al. "A cellulose synthase-like protein involved in hyphal tip growth and morphological differentiation in _Streptomyces_." _Journal of Bacteriology_ 14 (2008): 4971-4978.
3. Xiaobo Zhong et al. "Role for a lytic polysaccharide monooxygenase in cell wall remodeling in Streptomyces coelicolor." _mBio_ (2022): e00456-22.
4. Dennis Claessen and Jeff Errington. "Cell wall deficiency as a coping strategy for stress." _Trends in Microbiology_ 12 (2019): 1025-1033.
5. Yoshikazu Kawai et al. "Lysozyme counteracts β-lactam antibiotics by promoting the emergence of L-form bacteria." _Cell_ 5 (2018): 1038-1049.
6. Marjolein Crooijmans et al. "Enhanced extracellular matrix production provides protection to cell wall-deficient _Escherichia col_i." _bioRxiv_ (2024): 2024.11.06.622226; doi: https://doi.org/10.1101/2024.11.06.622226
Membrane microdomains facilitated by StlP are observed at hyphal tips of the wild-type strain M145, but are absent in the _stlP_mutant (**A**). Panel **B** shows the extrusion of cell wall deficient-cells in the _stlP_ mutant, a phenotype not observed in the wild-type strain M145.