Abstract
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 hyperosmotic 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.
Highlights
The cell wall is a shape-defining structure that envelopes almost all bacteria and protects them from environmental stresses
The sites for the incorporation of new PG is a major difference between the planktonic firmicutes that grow by extension of the lateral wall, and Actinobacteria, which grow via apical extension and thereby incorporating new PG at the cell poles[9,10]
While the cell wall is considered an essential component of virtually all bacteria, most species can be manipulated under laboratory conditions to produce so-called L-forms that are able to propagate without their wall[14,15,16,17]
Summary
The cell wall is a shape-defining structure that envelopes almost all bacteria and protects them from environmental stresses. Bacterial cells typically respond to osmotic changes by rapidly modulating the osmotic potential within the cell, either by importing or exporting ions and compatible solutes[1] While these responses typically occur immediately after cells have been exposed to the changed environment, they are able to tune the expression of metabolic pathways or critical enzymes[2]. Actinobacteria display a wide diversity of morphologies, including cocci (Rhodococcus), rods (Mycobacterium and Corynebacterium) and mycelia (Streptomyces and Kitasatospora), or even multiple shapes (Arthrobacter)[11,12] Species belonging to these genera are able to change their morphology to adapt to extreme environments. Proliferation of L-forms is independent of the FtsZ-based division machinery[15,22] Instead, their proliferation can be explained solely by biophysical processes, in which an imbalance between the cell surface area to volume ratio leads to spontaneous blebbing and the subsequent generation of progeny cells[20]. Whether L-forms have functional relevance in modern bacteria, is unclear
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