Nonwalled

Nonwalled refers to organisms or cells that lack a rigid cell wall. In microorganisms, this means the absence of peptidoglycan or similar structural layers that normally define the cell boundary, leaving the cell membrane as the primary protective and shape-determining structure. Nonwalled forms can occur naturally or be induced under certain conditions, such as exposure to antibiotics or environmental stress.

In bacteria and archaea, nonwalled organisms include the well-known Mycoplasma and Ureaplasma species, which lack peptidoglycan and instead depend on cholesterol or other sterols in their membranes for stability. These organisms are typically more flexible in shape (pleomorphic) and are highly sensitive to osmotic changes. Another concept is the cell wall–deficient (CWD) or L-form state, in which bacteria temporarily lose their cell wall and adopt a wall-deficient form. L-forms can arise in response to antibiotics targeting cell wall synthesis and may revert to walled forms under favorable conditions, complicating treatment strategies.

In contrast, many eukaryotes do not have cell walls by default. Animal cells, some protozoa, and many cells in multicellular organisms rely on a surrounding extracellular matrix or membrane for structure, rather than a rigid wall. Plant cells, fungi, and certain algae typically possess cell walls made of cellulose, chitin, or related polymers; their lack is unusual and usually associated with animal cells or certain life stages of protists.

Term usage is descriptive rather than taxonomic. Nonwalled denotes a state with specific biological implications, including altered osmotic tolerance, changes in morphology, and differences in how the organism interacts with its environment and host.