Monodermal

Monodermal is an adjective used in biology to describe organisms or cells that possess a single biological membrane, in contrast to didermal organisms, which have two membranes (an inner membrane and an outer membrane). The term is most often applied to bacteria and, to a lesser extent, archaea, where it refers to envelope architecture rather than taxonomy.

In bacteria, monoderm refers to cells with a cytoplasmic (inner) membrane and a thick peptidoglycan layer, with no outer membrane. This envelope is typical of many Gram-positive bacteria, such as Bacillus and Clostridium species, although Gram staining alone does not always reliably indicate envelope structure. In contrast, diderm bacteria have two membranes and an exposed periplasm, a feature common in Gram-negative lineages.

In archaea, monoderm envelopes are also common, with one cytoplasmic membrane beneath diverse cell wall structures, including protein S-layers or pseudopeptidoglycan. The monoderm-diderm distinction is a functional and evolutionary framework rather than a strict taxonomic classification.

Evolution and classification: The division into monoderm and diderm is used to discuss the evolution of bacterial outer membranes. Hypotheses differ: some argue for a single ancestral diderm with subsequent loss in several lineages, while others propose multiple independent origins or different losses. Ongoing genomic and structural studies continue to refine the relationship between cell envelope architecture and phylogeny.

Monodermal is thus a descriptive term in microbiology that helps compare cell envelope organization across prokaryotes and is not tied to a formal rank.