Didermal

Didermal describes organisms whose cell envelope comprises two membranes: an inner cytoplasmic membrane and an outer membrane separated by a periplasm that contains a thin layer of peptidoglycan. The term is most often applied to bacteria that are typically Gram-negative, in contrast to monoderm bacteria that have a single membrane and a thicker peptidoglycan cell wall.

Key characteristics of didermal envelopes include an outer membrane enriched with lipopolysaccharide in many groups, porins that allow the diffusion of small molecules, and a defined periplasmic space housing various enzymes and transport proteins. This architecture provides a selective barrier that influences nutrient uptake, interaction with the environment, and intrinsic resistance to certain antibiotics. Outer-membrane proteins and complex secretory systems are common features of diderms.

Etymology: didermal combines di- meaning two with derma, used here to denote a membrane envelope rather than skin in the literal sense. In microbiology, the term highlights the presence of two membranes rather than the amount of peptidoglycan alone.

Didermal bacteria include most major Gram-negative lineages, such as many Proteobacteria and related phyla. The diderm versus monoderm distinction is used to discuss cell envelope evolution, with hypotheses ranging from a single origin of the diderm envelope followed by secondary simplification in some lineages to multiple acquisitions of outer membranes in different lineages. In practical terms, diderms often exhibit a broader repertoire of outer-membrane proteins, virulence factors, and mechanisms for interactions with hosts and environments.