nonflagellar

Nonflagellar is an adjective used in biology to describe cells, organisms, or structures that lack flagella, the slender whip-like appendages used by many bacteria and other organisms for locomotion or sensory purposes. The term is applied across biological contexts to distinguish flagellated from nonflagellated forms.

In microbiology, nonflagellar species are common and include many Staphylococcus aureus and Streptococcus species, as well as several nonmotile enteric pathogens. Nonflagellar does not necessarily imply immobility: some nonflagellated bacteria move by alternative means, such as gliding motility or twitching motility, which rely on surface-associated mechanisms like type IV pili. Others are truly nonmotile. The presence or absence of flagella can reflect evolutionary adaptation; flagellar genes may be lost, silenced, or conditionally expressed depending on environmental circumstances.

Some bacteria can switch between flagellated and nonflagellated states in response to cues, balancing the metabolic cost of producing a flagellum with the need for movement or chemotaxis. Beyond bacteria, the term also applies to eukaryotes, including plants, fungi, and many algae, where flagella occur only in certain life stages or are absent entirely. Nonflagellar traits thus intersect with taxonomy, ecology, and studies of cell motility and surface interactions.