A bacterium with a single flagellum at each of two opposite ends
Explanation
Amphitrichous bacteria are characterized by having one whip‑like flagellum at both ends of the cell. The term comes from the Greek prefix “amphi‑,” meaning both, and “-trichous,” referring to hair. Flagella are rotating appendages used for propulsion; in amphitrichous cells only one flagellum is typically active at a time so the bacterium can change direction efficiently. This arrangement is a form of polar flagellation distinct from monotrichous (single flagellum at one pole), lophotrichous (tufts of flagella at one or both poles) and peritrichous (flagella all over the cell). Amphitrichous motility enables bacteria to swim forward and reverse, benefiting survival in viscous or heterogeneous environments. When the flagellum at one pole rotates counter‑clockwise, it propels the cell forward; when the opposite flagellum engages, the organism reverses. This type of motility has been studied in curved and spiral bacteria where polarity and flexibility are important for swimming in host tissues and aquatic ecosystems. The distribution of flagella is genetically determined and can be influenced by environmental conditions. Laboratory observation through electron microscopy or flagella staining reveals one flagellum emerging from each end of amphitrichous species. Because both flagella originate at opposing poles, energy expenditure is balanced and changes in direction are precise. Understanding these arrangements aids microbiologists in identifying bacteria and interpreting their movement behavior.
Examples and notable facts
Several genera exhibit amphitrichous flagella. Alcaligenes faecalis, Aquaspirillum serpens and Nitrosomonas are classic examples. Pathogenic species such as Campylobacter jejuni and the photosynthetic Rhodospirillum rubrum also display this arrangement. Some Magnetospirillum species are amphitrichous. In these organisms, only one flagellum operates at a time, allowing smooth directional changes. Amphitrichous flagella are also observed in certain cyanobacteria and free‑living nitrifying bacteria where precise movement helps locate nutrients. The ability to switch between poles is advantageous in viscous environments or when moving along mucosal surfaces. Because amphitrichous cells can quickly reverse direction, they are often effective colonizers of host tissues or aquatic habitats. Observation of their motility patterns in the laboratory helps differentiate species and can provide insight into pathogenicity.
Amphitrichous flagellation refers to a cell with a single flagellum at both poles. This polar arrangement allows bacteria to alternate movement between ends for precise navigation. Recognizing amphitrichous motility aids in bacterial identification and in understanding how microbes adapt to diverse environments.
Related Terms: Monotrichous, Lophotrichous, Peritrichous, Polar flagellum, Flagella