Ultramicrobacteria

Ultramicrobacteria are extremely small bacteria defined by cell size, typically having diameters around 0.2–0.3 micrometers or volumes less than about 0.01 cubic micrometers. They represent some of the smallest self-replicating organisms known and are found in a variety of habitats, especially nutrient-poor or oligotrophic environments. Ultramicrobacteria can be free-living in aquatic systems or associated with particles, host organisms, or biofilms.

Morphology and genomes: These cells are often coccoid or diminutive rods and exhibit a high surface‑area‑to‑volume ratio. Many have streamlined or reduced genomes and rely on readily available nutrients from their surroundings; metabolic capabilities can be limited compared with larger bacteria, though strategies vary across lineages. Some ultramicrobacteria may participate in symbiotic or syntrophic interactions.

Detection and cultivation: Ultramicrobacteria are challenging to culture in the laboratory. Their study relies heavily on culture-independent approaches, including flow cytometry to identify small cells, fluorescence microscopy, and environmental genomics and single‑cell sequencing. The abundant marine SAR11 clade (for example, Candidatus Pelagibacter ubique) is a well-known example of ultra-small, free-living bacteria in the ocean.

Filterability and taxonomy: Because of their tiny size, ultramicrobacteria can pass through filters designed to retain typical bacterial cells, which historically led to confusion with viral particles or nanobacteria. Modern genomics and microscopy confirm their status as cellular bacteria.

Importance: Understanding ultramicrobacteria sheds light on minimal cell size, nutrient limitation, and microbial ecology in oligotrophic environments, contributing to models of global biogeochemical cycles and expanding knowledge of microbial diversity.