a hypothetical class of living organisms; specifically cell-walled microorganisms with a size much smaller than the generally accepted lower limits for life, 200 nm for bacteria.

Scientists have captured the first detailed microscopy images of ultra-small bacteria believed to be about as small as life can get. The bacterial cells have densely packed spirals that are probably DNA, a very small number of ribosomes, hair-like appendages, and a stripped-down metabolism that likely requires them to rely on other bacteria for many of life’s necessities.

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