Prokaryotes are generally 10 times smaller than eukaryotic cells, but researchers found evidence of bacteria even smaller than that. In fact, they believe these types of bacteria could be the smallest form of life physically possible. A new paper published in Nature Communications describes the detailed microscopy techniques used to analyze the bacteria.

“These newly described ultra-small bacteria are an example of a subset of the microbial life on earth that we know almost nothing about,” senior author Jill Banfield from University of California, Berkeley said in a press release.

So exactly how small is “ultra-small”? They’re about 150 times smaller than a single E. coli bacterium, which means that the end of a single hair could hold over 150,000. If that doesn’t qualify as “ultra-small,” nothing does. 

The bacteria were collected from groundwater and represent three disparate phyla. Because of where they were found, the researchers believe they could be fairly common. The bacteria were cryogenically frozen in order to protect them during transport for imaging. 

“They’re enigmatic. These bacteria are detected in many environments and they probably play important roles in microbial communities and ecosystems. But we don’t yet fully understand what these ultra-small bacteria do,” Banfield continued.

Image credit: Berkeley Lab

“There isn’t a consensus over how small a free-living organism can be, and what the space optimization strategies may be for a cell at the lower size limit for life. Our research is a significant step in characterizing the size, shape, and internal structure of ultra-small cells,” added lead author Birgit Luef, who is now at the Norwegian University of Science and Technology.

Researchers believe that life has a lower size limit because a number of organelles are required in order for an organism to live. Microscopic imaging revealed that these bacteria are incredibly simple, with very basic function. They contained genetic material, pili for locomotion, and a small number of ribosomes. There was concern that these cells were not true representatives of their species and were small due to unnatural influences, but the researchers found evidence of cell division, indicating health and full function of the bacteria.

The researchers were also curious about the bacterial genomes. With so few parts to each bacterium, it doesn’t seem like there’s much they need to encode. It is presumed that the bacteria mooch metabolic processes from other bacterial species, but that has yet to be confirmed. The bacteria have genomes roughly one million base pairs long, and future study will seek to better understand their scope. As a comparison, strains of E. coli have genomes that are about 4.6 million base pairs in length.

“We don’t know the function of half the genes we found in the organisms from these three phyla,” Banfield concluded, expressing the need for future research with these tiny organisms.