The question is one that scientists have hypothesized and debated over since Antonie van Leeuwenhoek first described the single-celled organisms he viewed through his handcrafted microscopes in the 17th century. Thankfully, techniques and equipment have improved exponentially since then, allowing researchers from the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California, Berkeley to view ultra-small bacteria that they believe are as small as life can get.     

The findings from this study were published recently in Nature Communications through an article entitled “Diverse uncultivated ultra-small bacterial cells in groundwater”.

The investigators found that the bacterial cells had an average volume of 0.009 cubic microns. To put that into perspective, approximately 150 of these bacteria could fit inside an E. coli cell or 150,000 of the tiny bacteria could fit on the tip of a human hair.

“These newly described ultra-small bacteria are an example of a subset of the microbial life on earth that we know almost nothing about,” said Jill Banfield, Ph.D., professor at UC Berkeley in the department of earth and planetary science and senior author on the current study. “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.”

Dr. Banfield and her team isolated the bacteria from groundwater, collected in Rifle, Colorado, using successively smaller filters, down to 0.2 microns, which is the size most commonly used by scientists to sterilize water. However, they found that instead of the resultant sample being sterile, it was enriched with the exceptionally small microbes. The scientists flash froze the enriched samples for visualization using 2D and 3D cryogenic transmission electron microscopy.

“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,” explained Birgit Luef, Ph.D., former postdoctoral researcher in Dr. Bainfield's lab and first author on the study.

Amazingly, Dr. Banfield’s team was able to capture many of the bacteria in the process of cell division, which was indicative of the cells being healthy and of normal size for their species. Moreover, the team revealed some interesting observations about the bacteria’s internal composition, such as densely packed spirals, which they concluded to be DNA, a few ribosomes and hair-like appendages called pili.  

The scientists were able to sequence genomic DNA samples taken from the bacteria and found a diverse range of organisms belonging to the phyla OP11, WWE3, and OD1. Additionally, they observed that the bacteria had stripped-down metabolisms that likely require them to depend on other bacteria for essential metabolic nutrients.

“We don't know the function of half the genes we found in the organisms from these three phyla,” concluded Dr. Banfield.

The researchers are energized by their findings nonetheless, which they believe highlight the existence of extremely small cells that form a unique biological niche.