Our gut microbiome is home to trillions of microbes, but they may not all be microscopic. Giant bacteria, which can reach a centimetre in length, are generally found in niche environments, such as mangroves and fish guts. But now, microbiologists have detected signs of DNA from , one of the largest known types of bacteria, in human gut microbiomes from around the world.
Bacteria are usually around 2 micrometres long, while giant ones are generally defined as being larger than 10 micrometres. But species belonging to the genus can reach up to 600 micrometres – more than half a centimetre – in length.
These are usually found in the intestines of surgeonfish, but Danny Ionescu at the Technical University of Berlin noticed DNA sequences related to in other fish species. To find out how wide-ranging they are, Ionescu and his colleagues have searched DNA sequences amplified from more than 600,000 microbiomes. Most of these were publicly available sequences, while the rest were from samples collected by Ionescu’s team.
Within the microbiomes, which came from a range of species from more than 80 countries, the researchers found “tens of thousands of positives” for , says Ionescu, in animals from shellfish to ferrets.
They also found species in thousands of human microbiomes. These generally accounted for only 1 to 2 per cent of the DNA sequences identified in a sample, but in rare cases, they reached up to 20 per cent.
In surgeonfish, species alter intestinal pH and digestive enzyme activity, but it is unclear what impact they might have in people. The researchers did find, however, that they were most abundant in the first year of life. They were also more likely to be found in people with gastrointestinal conditions associated with inflammation, such as colorectal cancer and irritable bowel disease.
Rather than causing these conditions, “we think the [bacterial] organism is taking the opportunity of imbalance in the microbiome”, says Ionescu. Giant bacteria have hundreds, or even thousands, of different chromosomes in every cell, and this huge genetic diversity helps them thrive in unstable conditions. They might therefore be useful as a marker of a dysregulated gut before symptoms appear, says Ionescu.
The presence of giant bacteria in our guts “would be a very big surprise”, says Nicola Segata at the University of Trento, Italy. He says that looking for DNA sequences might pick up unrelated bacteria that aren’t necessarily giant but happen to have similar genes. “I wouldn’t be surprised at all if what they are actually seeing is another, maybe unknown, bacterium.”
Ionescu counters that even if the genes don’t belong to , their owner is probably closely related, with similar physical characteristics. “I would not expect it to be normal scale,” he says.
Jean-Marie Volland at the University of California, Santa Barbara, who was part of a team that discovered the largest known bacterium in 2022, says he has noticed -related sequences in unexpected species, such as guinea pigs, and also suspects they have a far wider host range than we thought.
But to prove that the bacterial owners of the -related genes seen in people really are giant, scientists will have to see them directly through a microscope. Ionescu is now conducting studies using fluorescent probes designed to bind to the identified sequences to uncover these bacteria. “When we get the pictures, we’ll have the proof,” he says.