First Enterococci Appeared on Earth during Ordovician Period, Says Study
According to a new study published today in the journal Cell , the enterococci — gram-positive aerobic bacteria and a leading cause of hospital-acquired infections — arose from an ancestor that dates back 450 million years (Ordovician period), about the time when animals were first crawling onto land.
This digitally-colorized scanning electron microscopic image depicts Enterococcus faecalis . Image credit: Pete Wardell / CDC.
“By analyzing the genomes and behaviors of today’s enterococci, we were able to rewind the clock back to their earliest existence and piece together a picture of how these organisms were shaped into what they are today,” said Dr. Ashlee Earl, group leader for the Bacterial Genomics Group at the Broad Institute of MIT and Harvard and co-corresponding author of the study.
“Understanding how the environment in which microbes live leads to new properties could help us to predict how microbes will adapt to the use of antibiotics, antimicrobial hand soaps, disinfectants and other products intended to control their spread.”
The picture Dr. Earl and he colleagues from Harvard Medical School, Massachusetts Eye and Ear, the Massachusetts Institute of Technology (MIT), and the Broad Institute of MIT and Harvard, pieced together begins with the dawn of life.
Bacteria arose nearly 4 billion years ago, and the Earth has teemed with them ever since, including the sea.
Animals first arose in the sea during the time known as the Cambrian Explosion, 542 million years ago.
As animals emerged in a sea of bacteria, bacteria learned to live in and on them. Some bacteria protect and serve the animals, as the healthy microorganisms in our intestines do today; others live in the environment, and still others cause disease.
As animals crawled onto land about 100 million years later, they took their bacteria with them.
Dr. Earl and co-authors found that all species of enterococci, including those that have never been found in hospitals, were naturally resistant to dryness, starvation, disinfectants and many antibiotics.
Because enterococci normally live in the intestines of most — if not all — land animals, it seemed likely that they were also in the intestines of land animals that are now extinct, including dinosaurs and the first millipede-like organisms to crawl onto land.
Comparison of the genomes of these bacteria provided evidence that this was indeed the case.
In fact, the authors found that new species of enterococci appeared whenever new types of animals appeared.
This includes when new types of animals arose right after they first crawled onto land, and when new types of animals arose right after mass extinctions, especially the greatest mass extinction, the end-Permian extinction (251 million years ago).
From sea animals, like fish, intestinal bacteria are excreted into the ocean, which usually contains about 5,000 mostly harmless bacteria per drop of water. They sink to the seafloor into microbe-rich sediments, and are consumed by worms, shellfish and other sea scavengers.
Those are then eaten by fish, and the bacteria continue to circulate throughout the food chain.
However, on land, intestinal bacteria are excreted as feces, where they often dry out and most die over time.
Not the enterococci, however. These bacteria are unusually hardy and can withstand drying out and starvation, which serves them well on land and in hospitals where disinfectants make it difficult for a microbe.
“We now know what genes were gained by enterococci hundreds of millions of years ago, when they became resistant to drying out, and to disinfectants and antibiotics that attack their cell walls,” said lead author Dr. Michael S. Gilmore, Sir William Osler professor of ophthalmology at Harvard Medical School and senior scientist at Massachusetts Eye and Ear. He also serves on the steering committees of the Harvard Microbial Sciences Initiative, and the Infectious Disease Initiative of the Broad Institute of MIT and Harvard.
“These are now targets for our research to design new types of antibiotics and disinfectants that specifically eliminate enterococci, to remove them as threats to hospitalized patients,” said first author Dr. Francois Lebreton, from Massachusetts Eye and Ear and Harvard Medical School.