Challenge the endophytic view of bacterial evolution: in addition to host cells, chlamydia can also use the environment to evolve!

The phylum Chlamydia belongs to bacteria
.
Chlamydia phylum bacteria are smaller than ordinary bacteria, and some are smaller
than viruses.
Their growth takes place entirely within the cells of other organisms, and today all chlamydia live in host cells from amoebas to animals and are obligate parasites
.
Originally mostly classified into the genus Chlamydia, with the development of molecular biology, it is now divided into four families according to the phylogenetic tree, including Chlamydial pneumoniae, Chlamydial psittacit, Chlamydial trachomatis and Chlamydia bovine
.

A team of scientists from the University of Vienna and Wageningen University found that the ancestors of chlamydia may have lived inside host cells, but that amoeba-infected chlamydia later evolved
in unexpected ways for intracellular bacteria.
This study is an important step
in understanding the emergence and evolution of endosymbiotic bacteria, including human pathogens.

While all chlamydia found today live in a wide range of hosts, from small amoebas to animal cells, evidence suggests that the group of bacteria Chlamydia trachomatis evolved 1 billion years ago, even much earlier than the first animals
.

Since the first sequencing of the chlamydia genome 20 years ago, scientists have puzzled that the chlamydia genome of infected animals is smaller and similar to other endosymbiotes, while the amoeba-infected chlamydia genome is larger and more similar to free-living bacteria
.
However, the study of the evolution of this diverse bacterial population has been hampered
by the difficulty of growing these microorganisms in the laboratory.

The teams around Matthias Horn (University of Vienna) and Thijs Ettema (Wageningen University and Research Center) can bypass this problem: "We have only recently gained the ability to sequence genomes directly from environmental samples to explore the breadth of chlamydia diversity," the researchers explain
.
With this new data, they traced the evolution of chlamydia
.
Using state-of-the-art computational methods, they reconstructed the genomes
of the last common ancestor of all known chlamydia.
The researchers found that "this extinct microbe possesses all the genes
needed to become an endosymbiote.
" Even genes important for today's chlamydia animal pathogens may already exist
.
"This means that chlamydia has been infecting host cells for more than 1 billion years of evolutionary history
.
"

However, to their surprise, the team also found that the chlamydia infected with amoeba only acquired many metabolic genes at a later stage, although endosymbionts had fewer
opportunities to exchange genes with other bacteria.
"Our findings suggest that gene exchange occurs in some chlamydia than expected endosymbiotes," the authors explain, "including access to key metabolic genes.
"

This result challenges our view of
endosymbiote evolution.
But the researchers also come up with a solution to this puzzle: "It's not so surprising when you think about the environment in which these chlamydia live: amoeba amoebas often have multiple endosymbionts that feed on free-living bacteria, so having other microbes around adds to the accessible gene pool
.
" In addition, most chlamydia move between different hosts, and exposure to changing environments could explain why it may be beneficial
for these endosymbionts to maintain or even acquire additional metabolic genes.
”

Scientists are curious to know if this symbiotic evolutionary model is more prevalent
.
In any case, this study is an important step
in understanding the emergence and evolution of endosymbiotic bacteria, including human pathogens.