Amoebae receive surprising support in defense against viruses: The bacteria they are infected with prevent them from being destroyed by giant viruses. A research team of microbiologists has investigated how a virus infection proceeds when the amoebae are simultaneously infected with chlamydia.

The research team shows for the first time that intracellular bacteria known as symbionts protect their host against viruses. Amoebae are protists, single-celled microorganisms with a cell nucleus. Protists play a key role in food webs and ecosystem processes. Consequently, the results of the study suggest that the interaction between symbionts and viruses influence the flow of nutrients in ecosystems. The study is now published in the journal PNAS.

Under natural conditions, protists including the studied amoebae are often infected by bacterial symbionts, which include chlamydiae. Chlamydiae are primarily known as human pathogens. However, close relatives of these pathogenic chlamydiae have been discovered in a variety of animals and protists.

Since, according to current knowledge, chlamydial infection leads to a slower growth rate of the infected host, chlamydiae are commonly considered to be parasitic.

Parasitic bacteria have a negative effect on the host while symbionts with a positive effect are called mutualists. This new study shows that chlamydiae are actually mutualists rather than parasites here, since they protect the protists against lethal infections by giant viruses. After all, slower growth is better than dying.

Infections with bacteria, but also those with viruses, shape how populations of protists develop. The researchers wanted to know how a viral infection proceeds when protists are simultaneously infected with bacteria. In order to study a situation that might also occur in natural environments, the scientists first isolated amoebae, bacteria, and a giant virus from the same environmental sample.

Giant viruses have only been known for about 20 years. Their discovery challenges many previously held assumptions about viruses, for they are not only multiple times larger than all previously known viruses, but also possess genes that were previously thought to be characteristic of cellular organisms such as bacteria, animals, plants and fungi. According to current knowledge, they are completely harmless to animals and humans. Their natural hosts are single-celled organisms with a cell nucleus, the protists.

When giant viruses infect a host cell, they remodel the entire host cell and set up a so-called "virus factory." This viral factory produces hundreds of new virus particles until the host cell bursts and releases the new viruses. If the protist is infected with bacterial symbionts, this very process is blocked.

This study shows that the presence of the chlamydia does not prevent the virus from being taken up. However, the viruses subsequently cannot form a functional virus factory.

The interaction of the bacteria with the giant viruses inside the amoeba does not only have consequences for the host itself. A common hypothesis is that these intracellular interactions between giant viruses and bacterial symbionts have played a role in giant viruses becoming so complex. The study of virus-symbiont interactions could therefore provide answers to the question of how giant viruses evolved.

Defensive symbiosis against giant viruses in amoebae., Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2205856119