Protection Against Viruses – The Passive Version

As humans, we often focus on viruses and bacteria because of their role in causing various, sometimes severe, diseases. However, an equally fascinating but lesser-known phenomenon is the ongoing battle between bacteria and bacteriophages—viruses that specifically target and infect bacteria.

This struggle, which takes place in oceans, drives the co-evolution of these two populations. In some oceanic regions, viral infections are a major cause of bacterial mortality. Without the resistance mechanisms that bacteria have developed over time, bacterial populations might have been wiped out over evolutionary time.

A new study, published in Nature Microbiology by researchers from the Technion – Israel Institute of Technology, reveals a previously unknown mode of resistance. The study was led by Prof. Debbie Lindell, former Ph.D. student Dr. Sofia Zborovsky (currently a postdoctoral fellow in the UK), and Ph.D. student Ran Tahan.

Prof. Debbie Lindell and PhD student Ran Tahan

Prof. Lindell’s research group has been exploring this field for years and has already made dramatic discoveries about bacteriophage-bacteria interactions in marine environments. Their new study uncovers a passive defense mechanism based on an exceptionally low level of molecules involved in translation of genetic material, the process that leads to protein formation.

Dr. Sofia Zborovsky

The research focuses on the marine bacterium Synechococcus and its interaction with the bacteriophage Syn9. Synechococcus, a cyanobacterium, is a primary producer that generates food from inorganic substances and produces oxygen through photosynthesis. These bacteria are crucial for atmospheric oxygen production and are at the base of the oceanic food chain.

Without defense mechanisms, Synechococcus would likely have become extinct because of the threats it faces from bacteriophages like Syn9. The Technion researchers’ study describes an evolutionary scenario where protection arises from a reduced level of tRNA (transfer RNA), a molecule critical for gene translation.

“Studies on resistance often focus on genes providing active defense against infection,” explained Prof. Lindell. “However, not all defenses stem from active mechanisms; some, like the one we discovered, arise from ‘passive resistance.’ Our findings show that normal tRNA level reduce bacterial resistance to the virus, while low levels increase such resistance. This is a passive mode of resistance where the loss of a certain intracellular function leads to resistance against viral infection.”

In the scientific image: the virus attaches to the bacterial cell (left image) and kills it (right image). Photos: Lihi Shaulov and Gazalah Sabehi

Prof. Lindell added that this mechanism does not prevent the phage from entering the bacterial cell but halts the formation of new viruses, allowing the bacteria to survive. “The fact that certain Synechococcus strains possess multiple defense mechanisms, and that no virus can infect them, suggests that marine bacteria have evolved several layers of protection, some passive, enabling them to withstand a wide range of marine viruses. We believe this passive defense layer evolved gradually due to selective pressure, where bacteria with reduced tRNA levels survived better and passed on their traits to subsequent generations.”

This phenomenon of passive resistance, the researchers suggest, is likely more widespread than previously thought and not limited to Synechococcus-Syn9 interactions.

The study was supported by the Simons Foundation.

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