Mammal cells use some viruses like vitamins, study hints

On the menu tonight, a nice, nutritional, bacteria-killing virus. Sounds unappealing? It may not be to your cells.

In a new study, scientists revealed that a type of bacteriophage — a virus that infects and kills bacteria — found in the human gut helps mammal cells grow and thrive in what could be a symbiotic relationship. That's a surprise, as other bacteriophages (phages for short) are known to trigger inflammatory responses when they encounter mammalian cells.

This phenomenon, described Thursday (Oct. 26) in the journal PLOS Biology, was only demonstrated in cells in the lab. However, the authors hope the findings will aid future research that could impact human health, such as supplementing studies investigating phage therapy to treat infections with antibiotic-resistant superbugs.

"[The study] opens up a new area of symbiosis and symbiotic interactions between phages and mammalian cells," senior study author Jeremy Barr,  an associate professor of biological sciences at Monash University in Australia, told Live Science. "I think this study suggests that there may be a lot more that we're unaware of."

Related: Could bacteria-killing viruses ever prevent sexually transmitted infections?

Phages are the most abundant biological entities on the planet. They're extremely small, with most ranging in size from around 24 to 200 nanometers; to put that in perspective, a penny is about 19 million nanometers long. They're made up of a DNA or RNA genome surrounded by a protein shell. Although interactions between phages and bacteria are relatively well studied, the same can't be said for those between the former and mammalian cells.

In the study, the authors looked at a well-known phage species called T4 that normally infects Escherichia coli bacteria. They applied T4 to three types of mammalian cells in the lab: an immune cell called a macrophage that had been extracted from mouse tissue; and human lung and dog kidney cells derived from cancer cell lines.

The T4 phages didn't activate DNA-mediated inflammatory processes in the cells. Instead they triggered signaling pathways that promote cell growth and survival, resulting in increased cellular metabolism and the reorganization of actin, a protein that is found in the fluid-filled space inside mammalian cells. Actin reorganization is needed for cells to uptake material via macropinocytosis, a phenomenon also known as "cell drinking."

The broader health impacts of the study are still unknown, Barr said. The authors also only looked at one phage species, while estimates suggest there are as many as 10^15 phages in the gut). In addition, the results may be a side effect of using immortalized cancer cell lines, which are already more likely to grow and proliferate, he said.

Nevertheless, the find should spur follow-up research. Phage therapy is generally considered to be safe, although it's still early in the clinical trial process, and the current study now suggests that there's "many, many other potential impacts" that phages may have on human cells, Barr said.

Another avenue where the research could be applied is in the gut microbiome.

"There's some really interesting research showing that there's certain gut communities associated with inflammatory disorders — IBD [inflammatory bowel disease], Crohn's disease — that have virus signatures associated with them," Barr said.

"This is very much conjecture and extrapolation but it's interesting to think that maybe phages do play a role in this and there may be some inflammatory interactions, and maybe some also beneficial interactions in a more sort of homeostasis gut microbiome system," he said.

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