Bacteria in the gut helps prime the immune system to respond to early signs of viruses invading the lungs and suppress the infection, researchers, led by the Francis Crick Institute in London, found.
Tests in mice infected with influenza found they were three times more likely to die after receiving a course of antibiotics as those left to fight the illness alone.
This was “further evidence that antibiotics should not be taken or prescribed lightly” the researchers said.
Inappropriate use of the drugs, often given for viral infections where they are ineffective or to help fatten up livestock, wipes out beneficial and harmful bacteria and increases the pressure for the bugs to adapt.
This is leading to untreatable infections like “super-gonorrhoea” which can even withstand drugs held back as a last resort.
Gut bacteria ensure that antiviral genes in the lung lining stay active and are ready to react as a first line of defence when flu emerges, the researchers found.
The cells lining the lung "are the only place that the virus can multiply, so they are the key battleground in the fight against flu,” they reported in the journal, Cell Reports.
They added: “Gut bacteria send a signal that keeps the cells lining the lung prepared, preventing the virus from multiplying so quickly.”
A third of the mice survived when given antibiotics before becoming infected, compared to 80 per cent who survived without the drugs, in the study.
The virus lies dormant before symptoms emerge and two days after infection, mice which had received antibiotics had five times more virus in their lungs.
This means that when the immune system does identify the threat its response has to be much stronger.
This immune response can be just as harmful as the infection, as happens with sepsis which can cause organ failure, lost limbs and death.
“We found that antibiotics can wipe out early flu resistance, adding further evidence that they should not be taken or prescribed lightly,” says the study’s lead researcher, Dr Andreas Wack. “Inappropriate use not only promotes antibiotic resistance and kills helpful gut bacteria, but may also leave us more vulnerable to viruses.”
He said the findings were “not only relevant in humans” as it could show similar risks for livestock herds treated with antibiotics.
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