Patients suffering from undiagnosed respiratory infections are often prescribed antibiotics before it's clear what kind of infection they have.

If their lung congestion turns out to be the flu, a viral infection, the treatment can backfire. Antibiotics can actually slow recovery from influenza, as new research by doctors at Cedars-Sinai Hospital in Los Angeles makes clear.

Antibiotics are one of the great achievements of modern medicine. They have helped countless patients fight infections of all kinds. But they are only effective against bacteria. They cannot kill or slow a viral infection.

“When a flu patient has a confirmed bacterial infection, you definitely need to prescribe antibiotics. But when there is no direct evidence of such an infection, antibiotics should not be prescribed because they will not help, and could, in fact, be harmful.”

Influenza, better known as the flu, is caused by a virus. Viruses, unlike bacteria, enter the body's cells and multiply. They cannot live apart from a host cell. Bacteria, in contrast, are independent agents. They can and do survive both inside the body's cells or outside in the air, soil and water, and even in extreme environments such as under the Arctic ice or near active volcanoes.

Treating influenza with an antibiotic disturbs the good bacteria and fungi that live in our gut. This microbiome is the central headquarters for the immune response. Unnecessarily disrupting the microbiome with antibiotics causes the body to produce eosinophils, a type of white blood cell that mobilizes to fight infection. The problem is that these cells also stimulate inflammation.

The negative effect of antibiotics on flu patients occurs because the excess eosinophils in the lungs hinder the functioning of immune cells that clear out bad bacteria and viruses from the lungs.

Few flu patients will become ill with a secondary bacterial pneumonia and need antibiotics, but some vulnerable patients with other diseases, or who are older, or have weakened immune systems are prescribed antibiotics to reduce their risk of a secondary bacterial pneumonia infection.

For those who have the flu but are otherwise healthy, the study showed that the increased levels of eosinophils associated with antibiotics are associated with poor clinical outcomes and more systemic inflammation.

“We need to be aware that antibiotics are not innocuous and can have harmful side effects,” said the study's corresponding author, Peter Chen, the Medallion Chair in Molecular Medicine and the interim chair of the Department of Medicine at Cedars-Sinai. “To be clear: When a flu patient has a confirmed bacterial infection, you definitely need to prescribe antibiotics. But when there is no direct evidence of such an infection, antibiotics should not be prescribed because they will not help, and could, in fact, be harmful.”

Investigators examined laboratory mice that had first been infected with influenza and then were infected with a bacterial infection caused by MRSA (methicillin-resistant Staphylococcus aureus), a common cause of a secondary bacterial pneumonia. One group of mice received antibiotics before they were infected with MRSA, while the control group did not.

Viruses, unlike bacteria, enter the body's cells and multiply. They cannot live apart from a host cell. Bacteria, in contrast, are independent agents.

The research showed that the mice treated with antibiotics experienced greater weight loss and evidence of more lung damage after MRSA infection. These mice had higher levels of MRSA bacteria in their lungs and higher levels of immune cells (macrophages and eosinophils). In other words, antibiotic treatment during an influenza infection made the situation worse.

Then the investigators turned their attention to human patients, examining a group of influenza patients at Cedars-Sinai and critically ill patients at Northwestern University and the University of Pittsburgh who had been treated with antibiotics. They found these patients had more eosinophils and a longer hospital stay and higher systemic inflammation.

“This research also sheds light on the role that eosinophils play in regulating lung immunity,” said co-author David Underhill, chair of the Department of Biomedical Sciences, adding that eosinophil-depleting strategies may be useful as a therapy to improve outcomes in patients admitted with influenza and other viral illnesses.

The study is published in the Journal of Clinical Investigation.