What Are Antibiotics Good For?

By Eva Briggs, MD

Every day someone asks me why I am not prescribing an antibiotic for their viral infection.

Many people hear only the word infection and immediately conclude that only an antibiotic with fix them.

But antibiotics only work for bacterial infections and have no effect on viral infections.

The explanation hinges on understanding something about bacteria and viruses and how they differ.

Bacteria are single celled organisms. They vary widely in size, but the vast majority is too tiny to see without a microscope. They have a membrane that encloses the cell contents (cytoplasm).

Bacterial cytoplasm generally lacks membrane-bound structures (nucleus, mitochondria) that are found in cells of humans and other animals and plants. Surrounding the cell membrane is a cell wall, a structure lacking in plants and animals.

Viruses are tiny infectious agents, too small to see with a microscope, that reproduce only by living inside of and hijacking the machinery of a living cell. This is unlike bacteria, the vast majority of which are free-living. Viruses are bare-bones structures consisting of nucleic acid (RNA or DNA) surrounded by a protein coat called a capsid.

Antibiotics work by targeting the functions of a living bacterial cell. Because bacteria and viruses are so completely different structurally and biochemically, antibiotics can’t affect viruses.

One common class of antibiotics is penicillins, which includes penicillin, amoxicillin, methicillin and others. They work by first entering bacterial cells. Once inside the cell, they disrupt the chemical process the cell uses to make the cell wall. Bacteria are constantly remodeling their cell walls which are made from chemicals called peptidoglycans.

Penicillins block the step where peptidoglycans are cross-linked together. Since viruses do not have cell walls, penicillins can’t affect them.

Macrolides are another common antibiotic class that includes erythromycin and azithromycin (the popular Z-pak). Macrolides interfere with the ability of bacteria to make new proteins. Viruses don’t synthesize proteins. They commandeer living cells and get them to do that task. This is why a Z-pak cannot get colds, influenza or COVID-19 better.

Contrary to popular belief, most sinus infections and bronchitis are caused by viruses, too and won’t be helped by any antibiotic.

People often wonder why medical providers are reluctant to prescribe antibiotics “just in case.” It’s because there are real potential harms from antibiotic use. First, they may cause antibiotic resistance. Bacteria are everywhere around and inside us. When exposed to antibiotics, susceptible bacteria are killed leaving their resistant colleagues alive to reproduce. If those bacteria then either mutate to become virulent (better able to cause infection) or encounter someone with weak immunity, they can cause an infection resistant to the antibiotic.

Taking antibiotics can alter the balance of bacteria in the intestines. This may lead to diarrhea. It can allow the dangerous bacteria Clostridioides difficile to grow. C. diff infection causes severe diarrhea and is often hard to eradicate.

Antibiotics potentially cause allergic reactions. There is no need to take the risk by taking an unnecessary antibiotic.

There are some antiviral medicines to teat specific viral infections: HIV, hepatitis C, shingles, influenza, COVID-19. But there is no broad-spectrum antiviral drug for common viral respiratory infections.

In summary, antibiotics are only useful for bacterial infections. They are unhelpful for viral infections but can cause real harm when overused.

Eva Briggs is a retired medical doctor who practiced in Central New York for several decades. She lives in Marcellus.