Lysozyme is a small protein that acts as one of your body’s built-in antibiotics. Found in tears, saliva, nasal mucus, and other body fluids, it kills bacteria by breaking apart their protective cell walls. It’s part of your innate immune system, the defense network you’re born with rather than one you build through exposure to infections.
How Lysozyme Destroys Bacteria
Bacteria are surrounded by a rigid structure called peptidoglycan, essentially a mesh of sugar molecules linked together to form a protective shell. Lysozyme works by cutting the bonds between two specific sugar molecules in that mesh. Once enough bonds are severed, the cell wall loses its structural integrity, and the bacterium bursts open from its own internal pressure.
This mechanism is particularly effective against gram-positive bacteria, a broad category that includes strep, staph, and listeria. These bacteria have a thick peptidoglycan layer sitting right on their surface, fully exposed to lysozyme’s attack. Gram-negative bacteria, which include E. coli and salmonella, are harder to kill because they have an additional outer lipid membrane shielding their thinner peptidoglycan layer. That lipid coating acts like armor, preventing lysozyme from reaching its target. Beyond bacteria, lysozyme also provides some protection against viruses and fungi, though its primary role is antibacterial defense.
Where Your Body Produces It
Lysozyme shows up in nearly every body fluid that contacts the outside world. Tears contain high concentrations, which is one reason your eyes stay remarkably resistant to infection despite constant exposure to airborne microbes. Saliva carries it too, helping keep your mouth’s bacterial population in check. Nasal mucus, breast milk, and the mucous lining of your airways all contain lysozyme as well.
White blood cells, particularly neutrophils and macrophages, also produce and release lysozyme when they encounter pathogens. This means lysozyme works on two fronts: it guards your body’s entry points passively through secretions, and it participates actively in immune responses at infection sites. The enzyme isn’t unique to humans. It’s found across the animal kingdom, in birds, fish, insects, and even in some plants, which speaks to how fundamental this defense mechanism is.
How Fleming Discovered It
Alexander Fleming discovered lysozyme in 1922, six years before his more famous discovery of penicillin. The story goes that Fleming, who was suffering from a cold, transferred some of his nasal mucus onto a petri dish. Not known for keeping a tidy lab, he left the dish sitting on his cluttered desk for about two weeks. When he finally checked it, bacterial colonies had grown across the plate, except in the area where the mucus had landed. Something in the mucus was killing the bacteria. That something turned out to be lysozyme. The discovery didn’t attract much attention at the time because lysozyme’s antibacterial effect was considered too weak to be medically useful, but it laid the groundwork for Fleming’s later, paradigm-shifting work with penicillin.
Lysozyme as a Diagnostic Marker
Because white blood cells produce lysozyme, measuring its levels in blood plasma can reveal certain health conditions. When granulocytes or monocytes (types of white blood cells) multiply abnormally, as happens in certain leukemias, lysozyme levels rise sharply. Plasma levels above 200 micrograms per milliliter can signal acute or chronic leukemias involving these cell types. Doctors also use lysozyme levels to track whether treatment for these cancers is working: successful therapy brings levels back down.
Elevated lysozyme isn’t always a sign of cancer. Sarcoidosis, an inflammatory condition that can affect the lungs and eyes, often raises lysozyme levels, making it a useful screening tool for the eye-related form of the disease. Crohn’s disease, kidney transplant rejection, and various infections can also push levels higher. On the other hand, lymphocytic leukemia, which involves a different type of white blood cell, can actually depress lysozyme levels below normal. So the direction of change matters as much as the change itself.
Uses in Food Production
The food industry has adopted lysozyme as a natural preservative, primarily sourced from chicken egg whites. Because it targets gram-positive spoilage bacteria with low toxicity to humans, it offers an appealing alternative to synthetic preservatives. The European Union has approved it as a food additive under the code E1105.
Cheese production is one of the biggest applications. During the aging process, certain bacteria can produce gas inside the cheese, causing a defect called “late blowing” that ruins texture and flavor. Adding lysozyme to the cheese-making process prevents this by keeping those bacteria in check. Winemakers use it for similar reasons: to control bacterial growth that can spoil wine without resorting to heavy sulfite treatments. It also appears in the preservation of meat, fruits, vegetables, milk, and unpasteurized beer. Since lysozyme is a natural component of the human immune system rather than a synthetic chemical, many countries have approved its use in food, though people with egg allergies should be aware of its presence in products labeled with E1105.