Lysozyme: An Antibacterial Enzyme and a Cause of Egg Allergies
Functions and Uses of Lysozyme
Lysozyme is an antibacterial enzyme found in body secretions, including saliva, mucus, tears, and human milk. The activity of lysozyme is part of our body's first line of defence against invading microbes. The enzyme is very helpful in areas that aren't protected by a skin barrier.
Lysozyme is also a major component of egg white, where it helps to protect the environment around the developing chick. In addition, it's added to certain foods, medications, and vaccines as a preservative. Commercial lysozyme is generally obtained from egg white. Unfortunately, some people are allergic to this lysozyme. The enzyme is one cause of egg allergies.
Alexander Fleming and the Discovery of Lysozyme
The discovery of lysozyme may be a great example of serendipity in science. Serendipity is the act of making an unexpected or fortunate discovery while searching for something else. The enzyme was found by Alexander Fleming—the same scientist who discovered penicillin.
At the time of the discovery, Fleming was involved in studying microorganisms. He was known as an untidy scientist. His laboratory was filled with Petri dishes containing microbes growing on agar. They were piled on lab benches in a seemingly disorganized fashion. Fleming's collection actually helped him with his discoveries, however.
One day in 1921 when Fleming had a cold, a drop of mucus fell from his nose on to a Petri dish that he was examining. Although most accounts describe this event as accidental, which makes a nice story, Fleming may have deliberately added mucus to the dish out of curiosity.
Bacteria cultures were growing on the nutrient agar in the Petri dish that Fleming was examining. Agar is a jelly-like substance obtained from algae. When nutrients are added to agar, it becomes a good growth medium for bacteria. Fleming discovered that the bacteria around the mucus drop in the Petri dish died. After further investigation he realized that a protein in the mucus was responsible for the death of the bacteria. (Enzymes are a type of protein). Fleming named the protein lysozyme.
The word lysozyme comes from "lysis", which is the process in which a cell bursts, and "enzyme", which is a protein that causes or speeds up a chemical reaction.
The Gram Staining Method of Classifying Bacteria
Bacteria are categorized into two major groups based on their reaction to specific stains. The classification method was created in 1884 by Hans Christian Gram, a Danish scientist. Lysozyme is more effective against one of the bacterial groups than the other.
After the gram staining process has been performed, gram positive bacteria have a purple cell wall and gram negative bacteria have a pink one. The different results are due to the fact that gram positive bacteria contain more peptidoglycans in their cell wall. Not all bacteria can by differentiated by gram staining, but many can.
Peptidoglycans in the Cell Wall of a Bacterium
How Does Lysozyme Destroy Bacteria?
Lysozyme works by destroying the protective cell wall of bacteria. The cell wall is the outer covering of a bacterium. One of its jobs is to withstand the pressure created by the fluid inside the cell and to keep the cell intact.
The wall contains chains made of molecules called peptidoglycans. Lysozyme breaks some of the bonds that hold the peptidoglycans together. As a result, the wall weakens and the bacterium bursts.
Lysozyme is more effective against gram-positive bacteria than gram-negative ones because gram-positive bacteria contain far more peptidoglycans in their cell wall. Despite this limited action, the enzyme is a valuable part of the immune system.
The human immune system is complex, fascinating, and awesome. It normally does a wonderful job of protecting us from the potentially dangerous microbes that we are exposed to every day. Sometimes the system needs help in the form of prescribed medications, however. In some people it makes a mistake and attacks a substance or material that isn't a threat to the body.
When someone has an egg allergy, their immune system mistakenly attacks one or more proteins in eggs, producing an allergic response. In this type of response the system overreacts to a harmless stimulus. There are proteins in both egg yolk and egg white. Since the proteins are more abundant in the white, however, this part of the egg is usually the cause of an egg allergy. Lysozyme is one protein in egg white that can trigger an allergic response.
An Allergist Discusses Egg Allergies
Symptoms of an egg allergy range from mild to serious. Sensitive people may experience one or more of the following symptoms after eating or even touching eggs:
- skin rash
- runny nose
- difficulty in breathing
- stomach pain
Anyone who experiences symptoms such as those listed above should see their doctor to get a diagnosis and treatment advice.
Egg allergies are more common in children than in adults. The American College of Allergy, Asthma and Immunology says that 70% of children with an egg allergy no longer have the allergy by the time they reach the age of 16. Sometimes an allergy persists into adulthood, however. It may even appear for the first time when a person is an adult. This variation in the allergy's presence is why the questions in the egg allergy poll below refer to both the past and the present.
An Egg Allergy Survey
Have you ever had an egg allergy?
Egg Substitutions for Baking
An uncommon but very serious result of an egg allergy is anaphylaxis, which can sometimes develop rapidly. Anaphylaxis is a body-wide allergic reaction that involves a dramatic drop in blood pressure, severe difficulty in breathing, and sometimes loss of consciousness. The condition is life threatening and is a medical emergency.
People with a severe allergy may be prescribed an epinephrine auto-injector. They must carry this around with them in case they are accidentally exposed to an allergen, such as components of eggs. Epinephrine is a hormone that expands the airways, making breathing easier, and constricts blood vessels, counteracting the drop in blood pressure. The hormone is also known as adrenaline.
Egg allergies are usually caused by a protein other than lysozyme. Nevertheless, allergies to lysozyme from eggs do exist. If someone is allergic to the enzyme, they will have to do more than simply stop eating eggs. Lysozyme from hens' eggs is added to many prepared foods as well as to some types of wine, some medications, some influenza (flu) vaccines, and the yellow fever vaccine. The ability of the enzyme to destroy many bacteria makes it a good preservative in these products. In addition, any food with added eggs or egg whites contains lysozyme.
Anyone with a lysozyme allergy should check the ingredient list on a package of prepared food carefully or check with the product's manufacturer to ensure that the enzyme is absent. One problem with checking ingredient lists is that manufacturers may not be required to list lysozyme as an ingredient if it's present in a very small concentration. This may not matter in someone with a mild allergy, but it could be very important in someone with a severe one.
If someone is allergic to a component in a vaccine, this doesn't necessarily mean that the person should refuse to be vaccinated. It would be a good idea to receive the vaccine in a medical facility containing emergency medications and equipment instead of in a pharmacy or a workplace, however. As always, a doctor should be consulted about medical matters.
Albumen is another name for egg white. The word shouldn't be confused with albumin, which is a type of protein found in the white of an egg.
Cooked and Separated Eggs
Some people may be allergic to the lysozyme in uncooked eggs but not to the enzyme in eggs that have been cooked for a long time at a high temperature. Nobody with a severe allergy should experiment with this idea outside of a medical setting, however.
Another possibility is that someone who is allergic to lysozyme but to none of the proteins in egg yolk may be able to eat the yolks of eggs. This is unlikely, however, because it's almost impossible to completely separate an egg yolk from the egg white. It definitely shouldn't be attempted in someone with a severe egg allergy.
Unboiling an Egg
Lysozyme is involved in a process that sounds amusing but could have practical importance. Strange as it may sound, in 2015 scientists discovered how to unboil an egg—or in other words, how to change the solid egg white of a boiled egg back into a liquid again. Specifically, they learned how to obtain normal and useable lysozyme from cooked egg white.
Egg white is rich in proteins, which are made of chains of amino acids. The chains are folded into complex shapes. The shape of a protein is vital to its function. If the protein is misfolded or changes its shape, it can no longer do its job. This is a very serious condition inside the human body, which contains many vital proteins.
During boiling and some other stresses, proteins are denatured. "Denaturation" means a change in shape. Chemical bonds holding the protein chains in the correct shape are broken. The chains then unfold and become tangled. This process is responsible for egg white changing from an almost colourless liquid to a white solid when it's boiled or otherwise cooked.
The procedure for recycling lysozyme from cooked egg white was developed at the University of California, Irvine. The process is surprisingly quick and easy. It requires minutes to complete, compared to an older method that requires days.
In their experiment, the researchers added a chemical called urea to cooked egg white, which turned the white into a liquid. They then put the liquid in an instrument called a vortex fluid device, which applied shear stress to the egg white proteins. As a result, the lysozyme chains in the egg white untangled and refolded correctly.
The Significance of Untangling Lysozyme
The value of the discovery described above is not that an egg can be unboiled, which might seem like an unnecessary action. Instead, its value lies in the fact that it demonstrates that the tangled amino acid chains of lysozyme—and perhaps the tangled chains of other proteins—can be quickly returned to their normal state. This could have valuable applications in both industry and medicine.
Lysozyme is an interesting and useful enzyme. Studying the protein and its behaviour could have important benefits for health and help us to better understand the fascinating world inside the human body.
The American College of Allergy, Asthma and Immunology website has more information about egg allergies.
This Mayo Clinic webpage lists the major allergenic proteins in eggs and gives suggestions for preventing egg allergy symptoms.
Unboiling an egg information is described at the University of California, Irvine website.
© 2015 Linda Crampton