Lysozyme, or 1,4-β-N-acetylmuramidase, is an enzyme found in tears, saliva, and cervical mucus. Tears, saliva and mucus are secretions from the body that first come into contact with the environment. This environment is filled with bacteria. Lysozyme is the first line of defence against bacteria.
Most bacteria contain a thick cell wall that enables them to survive the harsh external environment. Without a cell wall, water enters the bacterial cell and causes the bacteria to swell and finally burst and die. Many common antibiotics, such as penicillins, target the cell wall of the bacteria. In fact, before Alexander Fleming discovered that mucus could kill bacteria before he discovered that penicillin can kill bacteria!
The bacterial cell wall is made up of carbohydrates, in particular the carbohydrate peptidoglycan. The enzyme lysozyme breaks down peptidoglycan and destroys the cell wall of the bacteria.
Therefore the common routes of entry of bacteria into the body, such as the eyes, nose, mouth and genital tract, are guarded by the enzyme lysozyme, present in our secretions. It is so important to guard all the exposed routes of entry of bacteria into the body because once the bacteria gets into the bloodstream, it can then travel in the blood to any part of the body, including important organs such as the brain, the heart, the kidneys and the liver.
Lysozyme is not one hundred percent effective against bacteria. Using hands filled with bacteria to rub the eyes introduces bacteria into the eyes. The lysozyme present in tears may not be enough to kill all the bacteria. When this happens, an eye infection can occur. However, in the eye, there are also second lines of defence against bacteria such as immune cells known as macrophages and neutrophils.
The lysozyme in saliva kills the bacteria in the food remaining in the mouth. The bacteria that travels with the food into the stomach is mostly killed by the acidic environment in the stomach. Some people who suffer from dry mouth and have very little saliva production may have bad breath due to the bacteria multiplying in the mouth.
Sexual intercourse can introduce bacteria up the genital tract and the mucus produced by the cervix also contains lysozyme to fight bacteria.
Lysozyme is also present in breast milk, as are the immune cells and immune antibodies that help fight bacteria. Therefore, breastfeeding also helps to improve the baby’s immune defence against bacteria.