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An antimicrobial peptide, or host defense peptide, is part of the innate immune system present from birth to protect the body from infection. Structurally, it is a small molecule consisting of a chain of amino acids, the units from which proteins are made. There are a number of different types of antimicrobial peptide and they are found in all living creatures, where they act against microbes such as bacteria and viruses. As more microorganisms become resistant to antibiotics, it is thought that the use of antimicrobial peptides could provide alternative forms of treatment. Potentially, custom peptides could be made to treat infections, increase the immune response and neutralize the toxins produced by microbes.
The antimicrobial peptide molecule consists of a chain of amino acids, varying from six to 100 units in length. Over 800 different kinds of antimicrobial peptides have been recognized. They have been divided into four main classes according to their overall shape. These classes are known as α-helical, β-sheet, extended and loop peptides. The most common human peptide types are the histatins, which are found in saliva, and the defensins and cathelicidins, which are produced by immune system cells.
In most cases, an antimicrobial peptide attacks a microorganism by changing its cell membrane. Holes are created in the membrane that allow important substances such as nutrients to flow out of the cell. Although the details are not fully understood, there are thought to be a number of different membrane-changing mechanisms used by different types of peptides.
Research involving synthetic versions of histatins has shown that they can act against the yeast known as Candida albicans. This suggests that artificial histatins could be used to treat the yeast infection known as candidiasis that affects the mouths of HIV patients. Other research has suggested that histatins could be effective in treating some bacterial infections that occur in burns and skin wounds.
Thousands of different kinds of antimicrobial peptide could be manufactured to treat a variety of infections, but so far relatively few have been developed and trialled. One problem has been that, in clinical trials involving patients, antimicrobial peptides often appear less effective than they seem to be when tested in the laboratory. Another drawback is that synthetic antimicrobial peptides are expensive to produce. Advantages of developing antimicrobial peptides as drugs include that they are not harmful to human cells and they are not associated with resistance developing in the way that antibiotics are.