Antibacterial peptides are short strings of amino acids that the body uses to kill infectious bacteria. They are part of the body's innate immune response, so they are non-specific in their action. Sometimes these peptides are called antimicrobial peptides, because they are capable of killing other organisms like viruses and fungi, as well as tumor cells.
On average, antibacterial peptides consist of chains of just 15 to 45 amino acid residues. They are acidic, so they have a positive charge. The size and charge of these peptides have important implications for their role in immunity. Manufacturing these peptides inside of immune cells can take place quickly due to their small size. This allows the body to create large numbers of peptides to fight large numbers of rapidly dividing bacteria.
Most bacteria have a negatively charged cell membrane. Antimicrobial peptides, having a positive charge, are therefore attracted to them. The body's host cells have surface cholesterol that causes them to be neutral, so these peptides do not target them.
Due to their structural differences, antibacterial peptides have many ways of killing bacteria. They may open up pores in the bacterial cell membrane, or they may enter the membrane and act inside. Once inside of a bacterium, these peptides can damage bacterial organelles, prevent DNA replication, or interfere with cell division.
With few exceptions, these peptides always conclude their actions by causing a breakdown of the cell membrane known as lysis. This process destroys the bacteria completely. Many times, these peptides actually have killed the bacteria through other means before lysis occurs, however.
There are other functions that are performed by antibacterial peptides aside from the direct killing of infectious organisms. Peptides can help wounds to heal, cause nearby immune cells to express different genes, or to take certain actions against nearby germs. They may act as signaling molecules that cause immune cells to encourage more cells to arrive at an injury site. All of these actions contribute toward clearing infection from the body.
Bacteria can become resistant to some peptides, just as they can become resistant to antibiotic drugs. Resistance usually takes the form of changes to their cellular surface, making it more difficult for peptides to recognize and attack them. Generally, the body can work past these defense mechanisms, though. There are hundreds of types of antibacterial peptides produced in the body, with slight variations in form and function. Such a variety allows the body to fight off infection even in the face of resistance.