Wound infections can occur when bacteria from the skin or from the environment are introduced into damaged tissues. Crucially, all wounds are colonised by bacteria but under certain conditions these bacteria can multiply unchecked, growing to reach numbers that overwhelm the immune system.
Most wound infections can easily be treated with topical antimicrobials, or in more serious cases, by a course of antibiotics. They are also most commonly seen in clinical environments where, for example, infection-causing bacteria are inadvertently introduced into surgical incision sites after surgery.
Despite rigorous cleaning and hygiene regimens, infections are often unavoidable because of the prevalence of bacteria in the environment and as part of normal human flora.
When you add in that antibiotic resistant bacteria are rife within clinical environments, this can make these wound infections exceptionally difficult to treat.
Chronic, inflammed states
Without successful treatment, bacteria can remain within the wound, creating a non-healing or chronic state. When this occurs the wound becomes “stuck” in an inflammatory stage where damage can happen to the tissues. Any bacteria in the wound will then continue to grow and aggravate it.
Within chronic wounds, these bacteria exist as complex multi-species communities known as biofilms. These are comprised of layers of bacteria which are stuck to human proteins at the wound surface and which form complex, microscopic structures through which nutrients can flow.
Biofilm bacteria also protect themselves by secreting a sugary-layer that covers the entire microbial community which offers additional protection from antibiotic treatment and your immune response. Bacterial biofilms can remain in wounds despite numerous different types of antimicrobial treatment; in fact it is thought that 60% of chronic wounds are associated with these bacterial communities.
It is possible to impair biofilm growth and to disrupt established biofilms in the laboratory using topical antimicrobials. These antimicrobials function in a number of ways. They can act by preventing adhesion of bacteria to a surface – if bacteria cannot bind to a surface in the first place then they cannot colonise and so a biofilm cannot form. Many such antimicrobials act as an anti-biofilm agent as well as killing bacteria. They are often incorporated into wound dressings, and include compounds such as silver, which leach out of the dressing in time, providing an antimicrobial environment at the wound surface to prevent bacterial growth.
Some antimicrobials are effective against established biofilms. These ones must be able to diffuse through the sticky layer around the biofilm and penetrate the deeper layers to remove and kill the bacteria residing there.
Medical devices such as catheters often become infected, providing a ready-made “wound” or incision site where bacterial biofilm can grow. Biofilm can grow on the surface of these devices, which can be very problematic. To try to counter these problems, researchers are beginning to develop devices made from antimicrobial polymers that contain antimicrobial compounds within or on their surface, which are slowly released to prevent bacteria from adhering and to kill any bacteria in the vicinity of the device. The types of antimicrobial compounds used in this way are broad-spectrum, meaning that they are effective against a wide range of infectious bacteria making them highly versatile. The use of antimicrobial polymers also means that bacterial numbers are controlled which should prevent an infection occurring.
In an era where antibiotic resistance is increasing and outstripping the rate of antibiotic discovery, the development of antimicrobials is essential to counter these tough bacterial biofilms. Academic institutions are beginning to work closely with pharmaceutical industries to produce effective treatments for wound infection that are broad-acting and non-toxic – and which do not further antibiotic resistance.
While this will remain a significant challenge, the formulation of new antimicrobial and anti-biofilm treatments can not only improve treatment and prevention of infection, they may also help with the recognised threat of antimicrobial resistance.