Why do open fractures have increased risk for infection?
The presence of bacteria within an open wound increases the risk of colonization when hardware is used. Once the hardware is colonized, the bacteria grows rapidly. During the rapid growth phase, the bacteria secretes a polysaccharide sugar layer, called a “biofilm”, or slime layer that encases the bacteria. This biofilm provides protection to the bacteria against the body’s defenses and antibiotics.
Within the biofilm, there are channels that allow the bacteria to pass nutrients, messaging signals, and even DNA to each other. The bacteria pass on their DNA by:
Transformation is when a bacterial cell ruptures, releasing its DNA, which is then taken in by another bacteria. Transduction occurs when DNA is transferred from one bacterium to another by a virus. Phage DNA and proteins are made and bacterial chromosomes are broken up, completing the gene transfer. The phage release themselves from the host, carrying either bacterial or phage DNA. Conjugation occurs when two bacteria attach themselves together with a sex pilus and exchange their DNA.
How does the bacteria become resistant to antibiotics?
The bacteria can alter the genes they express by as much as 50-60%. By doing this, the bacteria can produce enzymes such as beta-lactamases, which destroy certain antibiotics before they can reach their target site. They can also make Efflux pumps which expel antibacterial agents from the cell before it can reach its target site. Finally, by expressing different genes, the bacterial cell wall can be altered to no longer contain the binding site of the antibiotic agent. Because the antibiotics cannot break through the biofilm and access the bacteria, the bacterium in the biofilm can become up to a thousand times more resistant to the antibiotics by the different mechanisms previously discussed.
If there is biofilm on the hardware, what can the physician do?
The only proven treatment, is to remove the hardware and wash the wound. However, removal of the hardware is a problem if the fracture is not healed and the fixation is needed. The physician may decide to suppress the infection, leaving the hardware until the fracture has improved. Or, the physician may decide the remove the hardware and seek an alternative method for stabilizing the fracture, such as an external fixator, and then using a biological material to help heal the fracture.
These are the issues that make infection with hardware so complex!