Mini-molecules keep bacteria at bay

17 July 2006

In the battle against infection, some of the most minuscule molecules have joined the front line. Infection-causing bacteria plague patients who undergo surgery for new joints. But by coating artificial joints with nano-sized particles, bio-engineers have dealt the bacteria a killer blow.

Antenna investigates...

Bacteria attack implants to form a slimy layer like this.

Image: NASA

The cutting-edge approach uses minuscule 'germ-fighting' molecules to fight off the bacteria that invade medical implants. When artificial joints are fitted, bacteria attack them, forming a slimy film that causes infection. But bio-engineers have designed a 'nano-surface' to coat implants, which stops the bacteria growing.
'This method of coating implants discourages bacteria growth significantly. We hope that this technique will lead to safer, longer-lasting implants.'
Thomas Webster, bio-engineer, Brown University

Thomas Webster, bio-engineer, Brown University.

Image: Brown University

Artificial hips help thousands of people stay active in the UK.

Image: iStockphoto/Jasmin Awad

Medical implants help people stay alive and active. Damaged joints such as shoulders, hips and knees are routinely replaced - last year over 125,000 people in the UK underwent surgery for a new knee or hip.
To protect implants from invading bacteria, the team used two materials, zinc oxide and titanium oxide. Zinc oxide is an antimicrobial agent which helps fight off bacteria. Titanium oxide is often used to make implants because it is strong and light.
They pressed tiny, nano-sized particles of these materials into 10p-piece-sized discs. These 'nano-surface' discs were tested alongside discs pressed from larger micro-sized particles. Under a microscope, the two types of disc look very different. Smaller bumps produced by the nano-sized particles make the surface much rougher.

The surface of the discs coated with micro-particles look like smooth plateaus, while the discs coated with nano-particles look like jagged mountains. Nano-particles are 1000 times smaller than micro-particles.

Image: Thomas Webster

The bacteria show up green in this image. Look at how many grew on the disc which didn't have nano-sized particles coating it.

Image: Thomas Webster

To test how well the discs fight off invading bacteria, the bio-engineers covered each disc with bacteria. After one hour, they took a swab of any bacteria that grew, and stained them green for counting.

Look at how few bacteria grew on the disc covered in the germ-fighting nano-particles.

Image: Thomas Webster

Astonishingly, after one hour, the discs containing nano-sized particles had 1000 times fewer bacteria growing on them, compared with the discs containing micro-sized particles.
Why does this happen?
'With the nano-surfaces we created, the surface area of the disc was increased by 25 to 35%. We think that this additional area exposes the bugs to more of the germ-fighting properties of the zinc oxide - surface area seems to be key.'
Thomas Webster, bio-engineer, Brown University

The nano-sized particles form minuscule bumps and craters, increasing the disc's surface area.

Image: Thomas Webster

One of the most important bacteria that causes infection in implants is called Staphylococcus epidermidis. These bacteria live harmlessly on our skin, but cause problems when they sneak into the wound created when a new implant is fitted.
Once inside a wound, Staphylococcus epidermidis multiplies, forming a bio-film around the implant. This slimy film is tough stuff which can't be shifted with antibiotics. Surgeons must remove the implant and either clean or replace it. By fighting off the bacteria, these 'nano-surfaces' will stop surgeons having to operate.

Staphylococcus epidermidis is the main culprit for causing implant infections.

Image: CDC/Janice Carr

But that's not all, the 'nano-surface' also helps the new implant attach to the patient's body. The nano-particles provide more surface area, meaning more places for cells from the patient to stick to.
Twice as many cells grew and formed bone on the nano-particle-covered discs. These 'bone-forming' cells help a new implant to anchor into the patient's body, making it last longer.
So bio-engineers look set to lead the way in the battle against bacteria using something even smaller than the bugs themselves. The next step is to try the technology out in patients.
.