Plasma sterilisation now a possibility

A new way of killing bacteria using a plasma at room temperature and pressure, which could have applications in decontamination and sterilisation, has been developed by researchers at Old Dominion University in Norfolk, Virginia, and the University of California in San Diego.

Most plasma at atmospheric pressure are at thousands of degrees Centigrade, making them impractical for everyday use. However, Dr Laroussi, Dr Mendis and Dr Rosenberg used a 'cold plasma' that was close to room temperature and atmospheric pressure in their experiments.

Cold plasma is a mixture of 97% helium and 3% oxygen, which is injected between electrodes that are charged with between 50 and 300W. This creates a new type of plasma, which is cheaper and easier to use because it is at room temperature and pressure and uses little power.

The researchers tested the effects of this plasma on two types of bacteria - the spore-forming Bacillus subtilis, which is similar to anthrax, and the vegetative Escherichia coli, which is a common cause of food poisoning

Exposure to the ultra violet radiation and the radicals damaged many parts of the bacteria.

In addition, the charged particles within the plasma can cause structural damage to the outer cell membranes in non-spore-forming bacteria. This happens because electrical charge collects on the outside of the cells, giving rise to a force acting on the cell membrane, tearing it apart. Spore-forming bacteria have a rigid membrane with a smooth outer surface, whereas non-spore-forming bacteria have a thin and bumpy outer surface.

Thicker and smoother membranes can better withstand the electric forces, so only the non-spore-forming bacteria end up with damaged membranes. The combination of all these factors make cold plasma lethal to both types of bacteria.

The researchers say it is likely that cold plasmas would not only kill other kinds of more dangerous bacteria, but also viruses.