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28 Sep 2007
Bacterial viruses can be made inactivate by a low-power visible femtosecond pulse, opening up new strategies for controlling micro-organisms.
A US group has found that firing a low-power femtosecond laser at bacterial viruses in solution can dramatically reduce the number of infectious viruses, in some cases by a factor of 1000. "I had to repeat the experiment several times to convince myself that the laser worked this well," said Shaw-Wei D Tsen of Johns Hopkins School of Medicine. (J Phys: Condens. Matter, 19, 322102.)
Tsen and the team, which also includes his father, believe that the pulses cause the viruses to vibrate by stimulating low-frequency Raman active vibrational modes. Under the right excitation, the vibration is sufficient to render the viruses inactive.
"The laser rapid pulses of the laser allow the solution surrounding the virus to cool off, reducing heat damage to the normal blood components," said Tsen.
The group used a diode-pumped continuous-wave mode-locked Ti-sapphire laser to fire femtosecond pulses at samples of M13 bacteriophage. Samples were typically irradiated for ten hours with 80 fs pulses each having an average power of 40 mW.
This method leaves the sensitive cells around the viruses unharmed and is much more selective than UV irradiation methods, which disinfect microorganisms but usually cause mutation. Lasers also pass through any water surrounding the viruses without absorption, unlike microwaves or ultrasonic vibrations.
Since the damage to the viruses is essentially mechanical, caused by structural vibrations rather than chemical attack, the technique should not trigger an immune response or evoke any drug-resistance. Consequently micro-organisms which are drug-resistant should be susceptible as well.
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