Effects of 810 nm laser irradiation on in vitro growth of bacteria: comparison of continuous wave and frequency modulated light.
OBJECTIVE
Low intensity laser therapy may modify growth of wound bacteria, which could affect wound healing. This study compares the effects on bacteria of 810 nm laser using various delivery modes (continuous wave or frequency modulated light at 26, 292, 1000, or 3800 Hz).


METHODS
Staphylococcus (S.) aureus, Escherichia (E.) coli, and Pseudomonas (P.) aeruginosa were plated on agar and then irradiated (0.015 W/cm(2); 1-50 J/cm(2)) or used as controls (sham irradiated); growth was examined after 20 hours of incubation post exposure.


RESULTS
There were interactions of species and modulation frequency in the overall effects of irradiation (P = 0.0001), and in the radiant exposure mediated effects (P = 0.0001); thus individual frequencies and each bacterium were analysed separately. Bacteria increased following 3800 Hz (P = 0.0001) and 1000 Hz (P = 0.0001) pulsed irradiation; at particular radiant exposures P. aeruginosa proliferated significantly more than other bacteria. Pulsed laser at 292 and 26 Hz also produced species-dependent effects (P = 0.0001; P = 0.0005); however, the effects for different radiant exposures were not significant. Bacterial growth increased overall, independent of species, using continuous mode laser, significantly so at 1 J/cm(2) (P = 0.02). Analysis of individual species demonstrated that laser-mediated growth of S. aureus and E. coli was dependent on pulse frequency; for S. aureus, however, there was no effect for different radiant exposures. Further tests to examine the radiant exposure effects on E. coli showed that growth increased at a frequency of 1000 Hz (2 J/cm(2); P = 0.03). P. aeruginosa growth increased up to 192% using pulsed irradiation at 1000-3800 Hz; whereas 26-292 Hz laser produced only a growth trend.


CONCLUSIONS
The findings of this study point to the need for wound cultures prior to laser irradiation of infected wounds. Similar investigations using other common therapeutic wavelengths are recommended.
Nussbaum
Ethne L
EL
Rehabilitation Services, Mount Sinai Hospital and Department of Physical Therapy, University of Toronto, Toronto, Ontario M5G 1X5, Canada. e.nussbaum@utoronto.ca
Lilge
Lothar
L
Mazzulli
Tony
T
D004307:Dose-Response Relationship, Radiation; D004334:Drug Administration Schedule; D004925:Escherichia / Q000254:growth & development* / Q000528:radiation effects*; D006801:Humans; D066298:In Vitro Techniques; D028022:Low-Level Light Therapy; D011550:Pseudomonas aeruginosa / Q000254:growth & development* / Q000528:radiation effects*; D013211:Staphylococcus aureus / Q000254:growth & development* / Q000528:radiation effects*; D014945:Wound Healing / Q000528:radiation effects; D014946:Wound Infection / Q000382:microbiology* / Q000628:therapy*