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36 Veterinary Laser Therapy in Small Animal Practice

don’t need to kill all the bacteria, just enough to help their differing results. Some in vitro experiments seem to
the patient deal with the rest. On the other hand, when show decreased microbial growth after exposure to laser
an incomplete kill of bacteria happens with antibiot- light, including common pathogenic and opportunistic
ics, resistance can be a problem, but this has not been species such as Candida albicans, [178] Staphylococcus
documented in laser therapy. aureus, [178, 181] Escherichia coli, [178, 181, 182] Pseudomonas
How does this happen? We certainly know it does aeruginosa, [183] Trichophyton rubrum, [178] and periodon-
not happen through genotoxicity: the photons of infra- topathic germs. [184] But others show no effect [185, 186] or
red and red light, by definition, just don’t have enough even increased growth after laser exposure. [182, 183, 185]
energy. However, several mechanisms of action have Some suggest pulsing frequency or pulse duration
been proposed for the antimicrobial effect. can have different effects at the same wavelength; for
instance, Karu et al. found 950 nm could enhance E. coli
• Light is absorbed by bacterial chromophores (por- growth at 26 Hz and decrease it at 5000 Hz. [182]
phyrins), producing an increase in intracellular ROS Results obviously depend on wavelength, dose, power
– the porphyrins act as endogenous photosensitiz- density, and other factors, and few studies carry out a
ers – and a decrease in transmembrane potentials, systematic parameter-by-parameter analysis. More
which are related to cell energy generation and recently, in vitro experiments have included biofilm
storage. [178] Because the types of porphyrin are not models rather than just bacterial suspensions. These
the same in all bacteria, their absorption peaks can may be more appropriate and include more complex
also be different, thus making the optimal wave- bacterial population dynamics, such as subpopulations
length for a wound potentially variable if we are of bacteria that are dormant, have a slower rate of
only talking about killing bacteria. Blue light seems growth, or are related to antibiotic resistance. [179]
to target porphyrins more than infrared. [179] In vivo studies have provided encouraging results in
• Of course, you could consider a thermal effect if you the treatment of infections with E. coli, [180] S. aureus, [187]
are using a high-power laser, but thermal killing of MRSA, [178, 188–190] P. aeruginosa, [181, 187] and some other
bacteria in wounds is not what we do in LT; that pathogenic species of bacteria [184] and fungal infections
sounds more like cauterization! The treatment such as T. rubrum [178, 191] and C. albicans. [178, 181] One
always takes place while maintaining physiological of these studies suggested that laser treatment could
temperature in the treated tissues. decrease/prevent pathogenic bacterial growth in burns,
• With a lower thermal effect, you could desiccate while increasing the number of non-pathogenic flora.
the surface of a wound, making it a less microbial- [187] Decreased bacterial counts have also been described
friendly environment. [180] But wound desiccation after irradiation of rats with an osteomyelitis model.
is not necessarily therapeutic; we try to make sure [188] Again, not all in vivo experiments show such pos-
the wound has just the right amount of moisture itive results: an experimental model of S. aureus septic
to physiologically heal – a very dry wound does arthritis in rats did not report a substantial benefit of
not heal better and does not encourage growth and LT, apart from a certain level of recovery in the articular
maintenance of healthy granulation tissue. cartilage and synovium. On the other hand, in this study
• Last but not least: in living tissue, increased oxygen- the power, wavelength, and dose used were quite low
ation in the area (which is part of what happens with and only one point was treated, [192] to be fair, so it would
LT) makes the microenvironment less friendly for not have any similarity to what you do in practice.
bacteria (most pathogenic species grow less well in These in vivo observations may assess the effect more
oxygen-rich environments) and more favorable for globally and realistically, since a culture plate does not
the aerobic metabolism of the host’s cells, includ- take into consideration the effect of laser irradiation on
ing leukocytes. When surgical site infections are immune cells, cytokine release, and the complex inter-
studied, the most commonly retrieved bacteria are actions that occur in a living tissue. The increased vas-
those susceptible to oxidative killing. cularization and oxygenation of tissue also play a role
in the fight against infection – a tissue with poor blood
Again, the different technical parameters (wave- flow and oxygenation is more likely to get infected.
length, energy, power density, etc.) used in the published More research needs to be done, though, to try to
literature make it difficult to compare such studies and clarify which parameters have more effect on particular

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