Another area for PDT treatment...
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Killing of Cutaneous Microbial Species by Photodynamic Therapy
Zeina B, Greenman J, Purcell WM, et al
Br J Dermatol. 2001;144:274-278
Emerging antibiotic resistance among common bacterial pathogens poses a growing public
health problem. Furthermore, oral antibiotic therapy is not without its risks, ranging from the
annoying (such as gastrointestinal disturbances) to the life-threatening (such as toxic epidermal
necrolysis and fulminant hepatitis). Clearly, a niche exists for an alternative method of microbe
eradication.
One especially promising method, known as antimicrobial photodynamic therapy (APDT),
employs light and photosensitizing agents such as methylene blue to generate bactericidal levels
of reactive oxygen species. Attractive features of APDT include safety, low cost, ease of
application, and lack of systemic effects. Scientists have known that photosensitizing
compounds such as methylene blue, toluidine blue, and hematoporphyrins could kill microbes
in the presence of sunlight for over a century (Raab, 1900). Now Zeina and colleagues attempt
to quantify this effect on a variety of common skin pathogens.
Study Design
The goal of the trial was to study microbial killing in vitro using methylene blue and visible light
from 2 different sources: a slide projector (polychromatic light, 400-700 nm, 42 mW/cm2) and
midday sunlight (estimated visible light intensity of 22.2 mW/cm2). Both sources are good
photoactivators of methylene blue dye, which has an estimated absorption peak of 668 nm.
To test the photodynamic killing, microbial cells from common skin pathogens in broth culture
were subjected to varied time periods and intensities of polychromatic light. Pathogens tested
included Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pyogenes,
Corynebacterium minutissimum, Propionibacterium acnes, and the yeast Candida
albicans. Kill rates for each pathogen were measured using a set concentration of methylene
blue (100 micrograms/mL final concentration) at varying light intensities. Viable bacterial and
yeast counts were plotted as the log of colony-forming-units per mL against time, and these
data were used to calculate pathogen-specific D-values, defined as the time required in
seconds to reduce the population by 1 log-fold. Control conditions were used for each trial
(methylene blue without light and light without methylene blue).
Results
All tested microbial species were susceptible to APDT, although the 1 eukaryotic pathogen
(the yeast C albicans) proved more resistant, showing a slower reduction in viability and a
20-minute lag time to effect. All bacterial species, in contrast, showed immediate viable cell
count reductions with a steeper slope. As expected, the kill rates increased in direct proportion
to light intensity, and the efficacy of sunlight (tested against S aureus and S epidermidis) was
comparable to that of the polychromatic artificial light source.
No bacterial microbes showed D-values of more than 2 minutes; however, P acnes proved
most susceptible to APDT, while the anaerobe C minutissimum was most resistant (D-values
of 30 seconds and 120 seconds, respectively). By contrast, the only eukaryotic organism
tested -- C albicans -- showed a D-value of 660 seconds.
Neither control condition (nonirradiated cell suspensions with methylene blue and irradiated cell
suspensions without methylene blue) showed any significant reduction in viable organism
counts.
Discussion
In sum, APDT shows impressive antimicrobial efficacy in vitro against a wide range of common
skin pathogens, including S aureus and P acnes. By contrast, eukaryotic organisms such as the
yeast C albicans show relative resistance to oxidative damage and ensuing cell death. PDT
acts by generating reactive oxygen species, which in turn react with and damage crucial cellular
target molecules such as membrane lipids, cytosolic enzymes, and nucleic acid. While
eukaryotic organisms have a nuclear membrane protecting their genetic material, prokaryotes
lack this barrier, perhaps accounting for the observed relative APDT resistance demonstrated
by the eukaryotic C albicans.
APDT has recently shown promise in the treatment of acne vulgaris, which may be due in part
to its antimicrobial effect against P acnes.[1] Perhaps even more appealing, however, is the fact
that APDT may represent a new, powerful weapon against the scourge of antibiotic-resistant
bacterial strains such as S aureus now threatening patients and hospitals worldwide.[2,3] |
