Photodynamic antimicrobial chemotherapy (PACT) is based on the principal that microorganisms will be inactivated
using a light source combined to a photosensitizing agent in the presence of oxygen. Oxidative damage of cell
components occurs by the action of reactive oxygen species leading to cell death for microbial species. It has been
demonstrated that PACT is highly efficient in vitro against a wide range of pathogens, however, there is limited
information for its in vivo potential. In addition, it has been demonstrated that sublethal photodynamic inactivation may
alter the virulence determinants of microorganisms. In this study, we explored the effect of sublethal photodynamic
inactivation to the virulence factors of Candida albicans. Methylene Blue (MB) was used as photosensitizer for sublethal
photodynamic challenge on C. albicans associated with a diode laser irradiation (λ=660nm). The parameters of
irradiation were selected in causing no reduction of viable cells. The potential effects of PACT on virulence determinants
of C. albicans cells were investigated by analysis of germ tube formation and in vivo pathogenicity assays. Systemic
infection was induced in mice by the injection of fungal suspension in the lateral caudal vein. C. albicans exposed to
sublethal photodynamic inactivation formed significantly less germ tube than untreated cells. In addition, mice infected
with C. albicans submitted to sublethal PACT survived for a longer period of time than mice infected with untreated
cells. The oxidative damage promoted by sublethal photodynamic inactivation inhibited virulence determinants and
reduced in vivo pathogenicity of C. albicans.
Antimicrobial photodynamic inactivation (APDI) is based on the utilization of substances that can photosensitize
biological tissues and are capable of being activated in the presence of light. Cryptococcus neoformans is an yeast
surrounded by a capsule composed primarily of glucoronoxylomannan that plays an important role in its virulence. This
yeast causes infection on skin, lungs and brain that can be associated with neurological sequelae and neurosurgical
interventions, and its conventional treatment requires prolonged antifungal therapy, which presents important adverse
effects. The aim of this study was to evaluate the protective effect of Cryptococcus neoformans capsule against reactive
oxygen species generated by APDI. Cryptococcus neoformans KN99α, which is a strain able to produce capsule, and
CAP59 that does not present capsule production were submitted to APDI using methylene blue (MB), rose bengal (RB),
and pL-ce6 as photosensitizers (PS). Then microbial inactivation was evaluated by counting colony form units following
APDI and confocal laser scanning microscopy (CLSM) illustrated localization as well as the preferential accumulation of
PS into the fungal cells. C. neoformans KN99α was more resistant to APDI than CAP59 for all PSs tested. CLSM
showed incorporation of MB and RB into the cytoplasm and a preferential uptake in mitochondria. A nuclear
accumulation of MB was also observed. Contrarily, pL-ce6 appears accumulated in cell wall and cell membrane and
minimal florescence was observed inside the fungal cells. In conclusion, the ability of C. neoformans to form capsule
enhances survival following APDI.
Photodynamic therapy has emerged as a possible supplement to the existing protocols for endodontic
disinfection. Microbes are known to gain significant ecological advantage when they survive as
coaggregates and biofilms in an infected tissue. Such microbial coaggregates and biofilms have been
confirmed to play a key role in the pathogenicity of many infections. So far, not many studies have
correlated the efficacy of antimicrobial photodynamic inactivation (APDI) to the different modes of
bacterial growth. This study aims to evaluate the APDI of 3 strains of Enterococcus faecalis in planktonic
phase, in a co-aggregated suspension and in a 4-day old biofilm. The results showed that the biofilm mode
of growth offered the greatest resistance to APDI and the inclusion of an efflux pump inhibitor significantly
increased the APDI of biofilm bacteria. From this study, we conclude that APDI of bacteria in biofilms is
the most challenging and that the use of bacterial efflux pump inhibitors enhances its photodynamic antibiofilm
efficacy.
Multi-drug resistant Acinetobacter baumanii infections represent a growing problem, especially in traumatic wounds and
burns suffered by military personnel injured in Middle Eastern conflicts. Effective treatment using traditional antibiotics
can be extremely difficult and new antimicrobial approaches are being investigated. One of these antimicrobial
alternatives could be the combination of non-toxic photosensitizers (PS) and visible light known as photodynamic
therapy (PDT). We report on the establishment of a new mouse model of full thickness thermal burns infected with a
bioluminescent derivative of a clinical Iraqi isolate of A. baumannii and its PDT treatment by topical application of a PS
produced by covalent conjugation chlorin(e6) to polyethylenimine followed by illumination of the burn surface with red
light. Application of 108A. baumannii cells to the surface of 10-second burns made on the dorsal surface of shaved
female BALB/c mice led to chronic infections that lasted on average 22 days characterized by a remarkably stable
bacterial bioluminescence. PDT carried out on day 0 soon after applying bacteria gave over three logs of loss of bacterial
luminescence in a light exposure dependent manner, while PDT carried out on day 1 and day 2 gave approximately a
1.7-log reduction. Application of PS dissolved in 10% or 20% DMSO without light gave only modest reduction in
bacterial luminescence from mouse burns. Some bacterial regrowth in the treated burn was observed but was generally
modest. It was also found that PDT did not lead to inhibition of wound healing. The data suggest that PDT may be an
effective new treatment for multi-drug resistant localized A. baumannii infections.
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