Photoinactivation of microorganisms using bacteriochlorins as photosensitizers.

In recent years, some microorganisms have shown resistance to conventional treatments. Considering this increase in resistant pathogens, treatment alternatives are needed to promote greater treatment efficiency. In this sense, antimicrobial photodynamic therapy (aPDT) has been an alternative treatment. This technique uses a photosensitizer that is activated by light with a specific wavelength producing reactive species, leading to the death of pathogenic microorganisms. In this study, bacteriochlorophyll derivatives such as bacteriochlorin metoxi (Bchl-M) and bacteriochlorin trizma (Bchl-T) obtained from purple bacterium (Rhodopseudomonas faecalis), were evaluated as photosensitizers in the aPDT. Photodynamic inactivation (PDI) of the microorganisms Staphylococcus aureus, Micrococcus luteus, Candida albicans and Pseudomonas aeruginosa was investigated with both bacteriochlorins (Bchl-M and Bchl-T) at different concentrations (1, 15 and 30 µM for S. aureus; 1, 15, 30, 45, 60 and 75 µM for M. luteus; 30, 60, 90, 105, 120 and 150 µM for C. albicans; and 200 µM for P. aeruginosa) and different doses of light (20 and 30 J/cm2 for S. aureus and M. luteus; 30 and 45 J/cm2 for C. albicans; and 45 J/cm2 for P. aeruginosa) to inactivate them. Both photosensitizers showed good activation against S. aureus and for M. luteus, we observed the inactivation of these microorganisms at approximately 3 log, showing to be a good photosensitizers for these microorganisms.

Inhibition of bacterial growth through LED (light-emitting diode) 465 and 630 nm: in vitro.

Photobiomodulation has been used to inactivate bacterial growth, in different laser or LED protocols. Thus, the aim of this study was to verify the inhibition of Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli, in ATCC strains and bacteria collected from patients with skin burns, after irradiation with LED; 300 µl of saline solution with bacterial suspension was irradiated at a concentration of 0.5-0.63, by the McFarland scale, after five serial dilutions, with evaluation of pre- and post-irradiation pH and temperature control. The cultures were placed in a bacteriological incubator at 37 °C for 24 h for later counting of colony-forming units (CFU). Data were analyzed by Shapiro-Wilk tests and single-factor ANOVA, with Tukey post hoc (p < 0.05). Both wavelengths and energy densities tested showed inhibition of bacterial growth. The comparison of the irradiated groups (ATCC) with the control group showed the following: S. aureus and P. aeruginosa 465 nm (40 J/cm2) and 630 nm (50 J/cm2) and E. coli 465 nm (40 J/cm2) and 630 nm (30 J/cm2). Among the ATCC S. aureus groups, there was a difference for 630 nm (30 J/cm2) and 465 nm (30, 40, 50 J/cm2). The bacteria from the burned patients were S. aureus (30 and 50 J/cm2) and P. aeruginosa (50 J/cm2). We conclude that different bacterial strains were reduced into colony-forming units after LED irradiation.

Light Modulates Important Pathogenic Determinants and Virulence in ESKAPE Pathogens Acinetobacter baumannii, Pseudomonas aeruginosa, and Staphylococcus aureus.

Light sensing has been extensively characterized in the human pathogen Acinetobacter baumannii at environmental temperatures. However, the influence of light on the physiology and pathogenicity of human bacterial pathogens at temperatures found in warm-blooded hosts is still poorly understand. In this work, we show that Staphylococcus aureus, Acinetobacter baumannii, and Pseudomonas aeruginosa (ESKAPE) priority pathogens, which have been recognized by the WHO and the CDC as critical, can also sense and respond to light at temperatures found in human hosts. Most interestingly, in these pathogens, light modulates important pathogenicity determinants as well as virulence in an epithelial infection model, which could have implications in human infections. In fact, we found that alpha-toxin-dependent hemolysis, motility, and growth under iron-deprived conditions are modulated by light in S. aureus Light also regulates persistence, metabolism, and the ability to kill competitors in some of these microorganisms. Finally, light exerts a profound effect on the virulence of these pathogens in an epithelial infection model, although the response is not the same in the different species; virulence was enhanced by light in A. baumannii and S. aureus, while in A. nosocomialis and P. aeruginosa it was reduced. Neither the BlsA photoreceptor nor the type VI secretion system (T6SS) is involved in virulence modulation by light in A. baumannii Overall, this fundamental knowledge highlights the potential use of light to control pathogen virulence, either directly or by manipulating the light regulatory switch toward the lowest virulence/persistence configuration.IMPORTANCE Pathogenic bacteria are microorganisms capable of producing disease. Dangerous bacterial pathogens, such as Staphylococcus aureus, Pseudomonas aeruginosa, and Acinetobacter baumannii, are responsible for serious intrahospital and community infections in humans. Therapeutics is often complicated due to resistance to multiple antibiotics, rendering them ineffective. In this work, we show that these pathogens sense natural light and respond to it by modulating aspects related to their ability to cause disease; in the presence of light, some of them become more aggressive, while others show an opposite response. Overall, we provide new understanding on the behavior of these pathogens, which could contribute to the control of infections caused by them. Since the response is distributed in diverse pathogens, this notion could prove a general concept.

Role of quorum sensing in UVA-induced biofilm formation in Pseudomonas aeruginosa.

Pseudomonas aeruginosa, a versatile bacterium present in terrestrial and aquatic environments and a relevant opportunistic human pathogen, is largely known for the production of robust biofilms. The unique properties of these structures complicate biofilm eradication, because they make the biofilms very resistant to diverse antibacterial agents. Biofilm development and establishment is a complex process regulated by multiple regulatory genetic systems, among them is quorum sensing (QS), a mechanism employed by bacteria to regulate gene transcription in response to population density. In addition, environmental factors such as UVA radiation (400-315 nm) have been linked to biofilm formation. In this work, we further investigate the mechanism underlying the induction of biofilm formation by UVA, analysing the role of QS in this phenomenon. We demonstrate that UVA induces key genes of the Las and Rhl QS systems at the transcriptional level. We also report that pelA and pslA genes, which are essential for biofilm formation and whose transcription depends in part on QS, are significantly induced under UVA exposure. Finally, the results demonstrate that in a relA strain (impaired for ppGpp production), the UVA treatment does not induce biofilm formation or QS genes, suggesting that the increase of biofilm formation due to exposure to UVA in P. aeruginosa could rely on a ppGpp-dependent QS induction.

Removal of contaminants of emerging concern (CECs) and antibiotic resistant bacteria in urban wastewater using UVA/TiO2/H2O2 photocatalysis.

The dispersion of pollutants and proliferation of antibiotic resistant bacteria in the aquatic environment are an emerging health concern worldwide. In this sense, it is essential to develop new technologies to increase the quality of wastewater treatment, which is spread throughout the environment. The present study has demonstrated evidence of the existence of antibiotic and mercury-resistant bacteria in the aquatic environment. The application of heterogeneous photocatalysis with UVA/TiO2 P25 slurry (200 mg L-1), UVA/TiO2-immobilized, and UVA/TiO2-immobilized/H2O2 were evaluated for the simultaneous elimination of a mixture of contaminants of emerging concern (acetamiprid (ACP), imazalil (IMZ) and bisphenol A (BPA)) and inactivation of antibiotic and mercury-resistant bacteria (Pseudomonas aeruginosa and Bacillus subtilis). UVA/TiO2-immobilized/H2O2 increased the inactivation and elimination of the contaminants. After the combined treatment, the mixture of BPA, IMZ and ACP decreased 62%, 21% and <5%, respectively, after 300 min at 13.10 kJ L-1 of accumulated UV energy. The Pseudomonas aeruginosa strain was inactivated after 120 min using 5.24 kJ L-1 of accumulated UV energy, whereas the Bacillus subtilis strain was shown to be extremely resistant, with a capacity to develop mechanisms to avoid the oxidation process.

Lethal Effect of Photodynamic Treatment on Persister Bacteria.

Persister bacteria tolerate bactericidal antibiotics due to transient and reversible phenotypic changes. As these bacteria can limit the effectiveness of antibiotics to eradicate certain infections, their elimination is a relevant issue. Photodynamic therapy seems suitable for this purpose, but phenotypic tolerance to it has also been reported for Pseudomonas aeruginosa. To test whether any phenotypic feature could confer tolerance against both antibiotics and photoinactivation, survivors from exposures to light in the presence of methylene blue were treated with ofloxacin, an antibiotic effective on nongrowing bacteria. Susceptibility to ofloxacin was normal in these bacteria in spite of their increased ability to survive photodynamic inactivation, suggesting the absence of cross-tolerance. It thus seemed possible to use one of these treatments to eliminate bacteria which had phenotypic tolerance to the other. To test this strategy, persister bacteria emerging from ofloxacin treatments were submitted to the action of light and methylene blue while the antibiotic remained in the bacterial suspension. Persisters lost their clonogenic ability under these conditions and the effects of the treatments seemed to be synergistic. These observations suggest that photodynamic antimicrobial therapy could be used as a complement to antibiotic treatments to eliminate persister bacteria from localized infections.

A Possible Phenomenon of Persistence in Pseudomonas aeruginosa Treated with Methylene Blue and Red Light.

Planktonic Pseudomonas aeruginosa cells harvested in stationary phase were exposed to red light in the presence of methylene blue to study the potential occurrence of persistence in bacterial populations submitted to photodynamic antimicrobial therapy. Survival curves revealed the existence of small subpopulations of cells exhibiting increased ability to tolerate the treatment. These subpopulations were detected even using high concentrations of photosensitizer, whether added in a single step or following a fractionated scheme, and when the irradiation medium was modified to delay the photodecomposition of methylene blue. When cells grown from survivors to the treatment were cultured and exposed to red light and dye, their responses were similar to that of the original strain. These results exclude exhaustion of the photosensitizer and selection of resistant mutants as explanations for the features of the survival curves. Cells able to tolerate the treatment were found even when radiation was imparted at a high-dose rate. They exhibit a response typical of persisters, which tolerate antimicrobial agents due to transient and reversible changes in their phenotype, suggesting that persistence is a factor to consider upon evaluating the efficacy of photodynamic antimicrobial therapy.

Exposure to low UVA doses increases KatA and KatB catalase activities, and confers cross-protection against subsequent oxidative injuries in Pseudomonas aeruginosa.

Solar UVA radiation is one of the main environmental stress factors for Pseudomonas aeruginosa. Exposure to high UVA doses produces lethal effects by the action of the reactive oxygen species (ROS) it generates. P. aeruginosa has several enzymes, including KatA and KatB catalases, which provide detoxification of ROS. We have previously demonstrated that KatA is essential in defending P. aeruginosa against high UVA doses. In order to analyse the mechanisms involved in the adaptation of this micro-organism to UVA, we investigated the effect of exposure to low UVA doses on KatA and KatB activities, and the physiological consequences. Exposure to UVA induced total catalase activity; assays with non-denaturing polyacrylamide gels showed that both KatA and KatB activities were increased by radiation. This regulation occurred at the transcriptional level and depended, at least partly, on the increase in H2O2 levels. We demonstrated that exposure to low UVA produced a protective effect against subsequent lethal doses of UVA, sodium hypochlorite and H2O2. Protection against lethal UVA depends on katA, whilst protection against sodium hypochlorite depends on katB, demonstrating that different mechanisms are involved in the defence against these oxidative agents, although both genes can be involved in the global cellular response. Conversely, protection against lethal doses of H2O2 could depend on induction of both genes and/or (an)other defensive factor(s). A better understanding of the adaptive response of P. aeruginosa to UVA is relevant from an ecological standpoint and for improving disinfection strategies that employ UVA or solar irradiation.

Red and infrared laser therapy inhibits in vitro growth of major bacterial species that commonly colonize skin ulcers.

Low-level laser therapy (LLLT) is used in chronic wounds due to its healing effects. However, bacterial species may colonize these wounds and the optimal parameters for effective bacterial inhibition are not clear. The aim of this study was to analyze the effect of LLLT on bacterial growth in vitro. Bacterial strains including Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa were suspended in saline solution at a concentration of 10(3) cells/ml and exposed to laser irradiation at wavelengths of 660, 830, and 904 nm at fluences of 0 (control), 3, 6, 12, 18, and 24 J/cm(2). An aliquot of the irradiated suspension was spread on the surface of petri plates and incubated at 37 °C for quantification of colony-forming unit after 24, 48, and 72 h. Laser irradiation inhibited the growth of S. aureus at all wavelengths and fluences higher than 12 J/cm(2), showing a strong correlation between increase in fluence and bacterial inhibition. However, for P. aeruginosa, LLLT inhibited growth at all wavelengths only at a fluence of 24 J/cm(2). E. coli had similar growth inhibition at a wavelength of 830 nm at fluences of 3, 6, 12, and 24 J/cm(2). At wavelengths of 660 and 904 nm, growth inhibition was only observed at fluences of 12 and 18 J/cm(2), respectively. LLLT inhibited bacterial growth at all wavelengths, for a maximum of 72 h after irradiation, indicating a correlation between bacterial species, fluence, and wavelength.

Blue Laser Inhibits Bacterial Growth of Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa.

OBJECTIVE: The purpose of this study was to analyze the influence of blue laser on bacterial growth of the main species that usually colonize cutaneous ulcers, as well as its effect over time following irradiation. BACKGROUND DATA: The use of blue laser has been described as an adjuvant therapeutic method to inhibit bacterial growth, but there is no consensus about the best parameters to be used. METHODS: Strains of Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 27853, and Escherichia coli ATCC 25922 were suspended in saline solution at a concentration of 1.5×10(3) colony forming units (CFU)/mL. Next, 300 µL of this suspension was transferred to a microtitulation plate and exposed to a single blue laser irradiation (450 nm) at fluences of 0 (control), 3, 6, 12, 18, and 24 J/cm(2). Each suspension was spread over the surface of a Petri plate before being incubated at 37°C, and counts of CFU were determined after 24 and 48 h. RESULTS: Blue laser inhibited the growth of S. aureus and P. aeruginosa at fluences >6 J/cm(2). On the other hand, E. coli was inhibited at all fluences tested, except at 24 J/cm(2). CONCLUSIONS: Blue laser light was capable of inhibiting bacterial growth at low fluences over time, thus presenting no time-dependent effect.

In Vitro Evaluation of Antimicrobial Photodynamic Therapy Associated with Hydroalcoholic Extracts of Schinopsis brasiliensis Engl.: New Therapeutic Perspectives.

OBJECTIVE: The aim of this study was to evaluate the photodynamic potential of extracts of Schinopsis brasiliensis Engl. on bacteria involved in several human infections. BACKGROUND DATA: Photodynamic therapy (PDT) involves the interaction of light with an appropriate and photosensitizer wavelength, and the prospect of existing photosensitive compounds in herbal extracts enhanced by the application of laser diode has been promising. METHODS: The antibacterial activity against Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Enterococcus faecalis was obtained by the disk diffusion method on agar. The laser diode InGaAIP was used with 660 nm wavelength, 100 mW, and 4 J/cm(2), and the application was performed in a timely manner for 34 sec on each disk tested. The groups tested were: Laser and bark extract (B+L+); bark extract only (B+L-); Laser and leaf extract (F+L+); leaf extract only (F+L-); Laser and malachite green (M+L+); malachite green only (M+L-); and laser only (L+). RESULTS: There were significant differences between the B+L- and B+L+ groups (p=0.029) and between the L+F- and L+F+ groups (p=0.029) at various concentrations of the nebulized extracts of bark and leaf. Among the tested pathogens, S. aureus showed the highest zone of inhibition, 24.55±0.36 mm in group B+L+, 500 mg.mL(-1). CONCLUSIONS: PDT with malachite green was effective, and groups B+L+ and F+L+ showed excellent activity on the bacteria tested, suggesting the presence of photosensitizers in extracts of S. brasiliensis Engl.

Survival of Pseudomonas aeruginosa exposed to sunlight resembles the phenom of persistence.

During exposure of Pseudomonas aeruginosa stationary phase cells to natural solar radiation, a reduction in the rate of loss of bacterial viability was observed when survival fractions were lower than 1/10,000. This reduction was independent of the growth medium used and of the initial bacterial concentration, and was also observed when irradiation was performed with artificial UVA radiation (365nm, 47Wm(-2)). These results indicate the presence of a small bacterial subpopulation with increased tolerance to radiation. Such a tolerance is non-heritable, since survival curves comparable to those of the parental strain were obtained from survivors to long-term exposure to radiation. The radiation response described here resembles the phenomenon called persistence, which consists of the presence of a small subpopulation of slow-growing cells which are able to survive antibiotic treatment within a susceptible bacterial population. The condition of persister cells is acquired via a reversible switch and involves active defense systems towards oxidative stress. Persistence is probably responsible for biphasic responses of bacteria to several stress conditions, one of which may be exposure to sunlight. The models currently used to analyze the lethal action of sunlight overestimate the effect of high-dose irradiation. These models could be improved by including the potential formation of persister cells.

Impact of environmental stress on biochemical parameters of bacteria reducing chromium.

Chromium pollution is produced in connection with industrial processes like in tanneries. It has been suggested that bioremediation could be a good option for clean up. The stress effect of variable chromate levels, pHs and growth temperatures on biochemical parameters of two Cr(VI) reducing bacterial strains Pseudomonas aeruginosa Rb-1 and Ochrobactrum intermedium Rb-2 was investigated. Transmission electrone microscopy (TEM) was performed to study the intracellular distribution of Cr(VI). It was observed that initial stress of 1000 µgmL(-1) caused significant enhancement of all studied biochemical parameters at pH 7.0 and growth temperature of 37 °C showing great bioremediation potential of the strains. Transmission electron microscopy revealed that the distribution of chromium precipitates was not uniform as they were distributed in the cytoplasm as well as found associated with the periplasm and outer membrane. Fourier transform infrared spectroscopy showed the possible involvement of carboxyl, amino, sulpohonate and hydroxyl groups present on the bacterial cell surface for the binding of Cr(VI) ions. Cr(VI) stress brought about changes in the distridution of these functional groups. It can be concluded that the investigated bacterial strains adjust well to Cr(VI) stress in terms of biochemical parameters and along that exhibited alteration in morphology.

Impact of environmental stress on biochemical parameters of bacteria reducing chromium

Chromium pollution is produced in connection with industrial processes like in tanneries. It has been suggested that bioremediation could be a good option for clean up. The stress effect of variable chromate levels, pHs and growth temperatures on biochemical parameters of two Cr(VI) reducing bacterial strains Pseudomonas aeruginosa Rb-1 and Ochrobactrum intermedium Rb-2 was investigated. Transmission electrone microscopy (TEM) was performed to study the intracellular distribution of Cr(VI). It was observed that initial stress of 1000 µgmL-1 caused significant enhancement of all studied biochemical parameters at pH 7.0 and growth temperature of 37 °C showing great bioremediation potential of the strains. Transmission electron microscopy revealed that the distribution of chromium precipitates was not uniform as they were distributed in the cytoplasm as well as found associated with the periplasm and outer membrane. Fourier transform infrared spectroscopy showed the possible involvement of carboxyl, amino, sulpohonate and hydroxyl groups present on the bacterial cell surface for the binding of Cr(VI) ions. Cr(VI) stress brought about changes in the distridution of these functional groups. It can be concluded that the investigated bacterial strains adjust well to Cr(VI) stress in terms of biochemical parameters and along that exhibited alteration in morphology.

Impact of environmental stress on biochemical parameters of bacteria reducing chromium

Chromium pollution is produced in connection with industrial processes like in tanneries. It has been suggested that bioremediation could be a good option for clean up. The stress effect of variable chromate levels, pHs and growth temperatures on biochemical parameters of two Cr(VI) reducing bacterial strains Pseudomonas aeruginosa Rb-1 and Ochrobactrum intermedium Rb-2 was investigated. Transmission electrone microscopy (TEM) was performed to study the intracellular distribution of Cr(VI). It was observed that initial stress of 1000 µgmL-1 caused significant enhancement of all studied biochemical parameters at pH 7.0 and growth temperature of 37 °C showing great bioremediation potential of the strains. Transmission electron microscopy revealed that the distribution of chromium precipitates was not uniform as they were distributed in the cytoplasm as well as found associated with the periplasm and outer membrane. Fourier transform infrared spectroscopy showed the possible involvement of carboxyl, amino, sulpohonate and hydroxyl groups present on the bacterial cell surface for the binding of Cr(VI) ions. Cr(VI) stress brought about changes in the distridution of these functional groups. It can be concluded that the investigated bacterial strains adjust well to Cr(VI) stress in terms of biochemical parameters and along that exhibited alteration in morphology.

Protective effect of low UVA irradiation against the action of lethal UVA on Pseudomonas aeruginosa: role of the relA gene.

The exposure of Pseudomonas aeruginosa cells to very low UVA fluences induces a growth delay, a phenomenon proposed in Escherichia coli as an adaptive mechanism related to protection against lethal and mutagenic effects of UVA. This paper reports that the treatment with low UVA irradiation fluences protects P. aeruginosa PAO1 strain from a subsequent lethal exposure. This phenomenon depends on the relA gene, coding for the main (p)ppGpp synthetase, and is unrelated to the induction of quorum sensing or catalase activity, two essential factors involved in the response of P. aeruginosa to UVA. Cross-protection between osmotic stress and UVA is observed when a great protective response to lethal UVA is caused by the induction of resistance to osmotic stress. The increase in resistance to osmotic shock observed in the pre-irradiated PAO1 strain but not in its relA derivative, unable to show photo-protection, leads us to hypothesize that the photo-protection could be attributed to an adaptive response to osmotic stress. It is concluded that the exposure of P. aeruginosa to low UVA doses induces a relA-dependent adaptive response that protects against cell death induced by high doses and causes an increase in the resistance to osmotic stress.

In vitro assessment of the bactericidal effect of low-power arsenium-gallium (AsGa) laser treatment.

The objective of the present study was to perform an in vitro evaluation of the bactericidal action of a low-power arsenium-gallium (AsGa) laser at a wavelength of 904nm and energy density of 6 J/cm(2). Ten petri dishes were seeded with Pseudomonas aeruginosa and another ten with Staphylococcus aureus. The dishes were then randomly divided into four groups with five plates in each group. Two groups were treated with AsGa laser once a day for 5 days, while the other two groups received no treatment. No halo of growth inhibition was found in any of the groups. It was therefore concluded that laser treatment (AsGa, 904nm, 6J/cm(2)) had no bactericidal effect.

Contribution of UVB radiation to bacterial inactivation by natural sunlight.

The contribution of different components of sunlight to the lethal action exerted by this radiation on bacteria was studied using Pseudomonas aeruginosa ATCC27853 as a model organism. When solar UVB was excluded from the incident radiation by filtering it through a naphthalene solution (cut off 327 nm), significant modifications were observed in the cell-death kinetics. These modifications were comparable to those expected for a reduction of 27-32% in the dose rate, according to the model used in the analysis of the survival curves, and were also observed when the effects of sunlight filtered through polyethylene terephthalate (cut off 331 nm) or polystyrene (cut off 298 nm) were compared. Viability of P. aeruginosa remained almost unchanged when the incident radiation was filtered through a sodium nitrite solution (cut off 406 nm) in order to exclude the UVA and UVB components of sunlight. Nevertheless, a delay in colony formation was detected in bacteria treated in this way, suggesting that a non-lethal effect was exerted by visible light. The results are not consistent with a generally accepted notion which attributes the lethal action of sunlight to the radiation with wavelengths above 320 nm. The characterization of UVB contribution to the lethal effect of sunlight on bacteria is relevant for understanding of the mechanism of cell death, and for improvement of dosimetry techniques and irradiation procedures.

Role of the quorum sensing mechanism in the response of Pseudomonas aeruginosa to lethal and sublethal UVA irradiation.

The role of quorum sensing (QS) in the response of Pseudomonas aeruginosa to UVA radiation was investigated in the PAO1 strain and derivatives defective in the synthesis of the QS signals 3OC12-HSL (lasI strain), C4-HSL (rhlI strain) or both (lasI rhlI strain). Cell viability measurements demonstrated that the double mutant was significantly more sensitive to UVA than single mutants, which in turn showed reduced cell survival with regard to the PAO1 strain. Irradiation under nitrogen atmosphere and chemiluminescence measurements indicated the oxidative nature of the UVA-induced damage. The activity of the antioxidant enzymes catalase and superoxide dismutase was assayed in these strains before and after irradiation, and a strong correlation between catalase levels and UVA sensitivity was observed. When a sublethal UVA dose was applied to PAO1, a growth delay was observed and this mechanism depended on the rhl system. Moreover, low doses of UVA irradiation triplicated the level of C4-HSL in log PAO1 cells. It is concluded that QS is fundamental in the defense against the toxic effects of UVA in P. aeruginosa. The induction of the QS system by UVA independently of cell density could function as an adaptative strategy to withstand this hostile environmental condition.

Microwave disinfection of complete dentures contaminated in vitro with selected bacteria.

PURPOSE: This study evaluated the effectiveness of microwave irradiation for disinfection of simulated complete dentures. MATERIALS AND METHODS: Seventy dentures were fabricated in a standardized procedure, subjected to ethylene oxide sterilization, individually inoculated (10(7) cfu/mL) with Staphylococcus aureus (n = 20), Pseudomona aeruginosa (n = 20), and Bacillus subtilis (n = 30) and incubated for 24 hours at 37 degrees C. After that, 40 dentures were selected for microwaving. For each microorganism, 10 dentures were submitted to microwave irradiation at 650 W for 3 minutes. In addition, 10 dentures contaminated with B. subtilis were irradiated for 5 minutes. Thirty non-microwaved dentures (n = 10 for each bacteria) were used as positive controls. Replicate aliquots (25 microL) of suspensions were plated at dilutions of 10(-3) to 10(-6) on plates of selective media appropriate for each organism. After incubation (37 degrees C for 48 hours), colonies were counted (cfu/mL). TSB beakers with the microwaved dentures were incubated at 37 degrees C for a further 7 days to verify long-term disinfection. The data were statistically analyzed by the Kruskal-Wallis test (alpha= 0.05). RESULTS: No evidence of growth was observed at 48 hours for S. aureus and P. aeruginosa on plates, and no turbidity was visible in the TSB beakers of these specimens after 7 days of incubation. Dentures contaminated with B. subtilis and irradiated for 3 minutes produced microbial growth on six plates and turbidity on all TSB beakers. Microwaving for 5 minutes resulted in survival of B. subtilis in two plates and two beakers. CONCLUSION: Microwave irradiation for 3 minutes at 650 W produced sterilization of complete dentures contaminated with S. aureus and P. aeruginosa. Dentures contaminated with B. subtilis were disinfected by microwave irradiation after 3 and 5 minutes at 650 W.