Journal of Antimicrobial Chemotherapy (2006) 57, 872–876
doi:10.1093/jac/dkl070
Advance Access publication 13 March 2006
Synergic antibacterial effect between visible light and
hydrogen peroxide on Streptococcus mutans
Osnat Feuerstein1*, Daniel Moreinos1,2 and Doron Steinberg2
1
Department of Prosthodontics, Hebrew University–Hadassah School of Dental Medicine, Jerusalem, Israel;
Institute of Dental Sciences, Hebrew University–Hadassah School of Dental Medicine, Jerusalem, Israel
2
Received 30 November 2005; returned 19 January 2006; revised 9 February 2006; accepted 14 February 2006
Objectives: To evaluate the possibility of enhancing the phototoxic effect on Streptococcus mutans using a
potentially antibacterial synergic effect between blue light and hydrogen peroxide (H2O2), and to investigate
the antibacterial mechanism involved.
Methods: Growth of S. mutans samples was determined after exposure to light in the presence and absence
of H2O2. The effect of such light on H2O2 degradation, on reactive oxygen species (ROS) generation and on
the exposed-medium temperature was examined.
Results: The combination of light exposure for 20 s (23 J/cm2) and a concentration of 0.3 mM H2O2 yielded
96% growth inhibition, whereas, when applied separately, light exposure decreased bacterial growth by 3%
and H2O2 by 30% compared with the control. The results showed no direct effect of the light on H2O2
degradation, a partial protective effect of ROS scavengers on S. mutans and a non-lethal increase in
the medium temperature after light exposure.
Conclusions: An antibacterial synergic effect between blue light and H2O2 was observed. The mechanism of
the phototoxic effect on S. mutans was basically a photochemical process, in which ROS were involved.
Application of such light in combination with H2O2 to an infected tooth could be an alternative to or serve as
an additional minimally invasive antibacterial treatment.
Keywords: light exposure, phototoxic effect, reactive oxygen species
Introduction
There is no dispute that topical antibacterial agents commonly
used in dentistry have a potential bactericidal effect on oral
bacteria. However, most agents have undesired side effects,
which can be minimized by reducing their concentration. The
synergic effect of certain antibacterial agents may enable their
concentration to be reduced without affecting their biological
activity.1–3
Conventional synergy is achieved by a combination of two
chemical antibacterial agents. The use of a chemical photosensitizer agent in conjunction with lethal light photosensitization
has been shown to be effective against bacteria.4–9 However,
photosensitizers have the disadvantages of possibly colouring
the surrounding tissues and of low availability. Hydrogen peroxide (H2O2) and near-ultraviolet (UV) radiation is another combination of chemical agent and light that may enhance the
damaging effect on microorganisms.10 This effect may be
explained by OH· production, from homolytic fission of the
H2O2 caused by UV light. This phenomenon has not yet been
investigated using visible light.
Blue non-coherent light sources, such as the plasma-arc curing
(PAC) light, the halogen lamp and the light emitting diode, are
often used in dentistry for photocuring resin composites. Previous
studies have shown that visible light at wavelengths of 400–
500 nm (blue light) induced an oxygen-dependent phototoxic
effect on the periopathogenic bacteria Porphyromonas
gingivalis11–13 and Fusobacterium nucleatum, in which reactive
oxygen species (ROS) such as hydroxyl radicals (OH·) were
involved.12 These ROS have been shown to cause damage to
proteins, lipids and nucleic acids.14,15 Indeed, although nonionizing, visible light (wavelengths 408–750 nm) causes
mutagenic and metabolic damage to Escherichia coli cells.16
In a recent study we found that the phototoxic effect of blue
light on Streptococcus mutans, which is associated with dental
caries, was lower than that on P. gingivalis and F. nucleatum.11
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*Corresponding author. Tel: +972-2-6776142; Fax: +972-2-6429683; E-mail: [email protected]
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872
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Antibacterial synergy of H2O2 and visible light
This is probably related to the fact that S. mutans is protected by
antioxidant defence enzymes such as superoxide dismutase
(SOD).17
The aim of the present study was to evaluate the possibility of
enhancing the relatively low phototoxic effect on S. mutans by
making use of a potentially antibacterial synergic effect between
blue light and H2O2, and to investigate the mechanism involved.
Materials and methods
Bacteria
S. mutans (ATCC 27351) was used in these experiments. The
bacteria were grown in brain heart infusion (BHI) broth (Acumedia
Manufacturers, Baltimore, MD, USA) and incubated at 37 C in 5%
CO2. All bacteria were subcultured at least twice before exposure to
light. The bacteria were then suspended in PBS (Sigma, Steinheim,
Germany), and a 50 mL suspension was placed in the wells of a 96well microplate.
Hydrogen peroxide (H2O2)
Before exposure to light, 50 mL of H2O2 was added to each well, at
the following final concentrations: 30 mM, 3 mM and 0.3 mM.
Control bacterial samples, in the absence of H2O2, were prepared
with the addition of 50 mL of PBS. The H2O2 concentrations used
were significantly lower than the MIC.
Light source
A xenon lamp with a combined filter for transmission of blue light
(450–490 nm) (MSq, Caesarea, Israel), the dental PAC light, was
applied. The distance between the light source tip and the exposed
sample was fixed to obtain a constant power density. An average
light power of 440 mW was measured using a power meter (Ophir,
Jerusalem, Israel) over a spot of 0.7 cm diameter. To calculate power
density, the average power was divided by the area of the light spot.
Effect of light exposure in combination with H2O2
on bacterial growth
The bacterial samples (100 mL) in the presence and absence of H2O2
were exposed to blue light with a power density of 1144 mW/cm2 for
20, 30 and 40 s and 10 min, equivalent to 23, 34, 46 and 686 J/cm2.
Following light exposure, 100 mL of BHI at twice the normal concentration was added to each well. The experiment was conducted at
room temperature under aerobic conditions, and the samples were
then immediately incubated for 24 h at 37 C in 5% CO2. Bacterial
growth was determined by measuring the optical density at OD650 of
each sample using a microplate reader (VERSAmax, Molecular
Devices, Sunnyvale, CA, USA). All experiments were conducted
in triplicate and repeated four times (n = 12).
To determine the synergic, additive or antagonist effect between
H2O2 and the light source, the minimal inhibitory dose (MID, i.e. the
minimum level of light exposure required to inhibit 90% of bacterial
growth) and the MIC of H2O2 were determined. The MIC of H2O2,
when applied separately, was established using a broth dilution
method similar to that described by Shani et al.18 Then, the fractional
inhibitory concentration index (FICI) was calculated, based on the
formula described by Giertsen et al.,19 as follows:
FICI = H2O2 (MIC) (in combination with light exposure)/H2O2 (MIC) +
Light exposure (MID) (in combination with H2O2)/Light exposure (MID)
An index value lower than 1.0 indicates that a synergic effect has
taken place. An index value equal to 1.0 indicates an additive effect.
An index value higher than 1.0 indicates an antagonistic effect
between H2O2 and the light exposure.
Direct effect of blue light on H2O2 degradation
The following experiment was performed to determine whether blue
light affects the homolytic fission of H2O2, which results in the
formation of ROS.
The degradation of H2O2 is enhanced in vivo in the presence of
trace amounts of transition metals. Samples (100 mL) containing
H2O2 to which a cocktail of three transition metals (cupric chloride,
ammonium ferrous sulphate and manganese chloride at final concentrations of 10 mM each), PBS or double distilled water was added
were placed in a 96-well microplate. Experimental samples were
exposed to blue light for 60 s, whereas control samples were not
exposed. The concentration of H2O2 in each sample was measured
using a modification of the ferrithiocyanate method described
by Thurman et al.20 Briefly, after exposure to the light, 10 mL of
10 mM ferrous ammonium sulphate and subsequently 5 mL of 2.5 M
potassium thiocyanate were added to each well. The absorption of the
red ferrithiocyanate complex formed in the presence of H2O2 was
measured at 480 nm using a microplate reader (VERSAmax,
Molecular Devices, Sunnyvale, CA, USA).
Effect of light on bacterial growth in the presence
of scavengers
This experiment was performed to determine whether generation
of ROS is involved in the phototoxic effect of blue light in the
absence of H2O2 on S. mutans. Before exposing bacterial suspensions
to light, a cocktail containing the following ROS scavangers was
added (final concentration): 20 U/mL catalase from bovine liver
(Sigma, Steinheim, Germany), 100 mM dimethylthiourea (DMTU)
(Sigma), 30 U/mL SOD from Escherichia coli (Sigma) and 30 mM
ascorbic acid (Sigma). Samples (100 mL) were placed in a 96-well
microplate and exposed to blue light at 686 J/cm2 (1144 mW/cm2 for
10 min) under aerobic conditions. Then, 100 mL of sterile broth was
added to the samples and the microplate was incubated at 37 C in 5%
CO2 for 24 h. Bacterial growth was determined as described above.
All experiments were carried out in triplicate and repeated four times
(n = 12).
Temperature change following exposure to light
An increase in temperature during exposure to light could affect
bacterial growth. The temperature was measured in triplicate
using thermocouple electrodes (Almemo, Holzkirchen, Germany)
placed in 100 mL of medium (PBS) in a 96-well microplate, before
and immediately after exposure to light for 20 s and 1, 2, 3, 4 and
10 min.
Statistical methods
To assess the effect of different combinations of H2O2 and light
exposure on bacterial growth, two-way ANOVA was applied. The
influence of scavengers on the effect of the light source on bacterial
growth was assessed using one-way ANOVA test.
The effect of exposure to the light source on the degradation of
hydrogen peroxide was assessed by comparing red ferrithiocyanate
complex formation between exposed and non-exposed H2O2 samples, using the t-test as well as the non-parametric Mann–Whitney
test. All the applied tests were two-tailed, and a P value of £0.05 was
considered statistically significant.
873
Feuerstein et al.
Results
Effect of blue light in combination with H2O2 on
bacterial growth
Bacterial growth was assessed following light exposure in combination with different concentrations of H2O2. Growth of the
non-exposed (control) bacterial samples, and exposed samples in
the absence and presence of H2O2, was expressed as the percentage OD650 of the control non-exposed bacterial samples in the
absence of H2O2 (100%) (Figure 1). Exposure of bacterial samples to blue light in the absence of H2O2 showed no effect upon
exposure for 20, 30, 40, 60 and 180 s. Only an exposure time of
10 min (686 J/cm2) caused a reduction in bacterial growth.
H2O2 at a concentration of 0.3 mM decreased bacterial growth
by 30% compared with the control. An exposure time of 20 s
(23 J/cm2) decreased bacterial growth by 3% compared with the
control. The combination of light exposure for 20 s and a concentration of 0.3 mM H2O2 yielded 96% growth inhibition compared with the control. Statistical analysis showed that H2O2
treatment, exposure to light and their interaction are responsible
for 95.9% of the variability in bacterial growth (coefficient of
determination R2 = 0.959). The FICI value of this combination
was 0.0501, suggesting that a synergic effect had taken place.
Direct effect of blue light on the degradation of H2O2
The concentration of H2O2 was determined in the non-exposed
samples and in the 60 s light-exposed H2O2 samples. H2O2 concentration was essentially the same in the exposed H2O2 samples
and in the control (data not shown).
Effect of light on bacterial growth in the presence
of scavengers
Figure 2 shows the growth of the control non-exposed
bacterial samples and of the light-exposed bacterial samples in
the presence and absence of ROS scavengers. Bacterial growth
was expressed as the percentage OD650 of the control nonexposed bacterial samples in the absence of ROS scavengers
(100%). Bacterial growth after exposure to light in the presence
of ROS scavengers was significantly higher than in their absence.
On the other hand, a comparison between samples exposed to
blue light with and without ROS scavengers showed that the
presence of scavengers did not completely eliminate the bactericidal effect of the blue light (P < 0.001 one-way ANOVA).
Temperature change following exposure to light and its
effect on bacterial growth
The bacterial medium temperature was measured before and
immediately after exposure to blue light for up to 10 min.
Increases in temperature of 1, 3.6, 4.6, 5.7 and 13.9 C after
exposures of 20, 60, 120, 180 and 600 s, respectively, were
measured when compared with the control at 25 C. There was
no difference in bacterial growth between samples incubated at
40 C for 10 min and the control samples (data not shown).
Discussion
The results of the present study show a synergic antibacterial
effect between blue light and H2O2. The combination of light
exposure for 20 s (23 J/cm2) and a concentration of 0.3 mM
H2O2 yielded 96% growth inhibition, whereas, when they were
applied separately, bacterial growth was decreased by 3% when
exposed to light and by 30% in the presence of H2O2 as compared
with the control.
The results do not support the assumption that most of the
damage to the bacterial cells was the result of the fission of H2O2,
caused by the visible light, similar to the mechanism of action of
120
Control
20 s
30 s
40 s
10 min
Bacterial growth (%)
100
80
60
40
20
0
0 mM
0.3 mM
3 mM
30 mM
H2O2 concentration
Figure 1. Bacterial growth following exposure to blue light in combination with different concentrations of H2O2. Growth of the non-exposed (control, black)
bacterial samples and the samples exposed to blue light at 1144 mW/cm2 for 20 s (horizontal lines), 30 s (vertical lines), 40 s (grey) and 10 min (white) in the absence
(‘0 mM’) and presence of H2O2 at a concentration of 0.3, 3 and 30 mM, expressed as percentage OD650 of the non-exposed bacterial samples in the absence of H2O2
(100%).
874
Antibacterial synergy of H2O2 and visible light
120
Scavengers
No scavengers
Bacterial growth (%)
100
80
*
60
40
20
0
No exposure
10 min exposure
No exposure
10 min exposure
2
Figure 2. Bacterial growth of the control non-exposed samples and of the blue-light-exposed samples (1144 mW/cm , 10 min) in the presence (black) and absence
(white) of ROS scavengers. Bacterial growth is expressed as percentage OD650 of the control non-exposed bacterial samples in the absence of ROS scavengers (100%).
*Significant difference between the group of samples exposed to light in the presence of scavengers and all the other groups (P < 0.001).
UV light.10 However, the synergy between blue light and H2O2
might be the result of the following mechanisms:
(i) Highly reactive OH· could be generated when H2O2 encounters ‘free Fe(II)’, via the Fenton reaction.10 Therefore, conditions under which bound Fe(II) is liberated, such as photooxidation, are extremely dangerous to metabolically active
Fe-containing cells, not only because of the generation of
OH· but also because the loss of Fe from iron-dependent
enzymes leads to failure of the biochemical pathways in
which they participate.21
(ii) OH·, being a potent oxidant, can react readily with macromolecules such as DNA or lipids in the cell membrane,22 a
principal site of photo-oxidative damage.23
(iii) H2O2 could increase the plasma membrane permeability24 of
the cells sublethally injured by exposure to light. This might
also lead to a higher penetration of H2O2, resulting in damage
to the intracellular organelles.
Overall, these results are in agreement with Khaengraeng and
Reed,25 who suggested that the sublethal damage to bacterial
cells caused by light leads to an ROS-sensitive state, since it
imposes an additional stress on these bacteria.
Indeed, our results showed a partial protective effect of ROS
scavengers on bacteria exposed to blue light alone, indicating that
the mechanism of the phototoxic effect on S. mutans was mainly
a photochemical process, in which ROS were involved. Those
results regarding S. mutans are similar to the results demonstrating the effect of blue light on P. gingivalis and F. nucleatum.12 In
both studies, the lack of complete protection by the scavengers
could be due to their partially inefficient access to the ROS
generated within the cells and their inability to scavenge the
highly reactive radicals.12,26 Involvement of a photothermal process in the mechanism of the phototoxic effect on bacteria27 can
be ruled out, since the increase in medium temperature following
light exposure was not lethal. However, the contribution of this
minimal temperature elevation to the photochemical toxic effect
cannot be excluded. The study showed that only a minute amount
of H2O2, which is most likely present in saliva and tissues, was
required to induce the synergic antibacterial effect between light
exposure and H2O2. Application of such light in combination
with H2O2 to infected tooth tissue could be an alternative to
or serve as an additional minimally invasive antibacterial treatment of dental caries or of root canal infection.
Planktonic bacteria may exhibit properties that are different
from those exhibited by biofilm bacteria.28 Therefore, testing this
effect in biofilm conditions of monoculture or mixed bacterial
culture is of interest as bacteria in the oral cavity are also present
in biofilms attached to tooth surfaces. The safety of applications
of blue light with or without the addition of H2O2, as an antibacterial treatment, should also be further investigated on various
tissues and under different physiological conditions.
In conclusion, this study shows a synergic antibacterial effect
between exposure to blue light and H2O2, based on a photochemical mechanism in which ROS are involved. Future studies exploring the molecular level at which the bacterial cells are affected
may help to elucidate this synergic mechanism.
Acknowledgements
This study is part of the PhD dissertation of Daniel Moreinos. This
research was funded in part by The Israel Health Ministry and was
performed in the Ronald E Goldstein Center for Esthetic Dentistry
and Dental Materials Research, Hebrew University-Hadassah
School of Dental Medicine.
Transparency declarations
None to declare.
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