Email updates

Keep up to date with the latest news and content from Journal of Biomedical Science and BioMed Central.

MST logoThe cost of publication in Journal of Biomedical Science is borne by the Ministry of Science and Technology, Taiwan.

Open Access Highly Accessed Research

The effects of the bacterial interaction with visible-light responsive titania photocatalyst on the bactericidal performance

Chia-Liang Cheng12, Der-Shan Sun34, Wen-Chen Chu5, Yao-Hsuan Tseng6, Han-Chen Ho7, Jia-Bin Wang1, Pei-Hua Chung1, Jiann-Hwa Chen8, Pei-Jane Tsai9, Nien-Tsung Lin10, Mei-Shiuan Yu10 and Hsin-Hou Chang234*

Author Affiliations

1 Department of Physics, National Dong-Hwa University, Hualien, Taiwan

2 Nanotechnology Research Center, National Dong-Hwa University, Hualien, Taiwan

3 Institute of Molecular Biology and Human Genetics, Tzu-Chi University, Hualien, Taiwan

4 Institute of Medical Science, Tzu-Chi University, Hualien, Taiwan

5 Department of Life Science, Tzu-Chi University, Hualien, Taiwan

6 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan

7 Department of Anatomy, Tzu-Chi University, Hualien, Taiwan

8 Institute of Molecular Biology, National Chung Hsing University, Taichung, Taiwan

9 Institute of Medical Biotechnology, Tzu-Chi University, Hualien, Taiwan

10 Institute of Microbiology, Immunology and Molecular Medicine, Tzu-Chi University, Hualien, Taiwan

For all author emails, please log on.

Journal of Biomedical Science 2009, 16:7  doi:10.1186/1423-0127-16-7


The electronic version of this article is the complete one and can be found online at: http://www.jbiomedsci.com/content/16/1/7


Received:24 October 2008
Accepted:15 January 2009
Published:15 January 2009

© 2009 Cheng et al; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Bactericidal activity of traditional titanium dioxide (TiO2) photocatalyst is effective only upon irradiation by ultraviolet light, which restricts the potential applications of TiO2 for use in our living environments. Recently carbon-containing TiO2 was found to be photoactive at visible-light illumination that affords the potential to overcome this problem; although, the bactericidal activity of these photocatalysts is relatively lower than conventional disinfectants. Evidenced from scanning electron microscopy and confocal Raman spectral mapping analysis, we found the interaction with bacteria was significantly enhanced in these anatase/rutile mixed-phase carbon-containing TiO2. Bacteria-killing experiments indicate that a significantly higher proportion of all tested pathogens including Staphylococcus aureus, Shigella flexneri and Acinetobacter baumannii, were eliminated by the new nanoparticle with higher bacterial interaction property. These findings suggest the created materials with high bacterial interaction ability might be a useful strategy to improve the antimicrobial activity of visible-light-activated TiO2.

Background

The widespread use of antibiotics and the emergence of more resistant and virulent strains of microorganisms [1-3] have caused an urgent need to develop alternative sterilization technologies. Using the superb photocatalytic effect of titanium dioxide (TiO2) is a conceptually feasible technology for this material is easy and inexpensive to produce in industrial scale. Photocatalytic TiO2 substrates have been shown to eliminate organic compounds and to function as disinfectants [4]. Upon ultraviolet (UV) light excitation, the photon energy excites valence band electron and generates pairs of electrons and holes (electron-vacancy in valence band) that diffuse and are trapped on or near the TiO2 surface. These excited electrons and holes have strong reducing and oxidizing activity and react with atmospheric water and oxygen to yield reactive species such as hydroxyl radicals (.OH) and superoxide anions (O2-) [5]. These radicals, .OH and O2- are extremely reactive upon contact with organic compounds. Complete oxidation of organic compounds and bacterial cells to carbon dioxide could be achieved [6,7]. Reactive oxygen species (ROS), such as .OH, O2-, and hydrogen peroxide (H2O2) generated on the light irradiated TiO2 surfaces, were shown to operate in concert to attack polyunsaturated phospholipids in bacteria [4]. Traditional TiO2 photocatalyst, however, is effective only upon irradiation of UV-light at levels that would also induce serious damage to human cells. This greatly restricts the potential applications of TiO2 substrates for use in our living environments. Recently, nitrogen or metal ion-doped anatase based TiO2 photocatalysts have been identified to be active upon visible-light illumination [8,9], offering the possibility to overcome this problem.

It is believed that nanometer-sized anatase phase particles have large surface area are efficient for the decomposition of pollutants in air and in water [10]. Furthermore, it is also found that the presence of anatase and rutile phases is important in some of the photocatalytic reactions where oxygen is used as electron acceptor [10]. Transmission electron microscopy studies also revealed that commercial TiO2 powder Degussa (P-25) consisting both anatase and rutile phases [11]. However, in these studies, the photocatalytic activities were induced under UV irradiation (wavelength < 380 nm). Previously, we have produced carbon-containing TiO2 in two different calcination temperatures (150°C and 200°C) resulted in two different nano-crystals (labeled as C150 and C200, respectively) with photocatalytic activity in the visible-light range [12]. These materials seem to be more convenient to apply in our living environment than the commercial UV responsive photocatalysts. The antibacterial activity of visible-light responsive photocatalysts has been reported by several groups [13-15]. Since photocatalyst-based anti-microbial technologies are still under development, the antibacterial activity of these materials does not match to that of conventional chemical disinfectants [13,16]. To improve the antibacterial activity, previous studies were mainly focused on the photocatalysis properties [17,18], while the photocatalyst-bacterial interactions were rarely discussed.

In this present study, scaning electron microscopy and confocal Raman spectrscopy were used to study different photocatalysts interact with pathogens. The photocatalyst-bacterial interaction properties were then compared to the bactericidal activity of respective photocatalysts. To further investigate whether the antibacterial effect can be generally applied to human pathogens, we tested several human pathogens including Staphylococcus aureus, Shigella flexneri and Acinetobacter baumannii. Among these bacteria, S. flexneri is a food-borne pathogen, which is usually found in contaminated water, plants, and sewage [19-22], and frequently leads to outbreaks in regions with poor sanitary conditions [21,23]. S. aureus is a exotoxin producing pathogen which can cause diseases such as food-borne diseases, soft tissue infections, and toxic shock syndrome in humans[19]. The emergence and rapid spread of multidrug-resistant A. baumannii isolates causing nosocomial infections are of great concern worldwide [24]. The antimicrobial performance of the visible-light responsive titania catalysts against these bacteria will be compared.

Materials and methods

Preparation of TiO2, C150 and C200 nanoparticles

Carbon-containing mixed phase nano-structured TiO2 powders were prepared using a modified sol-gel method. The produced powders were subjected to calcination at 150°C and 200°C, and named as C150 and C200, respectively. Details in preparation of C150 and C200, structural properties, the sizes of primary particles, light absorption, etc. have been reported elsewhere [12]. In our previous study [12], we found the C200 has a unique anatase/rutile mixed crystalline phases that exhibits strong visible-light absorption and photocatalytic effects. The photocatalytic studies have been reported previously [12,25,26]. In these TiO2, carbons exist in an amorphous form as seen in the Raman spectra, and the carbon contents were estimated using x-ray photoelectron spectroscopy to be approximately ~30 atomic % on the surface (data not shown). One commercially available TiO2 nanopowder (UV100, Sachtleben, Germany), that can exert the photocatalytic property only when illuminated by UV light, was used for comparison. Since C150 and C200 samples often aggregate into larger cluster due to surface charges, Van der Waals interactions, we dispersed the aggregates using sonication (Transsonic digital TP680DH, Ultrasonic cleaning Co. Singapore, Singapore) before the bacteria-killing or bacteria-photocatalyst interaction experiments.

Confocal Raman spectral mapping

Confocal Raman mapping was carried out with a confocal Raman spectrometer using 488 nm excitation wavelength (α-SNOM, Witec, Germany). The confocal Raman mapping has a spatial resolution of ~250 nm; typical scan were performed in an area of 10 × 10 μm2 area and in air. The mapping consisted of 0.2 μm in each step in both the x and y directions, with specific Raman signals of the interested sample components are plotted to form a 2-D map to reveal the structural distribution of the interested structures. The bacteria-nanoparticle images were taken after 20 μl of nanoparticles (10 mg/ml) and bacteria (1 × 106 CFU/ml) suspensions in H2O were spread on cover glasses and dry. Laser power were kept low (less than 1 mW) to avoid damaging the test samples, both the TiO2 and the bacteria.

Scanning electron microscopic imaging

Scanning electron microscopic (SEM) analysis was performed as previously described [27-30]. The images were obtained using a JEM-3010 scanning electron microscope (JEOL, Japan) equipped with energy dispersive x-ray spectrometer (EDS) for the chemical elemental analysis to observe the surface morphology of the tested TiO2 nanoparticles. To observe the interaction of microbes and TiO2 samples, bacteria and TiO2 powders were mixed and subjected to photocatalytic reaction as described in next sections. After the reaction, the samples were transferred to cover-glasses and fixed by 2.5% glutaraldehyde in 0.1 M phosphate buffer, then 1% osmium tetroxide in 0.1 M phosphate buffer, pH 7.3, and then subjected to a series of alcohol dehydration, critical point drying procedures, and gold coating [27] and observed under a scanning electron microscope at 15 kV (Hitachi S-4700, Hitachi, Japan). At least three different areas were randomly selected for photography at each magnification; representative data are shown.

Bacterial strains and culture

Basic bacterial cultural methods were performed as previously described [13,31]. Clinical isolated S. flexneri was collected from a shigellosis outbreak in central Taiwan in 1996 [23]. A. baumannii, pan-drug resistant A. baumannii and S. aureus were clinical isolates from Buddhist Tzu-Chi General Hospital in Hualien, Taiwan. All isolates were initially differentiated into Gram positive and Gram-negative strains by a standard staining procedure. The bacteria were cultured in tryptic soy broth supplemented with 0.5% yeast extract (TSBY) and LB at 37°C for 16 hr, and then identified by biochemical methods according to routine clinical laboratory procedures [32]. S. flexneri, A. baumannii and pan-drug resistant A. baumannii were maintained and grown in LB medium or LB agar at 37°C. Bacterium S. aureus was grown in TSBY broth or TSBY broth agar (MDBio, Inc. Taipei, Taiwan) at 37°C. All bacteria isolates were stored in 50% glycerol (V/V) in culture medium at -80°C before use. To reactivate bacteria from frozen stocks, 25 μl bacterial stock solution was transferred to a test tube containing 5 ml of freshly prepared culture medium and then incubated at 37°C under agitation overnight (16–18 hr).

Bactericidal effects of the TiO2 nanoparticles

In this study, bacterial concentrations were either determined by the standard plating method or inferred from optical density readings at 600 nm (OD600). For each bacterium, a factor for converting the OD600values of the bacterial culture to concentration (CFU/ml) was calculated as the followings. A fresh bacterial culture was diluted by factors of 10-1 to 10-7, and OD600 of these dilutions was measured. Bacterial concentrations of these dilutions were determined using standard plating method. The OD600 values were plotted against the bacterial concentrations' log values, and the conversion factors for particular bacteria were calculated. The conversion factor for S. aureus, for example, was calculated to be 1 × 108 CFU/ml per OD600 by this method.

In order to determine the bactericidal effects of the TiO2 nanoparticles, 200 μl of bacterial overnight culture was transferred into 5 ml of culture medium and incubated at 37°C until an OD600 of 0.3 to 0.6 (log phase) was reached. The bacterial concentrations were calculated using the conversion factor for the bacteria, and the cultures were diluted to 5 × 105 CFU/ml with culture medium. Fifty micro liters of the bacterial culture (2.5 × 104 CFU) were mixed with the TiO2 nanoparticles (200 μg/ml in 150 μl normal saline) using a plastic yellow tip and placed onto a 24-well cell culture dish. The cell culture dish was then placed under an incandescent lamp (Classictone incandescent lamp, 60W, Philips, Taiwan) for photocatalytic reaction, and a light meter (model LX-102, Lutron Electronic Enterprises, Taiwan) was used to record the illumination density. In the dose-dependence experiments, illuminations were carried out for 5 min at a distance of 5 and 15 cm from the lamp, corresponding to the illumination density of 3 × 104, and 5 × 102 lux (lumen/m2)(90 and 10 mW/cm2), respectively. In the kinetic analysis experiments, illuminations were carried out for 1, 5, 10, 20, and 40 min at a distance of 5 cm, corresponding to an illumination density of 3 × 104 lux (90 mW/cm2). After illumination, the bacterial solutions were recovered from the 24-well cell culture dishes, and an aliquot of fresh culture medium (250 μl) was used to flush the wells through repeatedly pipetting to further collect the residual bacteria on the wells of the culture dish. The two bacterial solutions were pooled to make a total of 350 μl. The bacterial concentration was determined by the standard plating method immediately after the bacterial collection, and percentage of surviving bacteria was calculated. Polystyrene latex beads were purchased from Sigma-Aldrich (Saint Louis, Mo, USA) and used as negative controls.

Statistical analysis

All results were calculated from data of three independent experiments. T-test was used to assess statistical significance of differences in results of the antimicrobial effects. A P value of less than 0.05 (P < 0.05) was considered significant. The statistical tests were carried out and output to graphs using the Microsoft Excel (Microsoft Taiwan, Taipei, Taiwan) and SigmaPlot (Systat Software, Point Richmond, CA, USA) software.

Results

Electron microscopic and Raman spectroscopic analysis

The interaction of the bacteria and TiO2 nanoparticles was observed using scanning electron microscope (Fig. 1). Fig. 1A, B depicts the SEM images of the aggregated C150 and C200 TiO2 nanoparticles. The sizes appear larger that the dispersed primary particles due to particle aggregations. Fig. 1C is the Energy dispersive x-ray spectroscopy (EDS) that indicates the elemental analysis of the investigated nanoparticles. As shown in the EDS spectrum, the investigated TiO2 nanoparticles contain carbons in addition to Ti and oxygen. The carbon contents was estimated to be 1 weight % or 10 atomic % from the EDS spectrum. In Fig. 1D–F, the SEM images revealed the interaction between the tested TiO2 samples and the S. aureus. As seen in these images, the nanometer-sized TiO2 can effectively interact with the bacteria S. aureus. However, commercial UV100 TiO2 that works as photosensitizer only in the UV range of the light spectrum, the morphology of the bacteria was not affected when interacted with the TiO2 upon visible-light illumination (Fig. 1D). For the C150 sample, already some effect was seen on bacterial morphology (Fig. 1E). As to the C200 sample, the morphology of the bacteria was strongly altered due to the interaction with the TiO2 under visible-light illumination (Fig. 1F). The SEM investigation showed that C200 sample upon visible-light illumination would spread over the bacterial surface, although bare C200 sample showed aggregation due to their nanometer sizes and strong van der Waals force interaction. This observation is consistent with our bacterial killing test for different strains of bacteria used, and the results will be shown in the following sections.

thumbnailFigure 1. Scanning electron microscope images of the TiO2 nanoparticles. (A) C150, (B) C200, (C) EDS elemental spectrum of C200, (D) S. aureus and UV100, (E) S. aureus and C150, and (F) S. aureus and C200. Scale bars: 100 nm.

However, the observation using the scanning electron microscope can only be achieved in high vacuum environment; and the samples required gold coating for imaging. This may cause the complication on the test bacterial samples. To analyze the bacterial samples in a relatively non-invasive way, the bacterial interaction with TiO2 was further observed with confocal Raman spectroscopic mapping in ambient. In Fig. 2, the Raman spectra of C150, C200, and the S. aureus (Fig. 2A–C) and the spectra of the corresponding positions indicated in the confocal Raman mapping images (Fig. 2E, F and Fig. 2H, I) are shown, respectively. In the Raman spectra, the spectral assignments are; 586, 682 cm-1 for anatase phase; 421, 461 cm-1 for rutile phase of the TiO2 crystal structures. The unique Raman peak at ~3000 cm-1 was used as a marker for imaging the position of bacteria S. aureus (Fig. 2C). Optical microscopy images show the typical examples of mixed aggregates of C150 and C200 with the S. aureus (Fig. 2D, G); and the signal of Raman mapping further reveals the distribution and the position of bacteria or TiO2 (Fig. 2E, F and Fig. 2H, I). For the C150 (Fig. 2F), the bright spots indicated the locked C150 Raman signals. It appears randomly across the bacteria S. aureus (the bright images in Fig. 2E). For the sample C200, the Raman mapping for both the S. aureus and the C200 completely overlapped, suggesting a uniform coverage of the TiO2 on the bacteria S. aureus. This observation is completely in agreement with the SEM observation (Fig. 1E, 1F). The result indicates C200 sample has better interaction with the observed bacteria S. aureus.

thumbnailFigure 2. Raman spectra and confocal Raman mapping of the interaction of S. aureus with TiO2 nanoparticles. The Raman spectra of (A) C150, (B) C200 and (C) S. aureus. Optical image of the aggregated bacteria S. aureus interacting with C150 (D), confocal Raman mapping of the S. aureus Raman signals (E) and confocal Raman mapping of C150 (F), optical image of the aggregated bacteria S. aureus interacting with C200 (G), confocal Raman mapping of the S. aureus signals (H) and confocal Raman mapping of C200 (I).

Killing of S. aureus by C150 and C200

To compare the bactericidal activities of the TiO2 nanoparticles, we mixed 2.5 × 104 CFU S. aureus with 30 μg of UV100, C150, or C200 in 200 μl solutions and irradiated the solutions with 3 × 104 lux visible-light for 5 min. After irradiation, bacteria solutions were recovered and the number of surviving bacteria was determined by standard plating-out method. Latex beads were used as a negative control. As shown in Fig. 3, C200 exhibited a significantly greater ability to reduce S. aureus number compared to latex beads and UV100 (Fig. 3, * P < 0.05, ** P < 0.01).

thumbnailFigure 3. Bactericidal activity of UV100, C150 and C200 against S. aureus. Illumination was carried out at a light density of 3 × 104 lux for 5 min. * P < 0.05, ** P < 0.01. Latex beads were used as negative controls.

To obtain dose dependent and kinetic data for S. aureus with C200 substrates, we further analyzed the effects of illumination by visible-light at various time points or at various distances (5 cm, 15 cm, and with different illumination intensities of 3 × 104 and 5 × 102 lux) (Fig. 4). The results show that C200 substrates could kill S. aureus in minutes when exposed to various degrees of illumination by visible-light (Fig. 4A, B). Even though the bacteria killing efficiency in both C150 and C200 groups were significantly greater than the comparing UV100 groups (Fig. 4A, 4B, ** P < 0.01; * P < 0.05), C200 still has superior performance when compared to C150 groups (Fig. 4A, 3 × 104 lux groups; 4B, 5 min to 40 min groups, + P < 0.05). The observation is in agreement with both the SEM and Raman observations that C200 sample exhibited distinct performance when interacting with the tested bacteria.

thumbnailFigure 4. Dose dependency and kinetics. Dose dependency (A) and kinetic (B) analyses of the visible-light induced bactericidal activity against S. aureus of TiO2-related photocatalyst substrates were shown. Illumination was carried out either at different light densities for 5 min (A) or at a light density of 3 × 104 lux for different times (B). In each illumination condition, the percentages of the surviving bacteria in C150 and C200 groups were normalized to the percentage of the surviving bacteria in the UV100 groups (100%). * P < 0.05 and ** P < 0.01 compared to the respective UV100 groups. + P < 0.05 compared to the respective C150 groups.

Bacteria-killing experiment for other pathogens

Bactericidal activities of C150 and C200 on other human pathogens including A. baumannii, pan-drug resistant A. baumannii and S. flexneri were also examined. C200 demonstrated significantly higher effectiveness in killing of all tested bacteria, as compared to C150 and UV100 (Fig. 5).

thumbnailFigure 5. Pathogen analysis. For each pathogen, the percentage of surviving bacteria on the C150 and C200 substrates was normalized to that on the UV100 substrates. Illumination was performed at light density of 3 × 104 lux for 10 min. * P < 0.05 and ** P < 0.01 compared to the respective UV100 groups. + P < 0.05 compared to the respective C150 groups.

Discussion

Urbanization, population growth and heavy traveling enable infectious diseases to quickly spread worldwide from one local area. Photocatalyst has the potential for use in a variety of settings to reduce the transmission of pathogens in public environments. The emergence of increasingly virulent and antibiotic resistant pathogens in hospital settings [1,3] provides another motivation for the development of alternative disinfection approaches using photocatalyst. There are several advantages to use the visible-light responsive photocatalyst such as titania. First, for safety consideration, visible light is a relatively safer light source as compared to UV irradiation [33]. Exposure to UV light at the necessary levels, would cause great damage of skin and eye tissues for humans [33-35]; thus limiting the use of conventional UV activated TiO2 substrates in environments where humans would be exposed. The visible-light activated photocatalyst offers a perfect alternative for use as a disinfectant in public areas. Second, because TiO2 is a chemically stable and inert material, it could continuously exert antimicrobial action when illuminated by light. Third, the bactericidal activity can be switched on and off or modulated by controlling the light intensity. In addition, from efficiency point of view, commercial titania absorbs only the UV range estimated 2–3% of solar light impinging on the Earth's surface [36] when used for an outdoor setting. The advantages of the visible-light responsive photocatalyst might be complementary to existing disinfectants and provide the potential for developing a variety of alternative antimicrobial applications. To extend the light-absorption into visible-light range, doping with transition ions and/or anions (negative ions) is a commonly used method. By which it creates intra-band gap states close to the conduction or valence band edges that induces visible-light absorption at the sub-band gap energies [9,36]. In some cases, the doped materials also are able to inhibit the charge recombination, thereby increase the photocatalytic activity [36-39]. Using such approach, many studies have shown to develop titania photocatalyst with antimicrobial activity in the visible-light range [13-15,38-46]. In these studies, anions such as sulfur, nitrogen and carbon, and metal ions such as neodymium, tungsten, and platinum were used for doping titania. The photocatalytic- and antimicrobial-performance of these dopants are different because the various roles of the doped materials in trapping electrons and/or holes on the surface.

Besides photocatalytic activity, there are still other unidentified factors affect the antimicrobial activity. For example, catalysts may have similar photocatalytic activity but with different bactericidal performance as observed in the study using titania-coated nickel ferrite [40]. Anatase-titania-coated nickel ferrite and brookite-titania-coated nickel ferrite have a similar photocatalytic reaction rate, while the former one has a superior bacterial-inactivation response [40]. This indicates the physical properties such as bacterium-catalyst interactions might influence the antimicrobial outcome.

To analyze the influence of bacterium-catalyst interactions on the antimicrobial performance, we used C150 and C200 titania catalysts. Previously we found that both C150 and C200 samples have a similar visible light absorption pattern [12]. C200 nanocrystals, however, contain mixed anatase and rutile phases that resulted in interface states in the mixed surface energy structures, as compared with uniform anatase phase structure of C150 nanocrystals [12]. The distinct bacterial interaction behaviors of the C150 and C200, as observed in the SEM and Raman mapping in this study, presumably are attributed to the existence of different structural complexity. The interactions between TiO2 with biomolecules were rarely discussed. It was shown that various sol-gel treatments can change the property of TiO2 surfaces [47,48]. It was also shown that different TiO2 crystal surfaces indeed have different affinities toward cellular protein fibronectin [49]. In addition, carbon-coated TiO2 samples showed high affinity and high photoactivity towards organic compound methylene blue [48,50]. Since bacterial surfaces express various organic components and proteins, it is not surprising that the bacteria would have a preferential interaction with specific catalyst. Using scanning electron microscopy and confocal Raman mapping techniques, here we successfully demonstrated that better bacterial-interaction is associated with better pathogen-killing performance when C200 samples were tested in bactericidal experiments.

In conclusion, we found that by generating mixed-phase TiO2 nanocrystals, the antibacterial activity of carbon-containing photocatalyst was significantly enhanced; and the photocatalysts can be used in the visible light settings. Although the bactericidal activity remains to be further improved and optimized, the unique property of C200 to interact with bacteria might provide a new perspective for developing more effective antibacterial photocatalysts.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

CLC carried out the Raman spectra and confocal Raman mapping, and participated in its design. DSS and WCC carried out the photocatalysis experiments. YHT participated in the synthesis of photocatalysts. HCH participated in the SEM analysis. JBW and PHC participated in the confocal Raman mapping. JHC, PJT, NTL and MSU participated in the analysis of pathogenic bacteria. HHC conceived of the study, and participated in its design, coordination and drafted the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors appreciate the financial support of National Science Council of Taiwan ROC under grant Nos. NSC 95-2314-B-320 -009 -MY3 and NSC 95-2120-M-259-003.

References

  1. Russell AD: Bacterial adaptation and resistance to antiseptics, disinfectants and preservatives is not a new phenomenon.

    J Hosp Infect 2004, 57:97-104. PubMed Abstract | Publisher Full Text OpenURL

  2. Russell AD: Biocide use and antibiotic resistance: the relevance of laboratory findings to clinical and environmental situations.

    Lancet Infect Dis 2003, 3:794-803. PubMed Abstract | Publisher Full Text OpenURL

  3. Aiello AE, Larson E: Antibacterial cleaning and hygiene products as an emerging risk factor for antibiotic resistance in the community.

    Lancet Infect Dis 2003, 3:501-506. PubMed Abstract | Publisher Full Text OpenURL

  4. Maness PC, Smolinski S, Blake DM, Huang Z, Wolfrum EJ, et al.: Bactericidal activity of photocatalytic TiO(2) reaction: toward an understanding of its killing mechanism.

    Appl Environ Microbiol 1999, 65:4094-4098. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  5. Fujishima A, Honda K: Electrochemical photolysis of water at a semiconductor electrode.

    Nature 1972, 238:37-38. PubMed Abstract | Publisher Full Text OpenURL

  6. Jacoby WA, Maness PC, Wolfrum EJ, Blake DM, Fennel JA: Mineralization of bacterial cell mass on a photocatalytic surface in air.

    Environ Sci Technol 1998, 32:2650-2653. Publisher Full Text OpenURL

  7. Legrini O, Oliveros E, Braun AM: Photochemical processes for water treatment.

    Chem Rev 1993, 93:671-698. Publisher Full Text OpenURL

  8. Iwasaki M, Hara M, Kawada H, Tada H, Ito S: Cobalt Ion-Doped TiO(2) Photocatalyst Response to Visible Light.

    J Colloid Interface Sci 2000, 224:202-204. PubMed Abstract | Publisher Full Text OpenURL

  9. Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y: Visible-light photocatalysis in nitrogen-doped titanium oxides.

    Science 2001, 293:269-271. PubMed Abstract | Publisher Full Text OpenURL

  10. Ohno T, Sarukawa K, Matsumura M: Photocatalytic Activities of Pure Rutile Particles Isolated from TiO2 Powder by Dissolving the Anatase Component in HF Solution.

    J Phys Chem B 2001, 105:2417-2420. Publisher Full Text OpenURL

  11. Ohno T, Sarukawa K, Tokieda K, Matsumura M: Morphology of a TiO2 Photocatalyst (Degussa, P-25) Consisting of Anatase and Rutile Crystalline Phases.

    J Catalysis 2001, 203:82-86. Publisher Full Text OpenURL

  12. Chou PW, Treschev S, Chung PH, Cheng CL, Tseng YH, et al.: Observation of carbon-containing nanostructured mixed titania phases for visible-light photocatalysts.

    Applied physics letters 2006, 89:131919. Publisher Full Text OpenURL

  13. Wong MS, Chu WC, Sun DS, Huang HS, Chen JH, et al.: Visible-light-induced bactericidal activity of nitrogen-doped titania photocatalyst in eliminating the human pathogens.

    Appl Environ Microbiol 2006, 72:6111-6116. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  14. Li Q, Xie R, Li YW, Mintz EA, Shang JK: Enhanced visible-light-induced photocatalytic disinfection of E. coli by carbon-sensitized nitrogen-doped titanium oxide.

    Environ Sci Technol 2007, 41:5050-5056. PubMed Abstract | Publisher Full Text OpenURL

  15. Yu JC, Ho W, Yu J, Yip H, Wong PK, et al.: Efficient visible-light-induced photocatalytic disinfection on sulfur-doped nanocrystalline titania.

    Environ Sci Technol 2005, 39:1175-1179. PubMed Abstract | Publisher Full Text OpenURL

  16. McDonnell G, Russell AD: Antiseptics and disinfectants: activity, action, and resistance.

    Clin Microbiol Rev 1999, 12:147-179. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  17. Egerton TA, Kosa SA, Christensen PA: Photoelectrocatalytic disinfection of E. coli suspensions by iron doped TiO2.

    Phys Chem Chem Phys 2006, 8:398-406. PubMed Abstract | Publisher Full Text OpenURL

  18. Vohra A, Goswami DY, Deshpande DA, Block SS: Enhanced photocatalytic inactivation of bacterial spores on surfaces in air.

    J Ind Microbiol Biotechnol 2005, 32:364-370. PubMed Abstract | Publisher Full Text OpenURL

  19. Salyers AA, Whitt DD: Bacterial pathogenesis: a molecular approach. Washington, D. C.: ASM Press; 1994:122-129.

  20. Martino MC, Rossi G, Martini I, Tattoli I, Chiavolini D, et al.: Mucosal lymphoid infiltrate dominates colonic pathological changes in murine experimental shigellosis.

    J Infect Dis 2005, 192:136-148. PubMed Abstract | Publisher Full Text OpenURL

  21. Lima AA: Tropical diarrhoea: new developments in traveller's diarrhoea.

    Curr Opin Infect Dis 2001, 14:547-552. PubMed Abstract | Publisher Full Text OpenURL

  22. Wong HC, Liu SH, Wang TK, Lee CL, Chiou CS, et al.: Characteristics of Vibrio parahaemolyticus O3:K6 from Asia.

    Appl Environ Microbiol 2000, 66:3981-3986. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  23. Chiou CS, Hsu WB, Wei HL, Chen JH: Molecular epidemiology of a Shigella flexneri outbreak in a mountainous township in Taiwan, Republic of China.

    J Clin Microbiol 2001, 39:1048-1056. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  24. Navon-Venezia S, Ben-Ami R, Carmeli Y: Update on Pseudomonas aeruginosa and Acinetobacter baumannii infections in the healthcare setting.

    Curr Opin Infect Dis 2005, 18:306-313. PubMed Abstract | Publisher Full Text OpenURL

  25. Treschev SY, Chou PW, Tseng YH, Wang JB, Perevedentseva EV, et al.: Photoactivities of the visible-light-activated mixed-phase carbon-containing titanium dioxide: The effect of carbon incorporation.

    Applied Catalysis B: Environmental 2007, 79:8-16. Publisher Full Text OpenURL

  26. Tseng YH, Kuo CS, Huang CH, Li YY, Chou PW, et al.: Visible-light-responsive nano-TiO2 with mixed crystal lattice and its photocatalytic activity.

    Nanotechnology 2006, 17:2490-2497. Publisher Full Text OpenURL

  27. Chang HH, Lin CH, Lo SJ: Recombinant rhodostomin substrates induce transformation and active calcium oscillation in human platelets.

    Exp Cell Res 1999, 250:387-400. PubMed Abstract | Publisher Full Text OpenURL

  28. Chang HH, Lo SJ: Full-spreading platelets induced by the recombinant rhodostomin are via binding to integrins and correlated with FAK phosphorylation.

    Toxicon 1998, 36:1087-1099. PubMed Abstract | Publisher Full Text OpenURL

  29. Sun DS, Lo SJ, Lin CH, Yu MS, Huang CY, et al.: Calcium oscillation and phosphatidylinositol 3-kinase positively regulate integrin alpha(IIb)beta3-mediated outside-in signaling.

    J Biomed Sci 2005, 12:321-333. PubMed Abstract | Publisher Full Text OpenURL

  30. Sun DS, Lo SJ, Tsai WJ, Lin CH, Yu MS, et al.: PI3-kinase is essential for ADP-stimulated integrin alpha(IIb)beta3-mediated platelet calcium oscillation, implications for P2Y receptor pathways in integrin alpha(IIb)beta3-initiated signaling cross-talks.

    J Biomed Sci 2005, 12:937-948. PubMed Abstract | Publisher Full Text OpenURL

  31. Kau JH, Sun DS, Tsai WJ, Shyu HF, Huang HH, et al.: Antiplatelet Activities of Anthrax Lethal Toxin Are Associated with Suppressed p42/44 and p38 Mitogen-Activated Protein Kinase Pathways in the Platelets.

    J Infect Dis 2005, 192:1465-1474. PubMed Abstract | Publisher Full Text OpenURL

  32. Murray PR, Baron EJ, Jorgensen JH, Pfaller MA, Yolken RH: Manual of clinical microbiology. Washington, D.C.: American Society for Microbiology; 2003.

  33. Sliney DH: Optical radiation safety of medical light sources.

    Phys Med Biol 1997, 42:981-996. PubMed Abstract | Publisher Full Text OpenURL

  34. Hearing VJ: Biogenesis of pigment granules: a sensitive way to regulate melanocyte function.

    J Dermatol Sci 2005, 37:3-14. PubMed Abstract | Publisher Full Text OpenURL

  35. Slominski A, Pawelek J: Animals under the sun: effects of ultraviolet radiation on mammalian skin.

    Clin Dermatol 1998, 16:503-515. PubMed Abstract | Publisher Full Text OpenURL

  36. Kisch H, Macyk W: Visible-light photocatalysis by modified titania.

    Chemphyschem 2002, 3:399-400. PubMed Abstract | Publisher Full Text OpenURL

  37. Hashimoto K, Irie H, Fujishima A: TiO2 Photocatalysis: A Historical Overview and Future Prospects.

    Japanese Journal of Applied Physics 2005, 44:8269-8285. Publisher Full Text OpenURL

  38. Rana S, Rawat J, Sorensson MM, Misra RD: Antimicrobial function of Nd3+-doped anatase titania-coated nickel ferrite composite nanoparticles: a biomaterial system.

    Acta Biomater 2006, 2:421-432. PubMed Abstract | Publisher Full Text OpenURL

  39. Sunkara BK, Misra RD: Enhanced antibactericidal function of W4+-doped titania-coated nickel ferrite composite nanoparticles: a biomaterial system.

    Acta Biomater 2008, 4:273-283. PubMed Abstract | Publisher Full Text OpenURL

  40. Rana S, Rawat J, Misra RD: Anti-microbial active composite nanoparticles with magnetic core and photocatalytic shell: TiO2-NiFe2O4 biomaterial system.

    Acta Biomater 2005, 1:691-703. PubMed Abstract | Publisher Full Text OpenURL

  41. Rana S, Srivastava RS, Sorensson MM, Misra RD: Synthesis and characterization of nanoparticles with magnetic core and photocatalytic shell: Anatase TiO2-NiFe2O4 system.

    Materials Science and Engineering B 2005, 119:144-151. Publisher Full Text OpenURL

  42. Rawat J, Rana S, Sorensson MM, Misra RD: Anti-microbial activity of doped anatase titania coated nickel ferrite composite nanoparticles.

    Materials Science and Technology 2007, 23:97-102. Publisher Full Text OpenURL

  43. Rawat J, Rana S, Srivastava RS, Misra RD: Antimicrobial activity of composite nanoparticles consisting of titania photocatalytic shell and nickel ferrite magnetic core.

    Materials Science and Engineering C 2007, 27:540-545. Publisher Full Text OpenURL

  44. Mitoraj D, Janczyk A, Strus M, Kisch H, Stochel G, et al.: Visible light inactivation of bacteria and fungi by modified titanium dioxide.

    Photochem Photobiol Sci 2007, 6:642-648. PubMed Abstract | Publisher Full Text OpenURL

  45. Rengifo-Herrera JA, Mielczarski E, Mielczarski J, Castillo NC, Kiwi J, et al.: Escherichia coli inactivation by N, S co-doped commercial TiO2 powders under UV and visible light.

    Applied Catalysis B: Environmental 2008, 84:448-456. Publisher Full Text OpenURL

  46. Kau JH, Sun DS, Huang HH, Wong MS, Lin HC, et al.: Role of visible light-activated photocatalyst on the reduction of anthrax spore-induced mortality in mice.

    PLoS ONE 2009, 4:e4167. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  47. Yu JXZ: Effect of surface treatment on the photocatalytic activity and hydrophilic property of the sol-gel derived TiO2 thin films.

    Materials Research Bulletin 2001, 36:97-107. Publisher Full Text OpenURL

  48. Tryba B: Increase of the photocatalytic activity of TiO2 by carbon and iron modifications.

    International Journal of Photoenergy 2008, 1-15. Publisher Full Text OpenURL

  49. Sousa SR, Moradas-Ferreira P, Barbosa MA: TiO2 type influences fibronectin adsorption.

    J Mater Sci Mater Med 2005, 16:1173-1178. PubMed Abstract | Publisher Full Text OpenURL

  50. Inagaki M, Hirose Y, Matsunaga T, Tsumura T, Toyoda M: Carbon coating of anatase-type TiO2 through their precipitation in PVA aqueous solution.

    Carbon 2003, 41:2619-2624. Publisher Full Text OpenURL