Photothermal Killing of Methicillin-Resistant Staphylococcus aureus by

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Photothermal Killing of Methicillin-Resistant Staphylococcus aureus by Bacteria-Targeted Polydopamine Nanoparticles with Nano-Localized Hyperpyrexia Dengfeng Hu, Lingyun Zou, Bochao Li, Mi Hu, Wanying Ye, and Jian Ji ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/ acsbiomaterials.9b01173 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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ACS Biomaterials Science & Engineering

Photothermal Killing of Methicillin-Resistant Staphylococcus aureus by Bacteria-Targeted Polydopamine Nanoparticles with Nano-Localized Hyperpyrexia Dengfeng Hu, Lingyun Zou, Bochao Li, Mi Hu, Wanying Ye and Jian Ji*

MOE Key Laboratory of Macromolecule Synthesis and Functionalization of Ministry of Education, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China. *Corresponding author *E-mail: [email protected] (Jian Ji) Abstract Bacterial infections caused by antibiotic-resistant pathogens have become intractable problems to public health. Therefore, it is an imperious demand to develop new-type approaches to effectively killing antibiotic-resistant bacteria. In this work, we report a kind of bacteriatargeted polydopamine nanoparticles exhibiting great photothermal killing ability to Methicillin-resistant Staphylococcus aureus (MRSA) by the nano-localized hyperpyrexia under low-power near-infrared (NIR) light irradiation. These bacteria-targeted nanoparticles (PDA-PEG-Van) are prepared by modifying polydopamine nanoparticles with thiolpolyethylene glycol (mPEG-SH) and vancomycin (Van) molecules. The PEG shell endows the

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nanoparticles with excellent long-term circulation stability. Due to the multivalent hydrogen bonds interaction between vancomycin and MRSA cell wall, the vancomycin modified polydopamine nanoparticles show specific target to MRSA rather than mammalian cells. These bacteria-targeted nanoparticles are employed as nano-localized heat source to kill MRSA via disrupting bacterial cell wall and membrane under the irradiation of low-power NIR light. More importantly, the surrounding healthy tissues suffer bare damage, owing to no target effect of PDA-PEG-Van to mammalian cells and the low power of NIR light used in the therapeutic process. Given the above advantages, the bacteria-targeted polydopamine nanoparticles proposed in this work show tremendous potentials to treat the MRSA infections, because it can effectively limit localized heating in the infection sites to kill bacteria and cut down damage to healthy tissues. Keywords: bacteria-targeted, polydopamine nanoparticle, vancomycin, photothermal killing, nano-localized heat Introduction The infections caused by pathogenic bacteria, especially those related to antibiotic-resistant bacteria, have resulted in numerous cases of morbidity and mortality clinically, posing great threat to public health beyond all doubt.1-4 As a common life-threatening pathogenic bacteria, methicillin-resistant Staphylococcus aureus (MRSA) can lead to many diseases such as sepsis and acute endocarditis, which is peculiarly problematic in hospital-acquired infections as well as community-acquired infections.5-7 Therefore, it is an urgent need to design novel strategies that can effectively kill MRSA.


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In order to treat the serious MRSA infections, plenty of strategies,8-18 including both chemotherapy by using multifold antibiotics and nonchemotherapy, for example, photodynamic therapy (PDT), sonodynamic therapy (SDT) and photothermal therapy (PTT),1923

have been come up and studied widely. Compared with the chemotherapy by only using

antibiotics, PTT would not result in the appearance of upgraded drug resistance of MRSA because it kills bacteria via physical heat, exhibiting outstanding superiority to treat the antibiotic-resistant bacteria infections. More recently, nanomaterial-based PTT which utilizes photothermal conversion agents, including noble metal nanomaterials, carbon-based nanomaterials, polymeric nanoparticles organic compounds and transition metal chalcogenides nanosheets to convert optical energy into heat under near-infrared (NIR) light irradiation have been widely researched.21,

24-29

With outstanding properties such as high photothermal

conversion efficiency, negligible cytotoxicity and good secondary chemical reactivity,30-31 polydopamine (PDA) has great potentials as an excellent photothermal therapeutic agent to treat cancer and bacterial infection.32-35 Generally, to effectively kill bacteria by PTT, the temperature of infected sites as well as the surrounding healthy tissues heated by photothermal conversion agents under NIR light irradiation commonly surpassed 50℃ even higher.19-20 However, it is a great challenge that the nonlocalized heating usually leads to immense damage to healthy tissues and brings much more pain to the patients when kills bacteria due to heat lacks bacteria target ability. Therefore, it is of great importance to endow photothermal conversion agents with great bacteria target ability to limit the heat in the bacterial infected sites rather than healthy tissues.


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In the present work, we design a kind of bacteria-target polydopamine nanoparticles (abbreviated as PDA-PEG-Van), which are fabricated by modifying polydopamine nanoparticles with thiol-polyethylene glycol (mPEG-SH) and vancomycin (Van) molecules on the strength of the michael reaction and schiff base reaction (Scheme 1). In healthy tissues, PDA-PEG-Van nanoparticles show excellent long-term circulation stability due to the PEG shell of the nanoparticles. While getting into the bacterial infected sites, PDA-PEG-Van nanoparticles can rapidly target and adhere to the surface of MRSA, because of the formation of the multivalent hydrogen bond interactions between PDA-PEG-Van nanoparticles and terminal D-alanyl-D-alanine moieties of MRSA cell wall.36-38 Even exposed with low-power NIR light irradiation, the bacteria in the infected sites are still largely killed, since plenty of PDA-PEG-Van nanoparticles stick to the surface of MRSA and each nanoparticle can act as nano-localized heating sources. More interestingly, because the NIR power density is low, the macroscopic temperature of the infected sites is very mild (~44°C). Owing to the great bacteriatarget ability and low NIR light power, PDA-PEG-Van nanoparticles display negligible injury to surrounding healthy tissues. In addition, PDA-PEG nanoparticles are also fabricated as a control to testify the ability of vancomycin triggering the effective target to MRSA and photothermal killing of bacteria. In consideration of the above advantages and straightforward preparation methods, PDA-PEG-Van nanoparticles developed in this work pave a new way for specific photothermal treatment of MRSA infections at mild temperature with minimal damage to surrounding healthy tissues.


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Scheme 1. (a) Synthetic route of PDA-PEG nanoparticles and PDA-PEG-Van nanoparticles; (b) Schematics of the mechanisms involved in photothermal eradication of MRSA associated infections by PDA-PEG-Van nanoparticles under NIR light. Experiment section Materials. Dopamine hydrochloride (Mw 189.64) and vancomycin hydrochloride (Mw 1485.71) were supplied by J&K Reagent Co., Ltd. (Shanghai, China) and mPEG-SH (Mw 1500) was

purchased

from

Peng

Sheng

Biological

Co.,

Ltd

(Shanghai,

China).

Tris(hydroxymethyl)aminomethane was provided by Sigma-Aldrich. Deionized water (DI, Millipore Milli-Q grade) was used in all the experiments. Synthesis of PDA nanoparticles. Polydopamine (PDA) nanoparticles were synthesized by self-polymerization of dopamine under oxidative and alkaline conditions. Briefly, 1 mL


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ammonium hydroxide (providing the alkaline conditions), 20 mL ethyl alcohol (as the retarding agent) and 45 mL deionized water were mixed to be the reaction solvent in 150 mL conical flask. Then the dopamine aqueous solution which was prepared by solving 250 mg dopamine into 5 mL deionized water was added into the conical flask. Under a two-day continuous stirring (800 rpm), the polydopamine nanoparticles were prepared. To terminate the selfpolymerization, the polydopamine nanoparticles were repeatedly centrifuged by ultrafiltration until to the pH of the solution to neutral. Preparation of PDA-PEG and PDA-PEG-Van nanoparticles. PDA nanoparticles solution (6.25 mg mL-1, 400 μL) was put into pH 8.5 Tris (10 mM, 16 mL) and then mPEG-SH solution (10 mg mL-1, 200 μL) was put into the above mixed solution. After 24-hour reaction under stirring (800 rpm), the PDA-PEG nanoparticles were obtained via repeatedly centrifugation with ultrafiltration tube (interception molecular weight of 3000) and resolution to remove unreacted mPEG-SH. And then vancomycin solution (3.55 mg mL-1, 1 mL) was put into PDAPEG nanoparticles solution (2 mg mL-1, 5mL) for another 24 h reaction under drastic stirring (800 rpm). Finally, the PDA-PEG-Van nanoparticles were prepared by repeatedly centrifugation with ultrafiltration tube (interception molecular weight of 3000) and resolution to remove unreacted vancomycin molecules. The amount of Van loaded on PDA-PEG-Van (MVan) could be calculated by the equation MVan = MVan1 - MVan2. In this equation, “MVan1” represented the quantity of Van used for coupling with the PDA-PEG, and “MVan2” represented the quantity of vancomycin in the supernatant after coupling and centrifugation to remove PDA-PEG-Van. The value of “MVan2” could be calculated by the standard absorption curve of


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Van (Fig. S5). Based on the above process, the coupling concentration of vancomycin was about 3 μg mL-1 for 1 mg mL-1 of normal PDA-PEG-Van. Bacterial culture. Methicillin-resistant Staphylococcus aureus (ATCC 44300) was used in the experiments. MRSA was cultured in tryptone soy broth medium (TSB). The concentration of MRSA was detected by OD595. Before PDA-PEG-Van target and antibacterial experiments, OD595 of MRSA solution was regulated to 0.8, corresponding to the concentrations of 2.25×109 cfu mL-1 for MRSA. MRSA-targeting effect of PDA-PEG-Van in MRSA solution detected by UV-Vis absorption spectrum. A 100 μL varying concentration of PDA-PEG or PDA-PEG-Van PBS solution (50, 100, 200 and 300 μg mL-1) and MRSA solution (2.25×109 cfu mL-1, 100 μL) were mixed and incubated in tube for different time (5, 10, 30 and 60 min). After incubation, the mixed solution was centrifugated at a low rotate speed (2000rpm) to separate the nanoparticles which were not adhering to MRSA surfaces. Subsequently, the products were resuspending with PBS, and then centrifugated. The above process was repeated for 3 times. The absorbance at 500 nm of outcomes were detected by UV-Vis absorption spectrum, and then the mass of nanoparticles adhering to MRSA was calculated by using the standard absorption curve of PDA. MRSA-target effect of PDA-PEG-Van in MRSA solution detected by SEM. A 100 μL PDA-PEG or PDA-PEG-Van PBS solution (100 μg mL-1) and MRSA solution (2.25×109 cfu mL-1, 100 μL) were mixed and incubated for different time. After incubation, the mixed solution was centrifugated at a low rotate speed (2000 rpm) to separate the nanoparticles which were not adhering to MRSA surfaces. Subsequently, the bacteria precipitation was


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resuspending with PBS. Subsequently the 2.5 % glutaraldehyde was employed to fix the products for 4 hours at 4°C. Following fixation with glutaraldehyde, the products were dehydrated in an alcohol series (20%, 50%, 80% and 95%). In the end, the samples were observed by SEM. Photothermal antimicrobial experiments. After incubation with 100 μL different concentration of PDA-PEG (50, 100, 200 and 300 μg mL-1) or different concentration of PDAPEG-Van (50, 100, 200 and 300 μg mL-1) for thirty minutes, the bacteria solution (2.25×109 cfu mL-1, 100 μL) was subsequently put into water bath for twenty minutes or irradiated by NIR light (808 nm, 0.78 W cm-2) for ten minutes, and the bactericidal ability was testified by standard plate counting assays. Surface temperature of PDA-PEG-Van nanoparticles calculated by heat balance equation. Under NIR light irradiation, the PDA-PEG-Van nanoparticles converted the energy of light into heat, thus the temperature of nanoparticles increased dramatically, at the same time, the solution was heated by the nanoparticles with high temperature. Finally, the temperature of the whole solution came form 37℃ (bacterial incubation temperature) to about 44℃ (the temperature under NIR light). It is obvious that the increased heat of the solution comes from the heat of nanoparticles. Thus, according to the heat balance equation (as follow), the highest surface temperature of PDA-PEG-Van nanoparticles is more than hundreds of centigrade, which provides great contribution to kill MRSA. C1m1(T1-T) = C2m2(T-T2)


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C1 is the specific heat capacity of PDA-PEG-Van, 1300 J/(kg·℃) (It is approximately equal to the specific heat capacity of polystyrene (1300 J/(kg·℃)) due to they have similar structure); C2 is the specific heat capacity of water, 4200 J/(kg·℃); m1 is the mass of PDA-PEG-Van, 20 ug; m2 is the mass of MRSA suspension solution, 200mg; T1 is the temperature of PDA-PEGVan nanoparticles, unknown; T2 is the temperature of MRSA solution before heated, 37℃; T is the temperature of the mixed solution after heated, 44℃; Bacterial cell membrane integrity detected by UV-Vis spectroscopy. According to the methods reported by the previous literature, membrane integrity of MRSA was investigated by measuring the supernatant OD260 value which suggested the released intracellular DNA and RNA. After different treatments, nanoparticles in the bacterial solutions were removed by centrifugation at high speed (15000 rpm). Then the supernatants were filtered by 0.22 µm filter membrane to further rid the MRSA and nanoparticles. Finally, the OD260 value was measured by UV-vis spectroscopy. Cytotoxicity assay. NIH 3T3 cell line was employed to test the cytotoxicity of nanoparticles. Firstly, cells were seeded in a 96-well plate and incubated in a 37°C incubator for 24 hours. Then cell medium was taken place by fresh cell medium including PDA-PEG-Van with concentrations of 50, 100, 200, 300 and 600 μg mL-1, respectively. After a thirty-minute incubation, the cell solution (containing cell and nanoparticles) was irradiated with NIR light (0.78 W cm-2, 10 min). Subsequently, cell medium was replaced again by fresh cell medium and incubated for another 24 hours. Later, cell medium was removed out and 20 µL MTT solution (0.1 mg mL-1) was put into every well to culture for another 4 hours. After that, the


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medium was extracted and 150 mL DMSO was added to each well. In the end, the absorbance value was measured by microplate reader. Animal Studies. To test the antibacterial effect of the PDA-PEG-Van under NIR light, a subcutaneous abscess was fabricated on the back of nude mice. In brief, a subcutaneous injection of MRSA (5×106 cfu mL−1, 100 µL) was performed on the back of the mice. After MRSA injection of 24 hours, the infected abscess was formed subcutaneously and then the different nanoparticle solutions (200 µg mL-1, 100 µL) were in injected into the mice via tail vein, respectively. After injection with nanoparticles for 2 hours, NIR light (808 nm, 0.78 W cm-2) was employed to irradiate the infected abscess for twenty minutes. The thermographic images of mice treated with different nanoparticles were harvested by IR thermal camera (ICI7330, infrared camera, Beaumont, TX, USA). To test their bactericidal effect of nanoparticles, the standard plate counting assays was performed. After NIR irradiation, the infected tissues were cut out and homogenized by the homogenizer (PRO25D Digital Homogenizer, 2000 rpm) for five minutes and then the serous fluid was sonicated for another five minutes to make bacteria fully disperse. Subsequently, the bacteria serous fluid was diluted by sterile PBS in series. Then 100 μL diluted bacteria suspension was painted on the tryptone soy agar (TSA) plates. After incubation at 37 °C for twelve hours, the bacteria colonies were statistically counted. After different treatments, other mice were sacrificed and these infected abscesses were harvested for histological H&E staining analysis.


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Statistical Analysis. The statistical analysis was conducted by three-sample Student’s t test unless otherwise noted, and the acquired quantitative data were reported as mean ± standard deviation (S.D.). A p value < 0.05 is regarded of significance. Results and discussion Preparation and characterization of PDA-PEG-Van nanoparticles By modifying polydopamine nanoparticles with the thiol-polyethylene glycol (mPEG-SH, Mw 1500) via michael reaction and schiff base reaction, the PDA-PEG nanoparticles were easily prepared. Subsequently, by vancomycin molecules conjugating with the phenyl group of polydopamine via michael reaction and schiff base reaction, the PDA-PEG-Van nanoparticles were also fabricated (Fig. S1). Following the preparation, the PDA-PEG-Van nanoparticles were firstly characterized by fourier transform infrared (FT-IR) and UV-vis spectrum. According to FT-IR spectrum, the peaks of -CH2 (2896 cm−1) and C-O-C (1109 cm−1) increased obviously and the peak of thiol rocking vibration at 835 cm−1 disappeared evidently, which demonstrated the mPEG-SH grafted onto the polydopamine nanoparticles through michel addition reaction. Furthermore, compared to PDA-PEG, peaks at 1493, 1565, 1735 cm−1 which correspond to the C-N, N-H, and C=N vibrations respectively, were patently enhanced, demonstrating that PDA-PEG-Van was successfully prepared (Fig. 1a). Furthermore, the absorbance peak at 273 nm of vancomycin in UV-vis spectrum also verified that vancomycin molecules were successfully grafted onto the PDA-PEG nanoparticles (Fig. S2). Referring to the calculation method previously reported,7 the grafting amounts of vancomycin onto PDA-PEG-Van nanoparticles (1 mg PDA-PEG-Van loaded with 3 μg


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vancomycin molecules) were calculated by using the standard absorption curves of polydopamine and vancomycin (Fig. S2). The dynamic light scattering (DLS) measurements of PDA-PEG-Van nanoparticles showed unimodal size distribution with an average diameter of 121±3.911 nm in phosphate-buffered saline (PBS, pH 7.4) buffer (Fig. 1b). Besides, PDAPEG-Van nanoparticles exhibited regular spherical morphology same as PDA and PDA-PEG nanoparticles, demonstrating by transmission electron microscopy (TEM) and scanning electron microscope (SEM) images (Fig. 1f and S3). The zeta potential of PDA-PEG-Van nanoparticles was about -3.8±0.85 mV (Fig. 1c). Moreover, no obvious precipitate in different medium or hydrodynamic size increasing was observed for PDA-PEG-Van nanoparticles during fifteen days of storage in phosphate-buffered saline (PBS, 10 mM, pH 7.4) buffer, suggesting the prepared PDA-PEG-Van nanoparticles showed high structural stability in aqueous environments (Fig. 1d and 1e).


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Figure 1. (a) FT-IR spectra of mPEG-SH, PDA, PDA-PEG, Van and PDA-PEG-Van; (b) DLS plot of PDA, PDA-PEG and PDA-PEG-Van in PBS (10 mM, pH 7.4); (c) Zeta potentials of PDA, PDA-PEG and PDA-PEG-Van in PBS (10 mM, pH 7.4); (d) Digital images of media only (1, 3, 5, 7, 9) and PDA-PEG-Van nanoparticles incubation in media at pH 7.4 (2, 4, 6, 8, 10). The media included phosphate-buffered saline (PBS, 10 mM, pH 7.4) buffer, 0.9% NaCl,


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bull serum albumin (BSA, 5 mg/mL, pH 7.4), 100% fetal bovine serum (FBS, pH 7.4) and cell culture medium (DMEM+10% FBS, pH 7.4) for a period of long time; (e) Hydrodynamic diameter of PDA-PEG-Van in PBS (10 mM, pH 7.4) during fifteen days of storage; (f) TEM and SEM images of PDA-PEG-Van in PBS (10 mM, pH 7.4). Photothermal conversion capability of PDA-PEG-Van nanoparticles In order to evaluate the NIR photoabsorption ability of PDA-PEG-Van nanoparticles, the UV-vis absorbance spectrum was obtained firstly. The strong NIR absorption of PDA-PEGVan nanoparticles provided them with outstanding ability for photothermal killing of bacteria (Fig. 2a). Subsequently, the photothermal conversion capability was evaluated. PDA-PEGVan nanoparticles were dispersed in phosphate buffer at concentrations from 50 to 200 μg mL−1, and then exposed to 808 nm NIR light with the powers from 0.53 to 1.32 W cm−2 for ten minutes. Pure PBS (10 mM) was employed as a negative control. Under NIR light irradiation, the temperature of all PDA-PEG-Van nanoparticles raised with the irradiation time, and the temperature increased more quickly with the increasing of concentration of nanoparticles and the NIR light power (Fig. 2b and 2c). After exposure to NIR light (1.32 W cm−2) for ten minutes, the temperature of PDA-PEG-Van nanoparticle solution (200 μg mL−1) rose to about 56.5°C, which was similar to PDA and PDA-PEG nanoparticle solution (200 μg mL−1) (Fig. 2e). These results indicated that there was no change of photothermal conversion capability for PDA-PEG-Van nanoparticles compared with PDA and PDA-PEG nanoparticles. Moreover, the photostability of PDA-PEG-Van nanoparticles was also evaluated. PDA-PEG-Van nanoparticle aqueous solution was exposed to 808 nm NIR light at 1.32 W cm−2 for ten minutes,


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and then the light was shut off for ten minutes and the process was repeated for three times. Before and after NIR light irradiation, the temperature change was recorded and the morphology of PDA-PEG-Van nanoparticle was checked. No obvious change of heating capacity of PDA-PEG-Van nanoparticles was detected between three-time heating and cooling cycle (Fig. 2d). As shown in SEM, PDA-PEG-Van nanoparticles maintained their morphology and size, demonstrating high photostability of PDA-PEG-Van nanoparticles (Fig. S4).


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Figure 2. (a) UV-vis absorbance of PDA-PEG-Van nanoparticles (100 μg mL-1); (b) Photothermal heating curves of PDA-PEG-Van with different concentrations under same NIR light (1.32 W cm-2, 10min); (c) Photothermal heating curves of PDA-PEG-Van at 200 μg mL1

under various NIR light power (808 nm, 10 min); (d) Photothermal effect of PDA-PEG-Van

nanoparticle solution (200 μg mL-1) for 10 min with NIR light (808 nm, 1.32 W cm−2) irradiation and then the light was shut off and the circulation was repeated for three times; (e) Thermographic images of PDA, PDA-PEG and PDA-PEG-Van nanoparticles (200 μg mL-1) at end of period of NIR light treatment (1.32 W cm-2). Strongly specific MRSA-targeting effect of PDA-PEG-Van nanoparticles In order to investigate MRSA-targeting effect of PDA-PEG-Van nanoparticles in bacterial suspension, PDA-PEG and PDA-PEG-Van nanoparticles were incubated with MRSA, respectively. After incubation with MRSA solution (2.25×109 cfu mL-1) for thirty minutes, PDA-PEG-Van nanoparticles obviously adhered to MRSA surface even though undergoing a three-times centrifugation by PBS (10 mM, pH 7.4), indicating excellent vancomycin-induced targeting effect to MRSA (Fig. 3a). In addition, the relationship between adhesion quantities of nanoparticles and their concentration or incubation time was investigated. It was observed that the adhesion quantities of PDA-PEG-Van nanoparticles were relevantly dependent on the concentration of nanoparticles and incubation time with MRSA. The adhesion quantities were increasing with the incubation time with MRSA (Fig. 3b). Furthermore, with the concentration of nanoparticles increasing, the adhesion amount of PDA-PEG-Van nanoparticles also increased. Approximately 60% amount of PDA-PEG-Van (12.71 μg) adhered to MRSA


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surface after incubation with MRSA for only five minutes, and the quantities of PDA-PEGVan came up to over 80% of total adding amount (20 μg) after thirty minutes (Fig. 3c). Nevertheless, as a control, there was no distinct adhesion of PDA-PEG to MRSA surface despite increasing its concentration or incubation time with MRSA because they lack bacteria target effect to MRSA. All these above results demonstrated that the vancomycin played a great role for PDA-PEG-Van nanoparticles to target and adhere to MRSA surfaces in bacterial solution, which provided highly promising for killing MRSA via photothermal effect.

Figure 3. (a) SEM images of MRSA incubation with PBS, PDA-PEG and PDA-PEG-Van (200 μg mL-1, 100 μL) for 30 minutes and then undergoing a three-times centrifugation by PBS (10 mM, pH 7.4), respectively (scale bar in images is 1 μm); (b) Adhesion quantities of different nanoparticles (200 μg mL-1, 100 μL) for different incubation time; (c) Adhesion quantities of


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different nanoparticles (100 μL) with different concentrations for 30 minutes in MRSA solution evaluated by UV-Vis absorption spectrum. In vitro bactericidal ability of PDA-PEG-Van under NIR light irradiation The bactericidal ability of PDA-PEG-Van nanoparticles without laser irradiation was firstly investigated. Because of the conjugation of vancomycin, PDA-PEG-Van showed the minimum inhibitory concentration (MIC) to MRSA around 900 μg mL-1 (equal to 2.7 μg mL-1 of Van) (Fig. 4a). In our following research, the concentration of PDA-PEG-Van was below 300 μg mL-1, at which PDA-PEG-Van nanoparticles without NIR light irradiation showed negligible antibacterial ability. In order to test the bactericidal ability of nanoparticles under NIR light irradiation, different nanoparticles (200 μg mL-1, 100 μL) were added into MRSA solution (5×109 cfu mL-1, 100 μL) and irradiated by NIR light with different power, respectively. As shown in Fig. 4b, under high-power NIR light (1.32 W cm-2, 20 min) irradiation, the bacterial suspension temperature could rise to about 56 °C. At this temperature, the bacterial survival rate of MRSA treated with PDA-PEG or PDA-PEG-Van nanoparticles was very low, indicating both PDA-PEG and PDA-PEG-Van exhibited outstanding bactericidal ability when irradiated with high-power NIR light. Whereas, the bactericidal ability of PDA-PEG and PDA-PEG-Van displayed great difference when irradiated with low-power NIR light. With the power of NIR light decreasing to 0.78 W cm-2, the temperature of the bacterial suspension decreased to 44 °C and the bacterial survival rate of PDA-PEG was around 77.5%, while the bacterial survival rate of PDA-PEG-Van was only 15.4%. It was inferred that the different bactericidal ability of these two kinds nanoparticles under low-power NIR light was mainly attributed to the different


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MRSA-target ability of them. Subsequently, the bactericidal ability of PDA-PEG and PDAPEG-Van nanoparticles at mild temperature was furtherly investigated. After NIR light (0.78 W cm-2) irradiation for twenty minutes, the MRSA incubated with PDA-PEG-Van nanoparticles were killed up to 84.6%. The bactericidal rate was increased with the concentration of the PDA-PEG-Van even though the solution temperature was only about 44°C. The bacterial survival rate decreased to 1.1% when the concentration of PDA-PEG-Van nanoparticles grew to 300 μg mL-1, indicating the remarkable photothermal bactericidal ability of PDA-PEG-Van under low-power NIR light even though at mild solution temperature (Fig. 3c). As controls, the MRSA incubated with PDA-PEG in water bath (~44°C), PDA-PEG-Van in water bath (~44°C) or PDA-PEG under same NIR light (0.78 W cm-2, 20 min), only exhibited very weak or negligible bactericidal effects. Although all groups were at mild solution temperature (~44°C), the surface temperature of MRSA treated with external water bath was different from that treated with PDA-PEG or PDA-PEG-Van under NIR light. The surface temperature of MRSA treated with water bath was basically homogeneous, approximately 44°C. In the group treated with PDA-PEG-Van under NIR light irradiation, the temperature of the whole solution increasing form 37℃ (bacterial culture temperature) to 44℃ was due to that nanoparticles transfer the heat to the solution continuously. Therefore, it was obviously that the surface temperature of nanoparticles was much higher than that of the solution before the temperature balance. Here the temperature of the surface of PDA-PEG-Van was calculated by heat balance equation (calculation details in experimental section).39-40 After calculation, it is found that the surface temperature of MRSA treated with PDA-PEG-Van


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under NIR light was more than 100°C before the heat balance, at which the bacteria were effectively killed. Compared with the PDA-PEG under NIR light, PDA-PEG-Van under NIR light exhibited much better photothermal therapy in the same solution temperature (~44°C), mainly due to the highly specific adhesion to MRSA surface, which was the key point to provide enough nano-localized hyperpyrexia to the surface of MRSA and then kill them effectively.

Figure 4. (a) Minimum inhibitory concentration (MIC) of PDA-PEG and PDA-PEG-Van nanoparticles towards MRSA; (b) Bacterial survival rate of MRSA treated with different nanoparticles (200 μg mL-1, 100 μL) under NIR laser with different power (20 min); (c)


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Bacterial survival rate of MRSA treated with PDA-PEG (200 μg mL-1, 100 μL) in water bath (44°C), PDA-PEG-Van (200 μg mL-1, 100 μL) in water bath (44°C), PDA-PEG (200 μg mL-1, 100 μL) under NIR light and PDA-PEG-Van (200 μg mL-1, 100 μL) under NIR light (0.78 W cm-2, 20 min); (d) Photographs of standard plate counting assay after different treatments (200 μg mL-1, 100 μL). Photothermal bactericidal mechanism of PDA-PEG-Van nanoparticles under NIR light To illustrate the antibacterial mechanism of PDA-PEG-Van nanoparticles under NIR light irradiation, laser scanning confocal experiments by employing Live/Dead Baclight assay kit was carried out. As a kind of green fluorescence, SYTO 9 could stain the nucleic acid of live as well as dead bacteria via infiltrating through the cell membranes, while propidium iodide (PI, a kind of red fluorescence dye) could merely stain dead bacteria with impaired cell membranes. Compared with other groups, MRSA treated with PDA-PEG-Van under NIR light irradiation had the strongest red fluorescence, indicating the effective destruction of MRSA cell membrane by nano-localized hyperpyrexia (Fig. 5a). Scanning electron microscopy (SEM) images (Fig. 5b) also demonstrated this result. The membranes of MRSA were wizened or disrupted after treatment with PDA-PEG-Van under NIR light irradiation, which demonstrated that the nano-localized hyperpyrexia from PDA-PEG-Van was high enough to destroy the cell membrane of MRSA even though the solution was just at mild temperature (~44°C). In addition, the damage of cell membrane could cause the divulgation of intracellular components and disorder the bacterial function, furtherly causing bacteria dead. As one of important symbols of the bacterial membrane damage, the leakage of intracellular components of nucleic acid


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(RNA and DNA) from MRSA cell membrane after various treatments were examined by UVVis characteristic absorption at 260 nm. After treated with PDA-PEG-Van nanoparticles under NIR light irradiation, MRSA exhibited remarkable nucleic acid (DNA and RNA) leakage from cytoplasm to the surrounding environments, suggesting the strong destruction of MRSA cell membrane by nano-localized hyperpyrexia on the surface of MRSA (Fig. 5c).


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Figure 5. (a) Laser scanning confocal images of MRSA after different treatments; (b) SEM images of the bacterial membrane changes after various treatments; (c) OD260 value for assessment of the leakage of intracellular components from MRSA after different treatments; (d) Cell viability of NIH-3T3 fibroblasts treated with PDA-PEG-Van nanoparticles with different concentrations under NIR light irradiation at mild solution temperature (~44°C). Cytotoxicity assay The cytotoxicity of PDA-PEG-Van nanoparticles under NIR light irradiation was performed by methyl thiazolyl tetrazolium (MTT) assay using mouse embryonic fibroblast cells (NIH3T3). After incubation with PDA-PEG-Van nanoparticles for thirty minutes, following NIR light irradiation for twenty minutes, the temperature of cell culture medium could also grow to around 44°C. Subsequently, cell viability was obtained after cultivation for 24 h at 37°C. There was no obvious influence to the cell viability of NIH 3T3 cells below 300 μg mL-1 PDA-PEGVan under NIR light irradiation, demonstrating PDA-PEG-Van had little cytotoxicity (Fig. 5d). In vivo bactericidal ability of PDA-PEG-Van under NIR light irradiation To testify the in vivo antibacterial activity of PDA-PEG-Van nanoparticles, the subcutaneous abscess in nude mice by the local injection of MRSA (5×106 cfu mL−1, 100 µL) was employed. After local injection of MRSA for 24 hours, the subcutaneous abscess was formed and then different nanoparticle solutions (200 µg mL-1, 100 µL) were in injected into the mice via tail vein, respectively. Following injection with nanoparticles for 2 hours, the heating effect of the PDA-PEG-Van nanoparticles was detected by IR thermal camera. In order to ensure the size of the laser spot covering the whole infected abscess, the lenses of the NIR light were


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dynamically regulated during the laser irradiation (0.78W cm-2, 20 min). On the basis of the thermographic images, the temperature of the mouse treated with PDA-PEG-Van nanoparticles irradiated with laser showed a rapid rise to about 44 °C in just three minutes. Whereas the surrounding healthy tissues just exhibited a very low temperature increasing to around 37 °C. These results indirectly demonstrated that the PDA-PEG-Van nanoparticles could effectively target and accumulate in the bacteria infected abscess. Different from the mouse treated with PDA-PEG-Van nanoparticles under NIR light irradiation, no obvious temperature rising was observed on the mouse treated with PDA-PEG nanoparticles irradiated with NIR light irradiation (Fig. 6a). Following the thermal imaging, the mice were sacrificed and their skins of infected region were cut out and exposed. It was distinct that plenty of PDA-PEG-Van nanoparticles were accumulated in the infected abscess site, where a remarkable temperature increasing had been found in the thermographic images, furtherly indicating they possessing great target ability to bacteria. However, very little PDA-PEG nanoparticles were observed in the infected abscess site, suggesting their insignificant target ability to MRSA (Fig. 6b). Subsequently, the bactericidal effect of nanoparticles under NIR light irradiation was investigated by counting the number of bacteria by employing standard plate counting methods. Compared with the bacterial survival rate of mice treated with PBS irradiated laser or PDAPEG nanoparticles irradiated with NIR light, the bacterial survival rate of mouse treated with PDA-PEG-Van nanoparticles was much lower, demonstrating stronger bactericidal effect of PDA-PEG-Van nanoparticles under NIR light (Fig. 6c). This was mainly attributed to the great target ability of PDA-PEG-Van nanoparticles. On day eleven after various treatments, the


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infected abscess on the mice that had been treated with PDA-PEG-Van nanoparticles irradiated with NIR light was disappeared and healed, while the infected abscess was still inflamed in the mice treated with PBS irradiated with NIR light or PDA-PEG nanoparticles irradiated with NIR light (Fig. 6d). In addition, the healing effect of these nanoparticles under NIR light irradiation was investigated by histological H&E staining analyses of skin sections. The histological analyses revealed that there were still severe infections in the mice treated with PBS irradiated with NIR light or PDA-PEG nanoparticles irradiated with NIR light, while a dramatically decreased inflammatory cells infiltration was observed in the mice treated with PDA-PEG-Van nanoparticles irradiated with NIR light, furtherly indicating their outstanding bactericidal ability (Fig. 6e). Moreover, the in vivo biocompatibility of PDA-PEG-Van nanoparticles was tested by histological analyses. On day eleven, there was only very little inflammatory cells in the surrounding healthy tissues in the mouse treated with PDA-PEG-Van nanoparticles under NIR light irradiation, suggesting very weak cytotoxicity of them. This mainly due to the great target of PDA-PEG-Van nanoparticles and mild temperature in the therapeutic process (Fig. 6f).


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Figure 6. (a) Thermographic image of nude mice with subcutaneous abscess under different treatments irradiated with NIR light (1.32 W cm-2); (b) Photographs of the exposed skin of subcutaneous abscess on nude mice (nanoparticles were marked with red arrow); (c) The quantitative results of bacterial CFU; (d) Photographs of infected skin of mice after different


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treatments for eleven days; (e) Histological photomicrographs of skin tissue sections of infected mice after various treatments (on day 11), healthy skin tissue on mice as a control; (f) Histological photomicrographs of infected site and its surrounding tissues that were retrieved at eleven days after treatment with PDA-PEG-Van nanoparticles under NIR light irradiation. Conclusion In summary, a kind of MRSA-targeting polydopamine nanoparticles for photothermal therapy at mild temperature was prepared. In this system, the MRSA-targeting was realized by modification PEGylated polydopamine nanoparticles with vancomycin molecules which could form multivalent hydrogen bonds interactions with MRSA cell wall. Due to the outstanding target ability of PDA-PEG-Van nanoparticles to MRSA, even if under low-power NIR light irradiation, the bacteria could still be killed by the nano-localized hyperpyrexia. Moreover, for lack of effective target effect and adhesion to healthy mammalian cells, PDA-PEG-Van nanoparticles displayed very little cytotoxicity even though under NIR light irradiation. With great biocompatibility and outstanding antibacterial activity at mild temperature, PDA-PEGVan nanoparticles provided a promising avenue to fight against MRSA associated infections. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Notes The authors declare no competing financial interest. Acknowledgement


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For Table of Contents Use Only

Photothermal Killing of Methicillin-Resistant Staphylococcus aureus by Bacteria-targeted Polydopamine Nanoparticles with Nano-Localized Hyperpyrexia Dengfeng Hu, Lingyun Zou, Bochao Li, Mi Hu, Wanying Ye and Jian Ji*


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Photothermal Killing of Methicillin-Resistant Staphylococcus aureus by

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