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Mannose-modificated Polyethyleneimine: A Specific and Effective Antibacterial Agent against Escherichia coli Mei Liu, Jiao Li, and Baoxin Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03556 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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Mannose-modificated Polyethyleneimine: A Specific and Effective

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Antibacterial Agent against Escherichia coli

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Mei Liu1, Jiao Li1, Baoxin Li 2

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1

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of Food Engineering and Nutritional Science, Shaanxi Normal University, Xi'an 710119,

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China

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Phone: 86-29-85310517, Fax: 86-29-85310517

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E-mail: [email protected]

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2

Key Laboratory of Analytical Chemistry for Life Science of Shaanxi Province, College

Key Laboratory of Analytical Chemistry for Life Science of Shaanxi Province, School

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of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710119,

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China.

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Phone: 86-29-81530726, Fax: 86-29-81530727

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E-mail: [email protected]

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Abstract

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Polyethyleneimine (PEI) has antimicrobial activity against Gram-positive

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(Staphylococcus aureus, S. aureus) and Gram-negative (Escherichia coli, E.

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coli), but it possesses highly cytotoxic and the selective antimicrobial

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activity against S. aureus is obviously better than E. coli. To reduce the

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cytotoxicity and improve the antibacterial activity against E. coli, we

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modified PEI by D-mannose through nucleophilic addition between primary 1

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amine and aldehyde group to get mannose-modificated polyethyleneimine

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copolymer particles (Man-PEI CPs). The use of mannose may provide good

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targeting ability towards E. coli pili. The antibacterial activity of Man-PEI

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CPs was investigated. Man-PEI CPs shows the specific and very strong

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killing capability against E. coli at the concentration of 10 µg/mL, which is

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the highest antimicrobial efficiency compared with unmodified PEI (220

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µg/mL). The antibacterial mechanism demonstrated that the enhancement in

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antibacterial activity is due to specific recognition of the mannose and

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destroying the cell wall of the bacteria by PEIs. Importantly, the Man-PEI

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CPs show less cytotoxicity and excellent biocompatibility. The results

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indicate that Man-PEI CPs have great potential as novel antimicrobial

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materials to prevent bacterial infections and provide specific application for

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killing E. coli.

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KEYWORDS:

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particles, mannose, antibacterial activity, specific recognition

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1. INTRODUCTION

mannose-modificated

polyethyleneimine

copolymer

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The prevalence of bacterial infectious diseases is one of the most common

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causes of morbidity in patients. After the emergence of antibiotics, it plays

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very important role in promoting the way for medical and social

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developments.1,2 However, multiple antibiotics-resistant bacteria has widely 2

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emerged among many species of pathogenic bacteria which led to some

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antibiotics no longer effective in controlling of infectious diseases.3 To

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discover and design new efficient antibacterial materials is a considerable

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attention for the treatment of microbial disease.4-6

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Among the antibacterial materials investigated, cationic polymers have

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emerged as a potential antibacterial material and have some obvious

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advantages,

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antibacterial features and better biocompatibility compared with small

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molecular antibiotics.7-9 Kuroda groups10 have investigated the antibacterial and

including

sustained

cytotoxicity

broad-spectrum

activity

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polyethyleneimine polymers (PEIs). They have found that PEIs antibacterial

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activity against E. coli and S. aureus depended on both the PEIs architecture

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and molecular weight (MW). Furthermore,the low MW PEIs are less

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cytotoxic to human cells than others, but the unmodified PEIs displayed

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selective activity against S. aureus over E. coli. This property of PEIs

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restricts its broad-spectrum antibacterial features, especially the antibacterial

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activity against E. coli. At present, a large number of studies have focused

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on improving the antibacterial activity of PEIs. The ways mainly recurs to

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modify PEIs by covalent linkage with other components, such as

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surface-grafted quarternized PEI,11 functional groups modified PEI

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microgels,12

functionalized

conventional

effect,

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PEI

of

inhibitory

silver

unmodified

nanoparticles,13

3

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phenylalanyl

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integrated PEI.14 Although there is some progress in enhancing the

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antibacterial activity of PEIs, these works do not involve the selective

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antibacterial activity and the cytotoxicity to cells of modified PEIs.

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On the other side, PEIs have been utilized as drug carriers in biomedical

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application because of their ability to give high gene transfection efficiency,

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but it is highly cytotoxic.15-17 There are some efforts on how to abate the

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toxic effects of PEIs so as to provide their potential applications in gene

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delivery. Various modifications of PEI have been introduced to alter the

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surface charge characteristics of PEI.18 Such as grafting PEI with

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poly(ethylene glycol),19 hyaluronic acid-PEI particles,20 chitosan-PEI,21

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galactosylated PEI22 and mannose PEI.23 However most of these

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modifications were tedious without controlling over composition and

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molecular structure, and achieved variable success. To the best of our

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knowledge, the investigation on how to lower the cytotoxicity of PEIs and

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meanwhile improve specific antibacterial features is still limited. Therefore,

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there is a need to modify PEIs in an easy and well controlled manner to

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achieve that low MW PEIs are likely to be less toxic while still exhibiting

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antibacterial behavior especially against E. coli.

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E.coli is a typical pathogenic bacterium, especially problematic because it

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only takes as few as 10 cells to infect humans and cause serious illnesses.24

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It is worthy of investigation for the purpose of protecting humans and the 4

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environment. In this study, we examined the sterilization potential of

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Man-PEI CPs which was modified by D-mannose through facile

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nucleophilic addition chemistry between primary amine and aldehyde group.

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The modification of PEIs resulted in a decrease in the cytotoxicity of PEI as

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that the primary amine groups of PEI was substituted and exhausted by the

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carbohydrate. Furthermore, the use of mannose may also provide targeting

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ability towards E. coli through specific and multivalent interactions between

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the mannose on Man-PEI CPs and FimH lectin pili on the surface of E.

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coli.25 Man-PEI CPs exhibit good biocompatibility, low cytotoxicity and

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efficient antibacterial ability, demonstrating safe antibacterial property in the

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application of healthcare.

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2. MATERIALS AND METHODS

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Materials and Measurements. Branched PEI (MW = 600, 1800, 10 000,

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99%), D-mannose and ascorbic acid were purchased from Aladdin Ltd.

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(Shanghai,

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bromide (MTT) was purchased from Sigma Chemical Company. The

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propidium iodide (PI) was purchased from Solarbio Ltd. (Beijing, China).

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The bacteria medium components and phosphate buffer saline (PBS, pH 7.4)

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were purchased from Sangon Biotech Co., Ltd. (Shanghai, China). The

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DMEM medium was purchased from HyClone Thermofisher (Beijing,

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China). E. coli K-12 and Staphylococcus aureus were purchased from the

China).

3-(4,5-dimethylthiazol-2-yl)-2,5–diphenyltetrazolium

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China General Microbiological Culture Collection Center (Beijing, China).

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Human cervix adenocarcinoma cells (HeLa) were purchased from KeyGen

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Biotech. Co. Ltd. (Nanjing, China). All other chemicals were of analytical

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reagent grade and used without further purification.

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UV-vis adsorption spectra were recorded on a U-3900H UV-Vis

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Spectrophotometer (Hitachi, Japan). Fluorescence spectra were measured on

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an F-4600 Spectrometer (Tokyo, Japan). Scanning electron microscope

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(SEM) measurements were performed with a FEI Quanta 200 scanning

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electron microscope (FEI, America). The nuclear magnetic resonance (NMR)

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spectra were collected on a Bruker AVANCE III 600 (600 MHz) (Bruker,

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Germany) with the freeze-dried product dissolved in D2O. Fluorescence

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images were recorded on a fluorescence microscope (Olympus, FV1200).

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Synthesis of Man-PEI Copolymer Particles.The synthetic method of

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Man-PEI CPs was carried out according to a previously reported procedure

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with slight adjustments.26 1 mL of PEI (0.1 g mL-1) was first dissolved in 7

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mL of PBS buffer (10 mM, pH 7.4) by stirring for about 1 min, and then 2

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mL of mannose (0.1 M) was added. Subsequently, after vigorous stirring for

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1 min, the mixture was heated at 90 ℃ for 40 min via hydrothermal

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treatment. Then the Man-PEI CPs solutions were dialyzed against ultrapure

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water for 24 h through dialysis bag (MWCO = 500 Da). The products inside

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the dialysis bag were collected to further study. In addition, PEI was 6

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modified with ascorbic acid to construct AA-PEI CPs according to the same

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method.

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Bacterial Cultivation. E.coli bacterial samples were transferred from

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-80 ℃ refrigerator onto agar slants (25 g of Lysogeny Broth and 15 g agar

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were dissolved into 1 L water) and incubated at 37 ℃for 16 h, then held at 4 ℃

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for up to 2 weeks. A single colony from the slants was cultured overnight in

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20 mL sterile medium for 14 h in a shaker at 37 ℃. After growth, the original

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E. coli was washed with 0.9 % sodium chloride solution twice to remove the

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medium. After centrifugation at 6000 rpm for 2 min, the remaining E.coli

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was redispersed in 0.9 % sodium chloride solution.

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Antibacterial activity experiments. The antibacterial activity of PEI,

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Man-PEI CPs was respectively investigated by incubation with bacterial

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cells suspensions in PBS buffer (10 mM, pH 7.4). E.coli with a

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concentration of 3×104 cells·mL-1 was mixed with different concentration

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PEI or Man-PEI CPs. After incubating at 37 ℃ for 3 h, 100 µL of the

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bacterial suspension was spread onto the solid LB agar plate. The colonies

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formed units (CFU) were counted after 16 h incubation at 37 ℃. The

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sterilization rate was determined by the following formula.

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Sterilization Rate % = (C0 - C)/C0 ×100%

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Where C is the CFU of the experimental group treated with PEI or Man-PEI

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CPs, and C0 is the CFU of the control group without any treatment. 7

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Fluorescent microscope measurements. The antibacterial efficiency of

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PEI or Man-PEI CPs was also proved by fluorescence microscopy. After

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treating suspensions of E. coli with PEI or Man-PEI CPs (the final

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concentration, 10 µg mL-1) at 37 ℃ for 3 h and staining them with PI for 15

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min, the bacteria were separated by centrifugation at 6000 rpm for 10 min,

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then precipitate was re-suspended in 20 µL PBS buffer (10 mM, pH 7.4).

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Fluorescent microscope samples were obtained by adding 10 µL of the

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pre-prepared mixed suspensions to clean glass slides and covering them with

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coverslips for immobilization. The color of PI is red and the type of light

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filter is BP 540−585 nm exciter, and DM 595 nm emitter. Magnification of

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object lens is 40×.

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Cell viability assay. Cytotoxicity against HeLa cells was evaluated

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according

to

the

3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium

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bromide (MTT) method.27 HeLa cells were seeded into a 96-well plate at a

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density of 1.0 × 103 cells per well in 100 µL of Dulbecco’s Modified Eagle’s

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Medium (DMEM) supplemented with 10% fetal bovine serum and incubated

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for 12 h at 37 ℃ in 5% CO2. The PEI and Man-PEI CPs were diluted with

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DMEM and added to wells at final concentrations of 10, 50, 100, 250, 500

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µg mL-1, with three replicates of each concentration. After culturing for 24 h,

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the cells were washed with PBS, and a 20 µL aliquot of MTT was then

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added into each well to remove PEI and Man-PEI CPs. Finally, the MTT 8

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was removed, then any formazan that formed was dissolved with dimethyl

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sulfoxide (DMSO) after 15 min of shaker. Absorption was measured at a

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wavelength of 490 nm.

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3. RESULTS AND DISCUSSION

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Preparation and Characterization of Man-PEI CPs. The preparation of

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Man-PEI CPs was conveniently accomplished by one step process.

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Following the previously reported protocol,26 we synthesized Man-PEI CPs

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through facile nucleophilic addition chemistry between primary amine on

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PEI and aldehyde group on D-mannose. The Man-PEI CPs were soluble in

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aqueous solution without the need of further modification. The absorption

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spectra of Man-PEI CPs, PEI and mannose in water are respectively shown

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in Figure 1. It shows that the Man-PEI CPs solution has a new absorption

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peak at 352 nm, whereas PEI and mannose have nearly no absorption at

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above 250 nm. On the other hand, Man-PEI CPs display an intense

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fluorescence at 460 nm. The intrinsic fluorescence emission closely

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resembles those of PAMAM and methylated PEI.28-30 And the inset in Figure

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1 displays the optical property of Man-PEI CPs exhibits faint yellow under

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daylight and bright blue fluorescence under ultraviolet lamp (365 nm).

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SEM was employed to characterize the morphological and structural

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characteristics of the obtained composite materials (Figure 2). Man-PEI CPs

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are monodisperse and exhibit rough spheres particles with a diameter of 9

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about 20 µm, which demonstrated that the copolymer particles were

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successfully prepared. To further verify the formation of a schiff base

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between PEI and mannose, we utilized Fourier transform infrared (FT-IR)

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spectroscopy and 1H NMR spectroscopy to investigate. As shown in Figure

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3, curves a, b, and c represent FT-IR spectra of PEI, mannose, and Man-PEI

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CPs, respectively. The raw PEI has absorption peaks at 2942, 2831, and

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1471 cm-1 corresponding to the stretching vibration and bending vibration of

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CH2 bonds, and characteristic absorptions at 3284 and 1577 cm-1 belong to

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the N-H bond. Compared to the spectrum of PEI and mannose, the bending

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vibration of the C=O groups of mannose at 1597 cm-1 disappeared and an

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obvious new peak at 1632 cm-1 was observed in the Man-PEI CPs spectrum,

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which can be attributed to the C=N bond.26,31,32 In addition, the absorption

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bands at 3437 and 1385 cm-1 are associated with the stretching vibration and

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bending vibration of O-H, respectively, and the stretching vibration of C-O

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is located at 1084 cm-1, which reveals the presence of C-OH. In addition, the

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1

H NMR spectra (Figure S1) of Man-PEI CPs has a new peak at 8.40 ppm

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belonging to H2C=N-protons,33 while PEI is no signal at this location. The

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results well confirm the favorably synthesis of the Man-PEI CPs.

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Evaluation of Antibacterial Activity.The molecular weight of PEI had a

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significant impact on antibacterial activity,10 so the antibacterial activity of

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Man-PEI CPs prepared by PEI with different molecular weight was 10

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investigated. The antibacterial activity against Gram-negative bacteria, E.

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coli K-12, was evaluated by colony counting. As shown in Figure 4, the

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antibacterial activity of PEI depended on its molecular weight. Increasing

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the molecular weight resulted in an increase in the antibacterial performance.

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The antibacterial activity of corresponding Man-PEI CPs was all improved

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compared with unmodified PEI. Importantly, when the molecular weight of

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PEI was 600, the antibacterial activity increased significantly (Figure 5a).

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Therefore, PEI with molecular weight of 600 was selected as the candidate

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to prepare Man-PEI CPs for next experiment.

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In addition, we further conducted fluorescence imaging tests to prove

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excellent bactericidal effect of Man-PEI CPs. After antibacterial experiments,

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PI was added to suspensions of E. coli with PEI and Man-PEI CPs for 15

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minutes, which can specifically stain damaged or dead bacteria. Figure 5b

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shows the fluorescence images of E. coli suspensions and the merged images

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under phase contrast bright-field and fluorescence filed. After incubation

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with Man-PEI CPs, all cells emit red fluorescence that means cells are killed,

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whereas less red fluorescence is observed when the bacterial cells were

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incubated with PEI. Man-PEI CPs exhibited highly efficient killing

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capability against bacteria than PEI at the same dosage concentration,

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implying that the antibacterial activity was obviously enhanced when PEI

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was modified with mannose. These results indicate that mannose played a 11

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vital role in the excellent bactericidal effect of Man-PEI CPs.

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In order to discuss the effect of mannose on the antibacterial activity, the

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antibacterial activity of Man-PEI CPs with different mass ratio (PEI :

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mannose) was evaluated by colony counting. As shown in Figure 6a, the

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sterilization rate of PEI (100 : 0 mg mL-1) only exhibited 9.29% ± 2.27%.

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After grafting with mannose, the antibacterial activity enhanced as the

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amount of mannose increased. At a mass ratio of 100 : 36 mg mL-1, the

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sterilization rate was 99.45% ± 0.6%. These observations were further

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demonstrated by the agar plates of E. coli (Figure 6b). In comparison with

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control plate, where a large number of E. coli colonies were observed,

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however, no colonies were found by treatment with Man-PEI CPs (100 : 36

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mg mL-1). These clearly demonstrated that the outstanding bactericidal effect

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of Man-PEI CPs is relevant to high surface mannose ligand content. The

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phenol-sulfuric acid method

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Table S2, Table S3). It can be seen from the result that the suitable

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surface-grafting degree of Man-PEI was achieved under the preferential

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molecular weight and mass ratio.

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was used to quantify sugars on PEI (seen in

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The antibacterial performance of the antibacterial agent is usually

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determined by the minimum bactericidal concentration (MBC). MBC is the

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minimum concentration of killing all bacteria with antibacterial agent. It is

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can be observed from Figure S2 that 99.9% sterilization rate was recorded 12

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for E. coli K-12 with 220 µg mL-1 PEI, whereas a same sterilization rate

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could be readily obtained with 10 µg mL-1 Man-PEI CPs. Furthermore, the

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MBC of Man-PEI CPs was markedly lower than some antibiotics and

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recently reported antibacterial polyethyleneimine materials (Table S1).

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All of above results indicate that the high antibacterial capability of

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Man-PEI CPs is resulted from the presence of mannose. So, we speculated

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that mannose was closely related to the adherence of PEI on the bacterial

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surface.

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Mechanism of Antibacterial Activity. The majority of E. coli strains

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possess a lot of fimbriae with different structure and function, which can be

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mediated bacteria on the target cell adhesion and infection. Type I fimbriae

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is the most common fimbriae of E. coli, consisting of four different subunits

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of FimA, FimF, FimG and FimH. It is well known that FimH is the

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determinant of the protein’s mannose-specific binding property; it possesses

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carbohydrate recognition sites, which produces a strong affinity for

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mannose.35-37 So, it is assumed that the excellent bactericidal effect of

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Man-PEI CPs is relevance to mannose, which improves the adherence of

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PEI on the bacterial surface.

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In our work, the Man-PEI CPs were incubated with E. coli for

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fluorescence-based agglutination assay to determine if the mannose

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molecules attached to Man-PEI CPs still retained their ability to bind with 13

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the FimH proteins of the pili in E. coli. From Figure S3, it can be shown that

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Man-PEI CPs can interact with the FimH proteins of the pili in E. coli by the

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better bacteria agglutination behavior. As a control experiment, PEI without

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mannose was also incubated with E. coli. PEI exhibited very little

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nonspecific binding to E. coli. These results show that the binding of

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Man-PEI CPs with E. coli is due to the interaction between the mannose and

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the FimH proteins rather than to nonspecific absorption to the Man-PEI

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CPs.38,39 The aldehyde group of mannose may consume the amino group of

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PEI and further reduce the nonspecific binding of Man-PEI CPs with E. coli.

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The enhancement of antibacterial ability can be attributed to the specific

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adherence of PEI on the surface of E. coli.

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To demonstrate the positive effect of mannose in Man-PEI CPs, we

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modified PEI with ascorbic acid to acquire AA-PEI CPs, which was selected

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as a model to make the control experiment. AA-PEI CPs could not

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specifically bind to E. coli as that there is no specific recognition between to

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ascorbic acid and E. coli. It can be seen from Figure S4, AA-PEI CPs hardly

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had antibacterial activity, whereas Man-PEI CPs exhibit excellent

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bactericidal effect. In the meantime, the antibacterial activity of PEI and

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Man-PEI CPs against Gram-positive bacteria, S. aureus, was studied at

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varying concentrations by colony counting. As shown in Figure S5, the

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activities of PEI and Man-PEI CPs against S. aureus showed 14

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concentration-dependent. However, compared with the unmodified PEI, the

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antibacterial activity of Man-PEI CPs did not exhibit any improvement. The

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major causes for this result are that Man-PEI CPs has nonspecific binding to

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S. aureus.

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These observations were demonstrated that the antibacterial ability of

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Man-PEI CPs mainly depend on electrostatic action of the positive charge

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PEI onto the negative charge bacterial surface, which causes the destruction

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of the cell wall and the leakage of the cytoplasmic constituents, thereby

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leading to the death of bacteria.40 The enhancement of antibacterial ability is

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due to specific recognition between the mannose and FimH, which improves

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the specific adherence of PEI on the surface of E. coli. Moreover, previous

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studies had shown that the schiff base ligands are used in antibacterial and

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antifungal material.41,42 Similarly, the 1H NMR spectra (Figure S1) had

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confirmed the formation of schiff base between PEI and mannose that is

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conducive to the enhancement of antibacterial activity.

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Cytotoxicity of PEI and Man-PEI CPs. The cell cytotoxicity of the PEI

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and Man-PEI CPs was tested using MTT assay against HeLa cells. As shown

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in Figure 7, Man-PEI CPs do not exhibit obvious cytotoxicity under the

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antibacterial condition (10 µg mL-1). The cell viability remained at ~75%

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after 24 h incubation even if the concentration of Man-PEI CPs was

21

increased to 500 µg mL-1. However, the PEI exhibits certain cytotoxicity 15

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under the antibacterial condition (220 µg mL-1). The cell viability decreased

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to 56% at 500 µg mL-1 PEI. Therefore, the characteristic of low cell

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cytotoxicity indicates that Man-PEI CPs has great potential application as an

4

antibacterial agent.

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4. CONCLUSION

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In this study, we have successfully modified PEI with mannose to

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construct Man-PEI CPs. By contrast with unmodified PEI, Man-PEI CPs

8

possess low cytotoxicity and excellent antibacterial activity on E. coli by the

9

specific recognition between FimH and mannose. Therefore, as an efficient

10

antibacterial agent, Man-PEI CPs efficiently broaden the antibacterial

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spectrum of PEI. In addition, the synthesis of Man-PEI CPs is simple, rapid

12

and cost-efficient without complicated chemical modification. Given the

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above advantages, Man-PEI CPs provide promising applications for

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combating multiple bacteria and also can be used as special agent for killing

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E. coli with low cytotoxicity.

16

ASSOCIATED CONTENT

17

Supporting Information

18

One table listing about antibacterial activity of antibiotics and PEI

19

nanomaterials, 1H NRM spectra of PEI and Man-PEI CPs, phenol-sulfuric

20

acid method to quantify sugars on PEI, fluorescence-based bacterial

21

aggregation assay, sterilization rate of PEI, Man-PEI CPs and AA-PEI CPs 16

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against E. coli and S. aureus.

2

AUTHOR INFORMATION

3

Corresponding Authors

4

*(M.L.)

5

[email protected]

6

*(B.L.)

7

[email protected]

8

Notes

9

The authors declare no competing financial interest.

10

Phone:

Phone:

+86-29-85310517;

+86-29-81530726;

Fax:

+86-29-85310517;

E-mail:

Fax:

+86-29-81530727;

E-mail:

ACKNOWLEDGMENTS

11

This work was supported by the National Natural Science Foundation of

12

China (No. 21405101) and the Shaanxi Science and Technology Plan

13

Projects (No. 2017NY-121).

14

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Figure captions

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Scheme 1. Schematic illustration of PEI and Man-PEI CPs antibacterial

5

strategy.

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Figure 1. UV-vis absorption spectra of PEI, mannose, Man-PEI CPs and

8

fluorescence emission spectra of Man-PEI CPs.

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Figure 2. The SEM images of (a) PEI and (b) Man-PEI CPs.

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Figure 3. FT-IR spectra of (a) PEI, (b) mannose, and (c) Man-PEI CPs.

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Figure 4. Sterilization rate of PEI and Man-PEI CPs at 10 µg mL-1

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according to the number of E. coli colonies on agar plates.

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Figure 5. (a) Agar plates of E. coli at a density of 3 × 104 treated with PEI

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and Man-PEI CPs for 3 h at 10 µg mL-1; plates were then incubated at 37 °C

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for 16 h.

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(b) Fluorescence microscope images of E. coli with PEI and Man-PEI CPs at

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10 µg mL-1 stained by PI after sterilization for 3 h. Unstained cells indicate 24

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live bacteria while red staining indicates dead bacteria. Scale bar, 50 µm.

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Figure 6. (a) Sterilization rate of 10 µg mL-1 Man-PEI CPs prepared by

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different mass ratio (PEI:mannose) according to the number of E. coli

5

colonies on agar plates.

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(b) Agar plates of E. coli at a density of 3 × 104 treated with 10 µg mL-1

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Man-PEI CPs for 3 h; plates were then incubated at 37 °C for 16 h.

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Figure 7. Cell viability of PEI and Man-PEI CPs against HeLa cells at

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different concentrations for 24 h. The error bars represent the standard

11

deviations of three parallel measurements.

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