Articles Cite This: ACS Chem. Biol. 2018, 13, 1890−1896 Unique Fluorescent Imaging Probe for Bacterial Surface Localization and Resistant Enzyme Imaging Hui Ling Chan,† Linna Lyu,† Junxin Aw,† Wenmin Zhang,†,‡ Juan Li,‡ Huang-Hao Yang,‡ Hirohito Hayashi,† Shunsuke Chiba,† and Bengang Xing*,†,‡ † Downloaded via DEPAUL UNIV on February 10, 2020 at 22:41:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Division of Chemistry and Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore ‡ College of Chemistry, Fuzhou University, Fuzhou, Fujian 350116, China S Supporting Information * ABSTRACT: Emergence of antibiotic bacterial resistance has caused serious clinical issues worldwide due to increasingly difficult treatment. Development of a specific approach for selective visualization of resistant bacteria will be highly significant for clinical investigations to promote timely diagnosis and treatment of bacterial infections. In this article, we present an effective method that not only is able to selectively recognize drug resistant AmpC β-lactamases enzyme but, more importantly, is able to interact with bacterial cell wall components, resulting in a desired localization effect on the bacterial surface. A unique and specific enzymeresponsive cephalosporin probe (DFD-1) has been developed for the selective recognition of resistance bacteria AmpC β-lactamase, by employing fluorescence resonance energy transfer with an “off−on” bioimaging. To achieve the desired localization, a lipid−azide conjugate (LA-12) was utilized to facilitate its penetration into the bacterial surface, followed by copper-free click chemistry. This enables the probe DFD-1 to be anchored onto the cell surface. In the presence of AmpC enzymes, the cephalosporin β-lactam ring on DFD-1 will be hydrolyzed, leading to the quencher release, thus generating fluorescence for real-time resistant bacterial screening. More importantly, the bulky dibenzocyclooctyne group in DFD-1 allowed selective recognition toward the AmpC bacterial enzyme instead of its counterpart (e.g., TEM-1 β-lactamase). Both live cell imaging and cell cytometry assays showed the great selectivity of DFD-1 to drug resistant bacterial pathogens containing the AmpC enzyme with significant fluorescence enhancement (∼67-fold). This probe presented promising capability to selectively localize and screen for AmpC resistance bacteria, providing great promise for clinical microbiological applications. W lactam drugs by resistance bacteria transmitting in human communities.11,12 So far, extensive investigations specific to class A Blas have been widely conducted.13−17 Although a similar serine hydroxyl group and geometries of active sites are observed in both class A and C Blas enzymes, their detailed secondary structures indicated a significant difference in the pocket size and the amino acid residue arrangement.18−20 Critically, recent outbreaks of resistance in bacterial species previously not expressing class C Blas have emerged due to conjugative transmission of a bacterial plasmid, which led to a heightened clinical risk of antibiotic resistance in hospitals globally.21−24 Therefore, a simple and unique approach that is capable of selectively identifying class C Blas enzyme activities will be clinically important in combatting microbial resistance. Over the years, various effective strategies have been developed to facilitate Blas drug resistant enzyme investiga- idespread antibiotic resistance among pathogenic bacteria is a critical problem currently faced by both hospitals and community health worldwide, thus placing individuals’ health at stake on both an international and national scale.1−3 One of the primary defense mechanisms that antibiotic resistance bacteria possess is the production of drug resistant enzymes, e.g., β-lactamases (Blas), a family of bacterial enzymes that specifically break down β-lactam antibiotic structure.4,5 Generally, such resistant Blas enzymes can hinder the antibiotic ability to inhibit penicillin-binding proteins for cell wall biosynthesis, hence rendering these drugs ineffective for antibacterial treatment.6,7 Therefore, extensive investigation is still required. Additionally, the development of specific approaches for selective recognition of resistance bacteria to gain a better understanding of the biochemical processes conferring bacterial drug resistance will be highly desirable. In terms of distinct expression patterns and related catalytic mechanisms, Blas enzymes can be categorized into four different Ambler classes: A, B, C, and D, each with its unique enzyme sequence conferring intrinsic antibiotic bacterial resistance.8−10 Among these enzyme classes, class A and C Blas are considered the most significant in hydrolysis of β© 2018 American Chemical Society Special Issue: Sensors Received: February 20, 2018 Accepted: March 29, 2018 Published: March 29, 2018 1890 DOI: 10.1021/acschembio.8b00172 ACS Chem. Biol. 2018, 13, 1890−1896 Articles ACS Chemical Biology tions.25−29 Among which, fluorescence optical imaging has been widely employed due to its promising possibilities in offering accurate and reliable results in real-time monitoring of biochemical functions and resolving drug-resistant bacterial pathogens in vitro and in vivo.30,31 Despite great dependability, most of the established methods through chemical-based probe molecules may have concerns for real-time monitoring of the dynamics of enzyme functions, or limited precision for the accurate detection of bacterially resistant processes within a targeted and localized region which is mainly attributed to the multidimensional, heterogeneous, and spatial complexity of the cellular environment. Moreover, the inevitable diffusion of the chemical probe during the imaging process may present a potential issue of specificity or limited spatial resolution for real-time imaging analysis in living systems. Although several localization strategies based on covalent labeling or enzyme triggered probe molecule aggregation have been previously proposed to selectively report different resistant enzyme expression in bacterial pathogens;32−35 these methods may potentially induce perturbation toward bacterial structure and functions in living settings. Therefore, the development of a simple and efficient method that allows effective probe localization on the bacterial surface, especially for the analysis of drug resistant bacteria, remains a technical challenge in the field, and extensive studies are required. Fatty acid lipid molecules are naturally expressed cell surface components that are well recognized as integral membrane structures with a close relation to cell wall stiffness and susceptibility to various types of cell functions.36,37 So far, incorporation of fatty acid lipidated groups with therapeutic moieties and contrast agents have been proposed to achieve an enhanced pharmacokinetic profile, improved treatment efficacy, as well as cellular structure-localized imaging to track real-time enzymatic dynamics and subcellular organelle position.38−47 Inspired by the promising capability of such unique cell surface components, in this study, we present an effective method that can selectively recognize class C drug resistant Blas and, more importantly, interacts with bacterial cell components, allowing the desired localization effect to be obtained (Scheme 1). Effective immobilization of the probe onto the bacterial surface was achieved through insertion of the lipidated fatty acid chain, which is a part of the cell membrane component’s favorable structure. Typically, a FRET pair, fluorescein isothiocyanate (FITC) fluorophore as a donor and 4-(4,2-dimethylaminophe- nylazo) benzoic acid (DABCYL) as a quencher, can be utilized to amplify enzyme activity. Under the presence of class C Blas (e.g., AmpC), the cephalosporin β-lactam ring in DFD-1 was cleaved, releasing the 3′-position conjugated DABCYL. Such enzyme triggered probe release led to fluorescence enhancement and allowed real-time visualization of resistance bacteria. To attain AmpC enzyme selectivity, a bulky dibenzocyclooctyne (DBCO) group was attached to the cephalosporin βlactam ring 7′-position to introduce steric hindrance, which may enable selective recognition toward AmpC Blas. More importantly, through fatty acid chain exploitation, insertion of the lipid into the bacterial surface can allow the observation of a desired localization. To achieve such a localization property, we employed a lipid-azide conjugate for its penetration into the bacterial cell surface, followed by copper-free click chemistry, which immobilized the fluorescence signal onto the bacterial surface. Such a unique strategy can promote efficient localization of the fluorescence probe DFD-1 and can thus greatly reduce the active diffusion of the probe molecules in bacterial structures, therefore providing great promise for performing precise and reliable screening of bacterial resistance in clinical practice. ■ RESULTS AND DISCUSSION Synthetic Strategy. The synthetic route of probe DFD-1 is depicted in Scheme 2. Cephalosporin β-lactam 1 was first linked with Mtt-Lys 3 at the 7′-position to yield Mtt-Lys-Lac 4 (step a). Next, at the 3′-position, cephalosporin was coupled with Thio-DABCYL 8 (prepared from trityl-protected 4aminothiophenol with DABCYL) to afford 9 (step b). Subsequently, at the 7′-position of 9, FITC was conjugated under basic reaction conditions, which was then followed by installation of the DBCO moiety through amide coupling with dibenzocyclooctyne-N-hydroxysuccinimidyl ester (DBCONHS ester) to give the desired product DFD-1. The reaction mixture was purified by high performance liquid chromatography (HPLC) and characterized by mass spectroscopy analysis (MS). Enzymatic Properties. To study the enzyme activity of the reporter molecule DFD-1, the fluorescence emission was recorded upon the addition of AmpC Blas in phosphate buffered saline (PBS) solution (0.1 M, pH = 7.4). As shown in Figure 1A, under the absence of the Blas enzyme, little fluorescence was observed with DFD-1 alone due to the efficient FRET quenching. Notably, with the treatment of the AmpC enzyme in PBS, significant enhancement in the fluorescence intensity at 516 nm was detected (∼67-folds) before and after the enzyme reaction. This indicates the ability of the AmpC Blas enzyme to efficiently cleave the β-lactam ring and release the quencher from the cephalosporin structure, thereby resulting in the observed intensity increment. Moreover, such enzyme cleavage is further supported by the LC-MS spectral analysis of the fragments corresponding to the hydrolyzed cephalosporin ring and quencher Dabcyl moieties at 1062.73 and 377.23, respectively (Supporting Information Figure S1). In addition, a low fluorescence enhancement was found in the presence of a typical AmpC inhibitor, aztreonam (AZT; Figure 1B). Such effective suppression of the AmpC enzymatic activity resulted in a substantial reduction in the fluorescence readout, which clearly demonstrates the specificity of DFD-1 to the AmpC enzyme instead of the spontaneous degradation of the probe or nonspecific interactions. As a control, a similar enzymatic analysis was carried out with the Scheme 1. Bacterial Surface Localization and Enzymatic Responsive Fluorescence Changes upon the Reaction of Probe DFD-1 and AmpC Blas 1891 DOI: 10.1021/acschembio.8b00172 ACS Chem. Biol. 2018, 13, 1890−1896 Articles ACS Chemical Biology Scheme 2. Synthesis of the Enzyme Responsive Probe DFD-1a Reagents and conditions: (a) Mtt-Lys 3, EDC·HCl, HOBt, TEA, CH3CN/dioxane (1:1), 0−23 °C, 12 h; (b) Thio-DABCYL 8, 2,6-lutidine, NaI, DMF 23 °C, 13 h; (c) (i) 2% v/v TFA, CH2Cl2, 23 °C, 1 h; (ii) FITC, TEA, DMF, 23 °C, 10 h; (d) (i) 12.5% v/v TFA, 2.5% v/v TIPS, CH2Cl2, 23 °C, 4 h; (ii) EDC·HCl, 4-DMAP, DBCO-NHS, DMF, 23 °C, 10 h. a microscopy with the excitation at 488 nm (Figure 2A). Furthermore, to examine whether the lipid moiety chain length Figure 1. Emission spectra of (A) probe DFD-1 (10 μM) before and after incubation with AmpC and TEM-1 Blas, respectively (30 nM). (B) Fluorescence enhancement of DFD-1 incubated with AmpC, TEM-1 enzymes, inhibitor AZT, and CA (100 μM) in 0.1 M PBS (pH = 7.4). use of TEM-1 enzyme, a typical class A Blas, to investigate the reaction selectivity. As shown in Figure 1A and B, treatment with the TEM-1 enzyme showed weak fluorescence activity after enzyme reaction. Further analysis of enzyme kinetics was also conducted to determine the activity of both AmpC and TEM-1 Blas (Supporting Information Figure S2). The probe DFD-1 cleavage was identified with the Michaelis constant, Km = 7.4 μM and 10.0 μM, and the catalytic constant, kcat = 142.9 min−1 and 47.6 min−1, for AmpC and TEM-1, respectively. Hence, these results clearly demonstrated that the synthesized DFD-1 exhibited a greater specificity and selectivity recognition toward AmpC Blas rather than to the TEM-1 counterpart, mostly attributed to the additional bulky DBCO moiety within the DFD-1 cephalosporin 7′-position that could accommodate well into the larger binding pocket located in the AmpC Blas enzyme structure.18−20 Ability of Lipids to Anchor onto Bacterial Surface. To investigate the feasibility of the enzyme responsive probe molecule to be efficiently anchored onto the bacterial surface for facilitated cell structure immobilization, we first employed a fatty acid molecule, an intrinsic lipid component of the bacterial surface as the targeting moiety modified with an azide group, for incubation with the bacterial pathogens. As a proof of concept in this study, two commonly used bacterial strains, the Gram-negative Pseudomonas aeruginosa PAO1 (P. aeruginosa PAO1) and Gram-positive Enterococcus faecium (E. faecium) were chosen. Both are well-known as typical pathogens for bacterial resistance studies.48−51 Upon bacterial incubation with the lipid, a simple fluorescence molecule, DBCO-FITC (DF), was subsequently conjugated through a copper-free click reaction. The fluorescence images were recorded to investigate the effective bacterial surface insertion by using confocal Figure 2. (A) Scheme of bacterial insertion with lipid of LA-18, LA12, and LA-6, followed by click reaction with DF. (B) Confocal imaging of lipid screening in P. aeruginosa PAO1 and E. faecium bacterial strains upon treatment of lipid moieties (LA-18, LA-12, and LA-6) (2 μM) and DF (2 μM) in 0.1 M PBS, pH = 7.4. Scale bar: 5 μm. may affect bacterial insertion, the azido-coupled fatty acids with different carbons (N-(2-azidoethyl) stearamide, N-(2-azidoethyl) dodecanamide, and N-(2-azidoethyl) hexanamide abbreviated as LA-18, LA-12, and LA-6, respectively; Supporting Information Scheme S2) were also synthesized, and their subsequent bacterial incubations were carried out for imaging analysis. As shown in Figure 2B, the P. aeruginosa PAO1 strain, there was a weak fluorescence observed in fatty acid moiety LA-18 incubated bacteria when compared to the pathogens treated with LA-6 and LA-12 structures. Although both bacteria treated with LA-6 and LA-12 moieties led to an increment in fluorescence signal, LA-12 exhibited a stronger fluorescence intensity than that of LA-6. Similarly, for the E. faecium bacteria, among three fatty acids moieties, LA-12 demonstrated the strongest fluorescence for lipid facilitated bacterial imaging (Figure 2B and Supporting Information Figure S3). The results indicated that the increased length of the carbon chain could likely enhance the immobilization of the probe onto the bacterial surface. The shorter hydrophobic chain in LA-6 would 1892 DOI: 10.1021/acschembio.8b00172 ACS Chem. Biol. 2018, 13, 1890−1896 Articles ACS Chemical Biology Figure 3. Fluorescence imaging of selective enzyme reaction and bacterial surface localization in living bacteria: antibiotic susceptible strains S. aureus and P. putida OUS82, TEM-1 Blas producing MRSA BAA-44 and E. faecium, as well as AmpC Blas producing P. aeruginosa PAO1 and E. cloacae, with subsequent treatment of LA-12 (50 μM) and DFD-1 (10 μM) in 0.1 M PBS, pH = 7.4. Scale bar: 5 μm. producing bacterial strains in P. aeruginosa PAO1 and E. cloacae. On the other hand, similar bacterial treatment with LA-12 and DFD-1 in the TEM-1 Blas enzyme expressing MRSA BAA-44 and E. faecium strains only resulted in weak fluorescence signals. Importantly, the pretreatment of targeted strains producing the AmpC enzyme (e.g., P. aeruginosa PAO1) with a typical class C Blas AZT inhibitor greatly reduced the fluorescence intensity in bacterial imaging, while similar incubation of P. aeruginosa PAO1 bacteria with class A TEM-1 enzyme inhibitor CA demonstrated little effect on the fluorescence signal (Supporting Information Figure S5). In the negative control, there was almost no fluorescence detected in antibiotic susceptible S. aureus and P. putida OUS82 bacteria under treatment with the probe molecule (Figure 3). Importantly, there was minimum effect observed in inhibition of bacterial growth during the bacterial imaging by using the DFD-1 probe molecule (Supporting Information Table S1). These results clearly enforced the capability of the designed enzymatic substrate DFD-1 as a safe probe molecule for the selective recognition of AmpC enzyme activities in live bacterial strains. Moreover, compared to the imaging of bacteria strains treated with the LA-12 lipid moiety and DF, the bright fluorescence signals observed in P. aeruginosa PAO1 and E. cloacae strains further confirmed the localization ability of LA-12 that facilitated immobilization of the enzyme responsive probe onto the bacterial surface (Figure 3 and Supporting Information Figure S6). Enzyme Activity and Click Reaction Contribution toward Imaging. In order to investigate the fluorescent staining effects of enzyme hydrolysis and click chemistry triggered bacterial localization contributed to live bacterial imaging, we compared these individual performances in the drug resistance bacteria (e.g., P. aeruginosa PAO1) with AmpC Blas expression. In this typical study, the bacterial strain P. aeruginosa PAO1 was first incubated with the LA-12 lipid moiety for 1 h at 37 °C, followed by subsequent addition of DF. The bacterial surface immobilization triggered by the copper-free click chemistry reaction was determined by measuring the fluorescence change at different time durations. Alternatively, the enzyme cleavage activity was also carried out in a similar manner, through incubation of LA-12 lipid-labeled lead to less staining, which could be easily washed away, giving decreased fluorescence intensity. Although LA-18 could also show an enhanced tracking ability on the bacterial surface, this azido-coupled lipid moiety indicated limited solubility in aqueous solutions, which may potentially affect cell activity and reduce the effective concentration for bacterial imaging. These fluorescent staining studies in both P. aeruginosa PAO1 and E. faecium bacteria clearly showed the lipid moiety LA-12 as the ideal tracer to specifically immobilize DF onto the bacterial cell surfaces, which may thus greatly facilitate dynamic visualization of drug-resistant enzymes in living bacteria. Additionally, staining of LA-12 treated bacterial strains with propidium iodide (PI) showed minimum bacterial perturbation (Supporting Information Figure S4). These studies unequivocally indicated the great possibilities of LA-12 as a promising localizing agent for bacterial imaging under living conditions. Imaging Resistant Bacterial with LA-12 and DFD-1. Encouraged by the specific enzymatic hydrolysis of DFD-1 as well as the promising bacterial immobilization ability of the LA12 lipid moiety, we investigated the possibility for localization of the reporter molecular DFD-1 onto the bacterial surface for real-time imaging of drug resistant enzymes in living bacterial pathogens (Figure 3). In this typical study, two antibiotic resistant bacterial strains, Enterobacter cloacae (E. cloacae, ATCC 13047) and P. aeruginosa PAO1 (ATCC 15692), were selected as our main targets primarily due to their native capability to produce class C AmpC Blas enzyme. Another two drug resistant bacterial pathogens E. faecium (ATCC 51559) and methicillin-resistant Staphylococcus aureus (MRSA BAA-44, ATCC BAA44) were utilized as control strains owing to their high expression levels of class A TEM-1 Blas. In addition, two antibiotic susceptible strains Pseudomonas putida OUS82 (P. putida OUS82) and Staphylococcus aureus (S. aureus, ATCC 29213) without Blas expression were selected as the negative control. Typically, these bacterial strains were separately treated with lipid LA-12 for 1 h at 37 °C. After washing and subsequent incubation with the DFD-1 probe for another 30 min, the bacterial samples were subjected to confocal microscopy for fluorescence imaging analysis. As shown in Figure 3, strong fluorescence emissions were observed upon the incubation of the DFD-1 probe with the AmpC Blas enzyme 1893 DOI: 10.1021/acschembio.8b00172 ACS Chem. Biol. 2018, 13, 1890−1896 Articles ACS Chemical Biology P. aeruginosa PAO1 bacteria with the probe DFD-1; the fluorescence change was recorded to evaluate surface localized enzyme activities. As shown in Figure 4B, under the same Figure 5. Flow cytometry analysis of resistant bacterial strains incubated with (A) lipid LA-12 (50 μM) and probe DFD-1 (10 μM) or (B) lipid LA-12 (50 μM) and inhibitor AZT (100 μM), followed by probe DFD-1 (10 μM) staining in 0.1 M PBS, pH = 7.4. Concentration of bacterial was approximately 108 cells mL−1. could efficiently be inserted into the bacterial surface and thus greatly facilitated the localization of the enzyme triggered fluorescent signal onto the bacterial surface. By taking advantage of the bulky DBCO group at the 7′-position of cephalosporin structure, the fluorescent probe DFD-1 could selectively recognize the AmpC Blas enzyme. The significant fluorescence enhancement toward selective detection of the AmpC Blas enzyme indicated a promising strategy for direct observation of resistant bacterial infections under living conditions. More importantly, such lipid facilitated surface localization of fluorescence labeling could also provide the promising capability for dynamic monitoring of bacterial development and assessing effective antibacterial therapeutics in vitro and in vivo. Figure 4. (A) Scheme of the different staining effects from the enzyme hydrolysis and click reaction toward fluorescence imaging in live bacteria. (B) Comparison of fluorescence intensity between the enzyme hydrolysis and click chemistry in live bacteria. LA-12 lipidated (50 μM) P. aeruginosa PAO1 strains were separately treated with DF and DFD-1 (10 μM), under different time intervals in 0.1 M PBS, pH = 7.4. reaction conditions, DF labeling triggered by click reaction exhibited a relatively higher fluorescence intensity than the fluorescence generated from the enzyme reaction. The fluorescence profiles implied that both enzyme hydrolysis and click chemistry triggered surface insertion would contribute to the bacterial imaging, and the click coupling would facilitate the imaging probe staining on the bacterial surface for effective imaging studies. (Figure 4). Flow Cytometry Analysis. Additionally, we studied the possibility to quantify the specific labeling of AmpC Blas expressing resistant bacteria (e.g., P. aeruginosa PAO1) using flow cytometer analysis (FCM). The antibiotic susceptible S. aureus and resistant MRSA BAA-44 strains with expression of TEM-1 Blas were used as the negative control (Figure 5). In this study, the different strains were first incubated with the LA12 moiety and subsequently treated with DFD-1 for flow cytometry analysis. Fluorescence signals were collected at 525 nm. As shown in Figure 5A, a stronger intensity of fluorescence was observed in the AmpC expressing P. aeruginosa PAO1 strain compared to the negative control S. aureus bacteria that do not produce β-lactamase. Meanwhile, a lower fluorescence change was detected for the TEM-1 producing MRSA BAA-44 strain. Moreover, a similar flow cytometry analysis with the treatment of AmpC enzyme inhibitor AZT showed a significant decrease in the fluorescence intensity (Figure 5B). Therefore, these data clearly demonstrate the specificity of DFD-1 as a reliable reporter molecule for quantification of AmpC activities in antibiotic resistant bacteria. In conclusion, this work presented a specific and selective approach toward efficient bacterial surface localization and realtime imaging of drug resistant bacteria with AmpC enzyme expression. In this study, the optimized lipid moiety (LA-12) ■ ■ METHODS See the Supporting Information for a detailed description of the experimental methods. ASSOCIATED CONTENT S Supporting Information * The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acschembio.8b00172. Supplemental experimental procedures, figures, data, and spectra for compounds (PDF) ■ AUTHOR INFORMATION Corresponding Author *Phone: +65-6316 8758. Fax: + 65-6316 6984. E-mail: bengang@ntu.edu.sg. ORCID Juan Li: 0000-0001-7175-2320 Huang-Hao Yang: 0000-0001-5894-0909 Shunsuke Chiba: 0000-0003-2039-023X Bengang Xing: 0000-0002-8391-1234 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was partially supported by NTU-AIT-MUV NAM/ 16001, Tier 1 RG110/16 (S), RG 11/13, and RG 35/15 1894 DOI: 10.1021/acschembio.8b00172 ACS Chem. Biol. 2018, 13, 1890−1896 Articles ACS Chemical Biology (20) Lahiri, S. D., Johnstone, M. R., Ross, P. L., McLaughlin, R. E., Olivier, N. B., and Alm, R. A. (2014) Avibactam and class C betalactamases: mechanism of inhibition, conservation of the binding pocket, and implications for resistance. Antimicrob. Agents Chemother. 58, 5704−5713. (21) Philippon, A., Arlet, G., and Jacoby, G. A. (2002) Plasmiddetermined AmpC-type beta-lactamases. Antimicrob. Agents Chemother. 46, 1−11. (22) Jacoby, G. A. (2009) AmpC beta-lactamases. Clin Microbiol Rev. 22, 161−182. (23) Doi, Y., and Paterson, D. L. 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