large-scale genomic studies conspicuously NANOMATERIALS avoiding the complexities of plasmid struc- ture.Genomic comparisons such as that described by Conlan et al.reveal how the Making strong nanomaterials dynamism in the structure and arrange- ment of resistance elements can only be realized by“closing”plasmid genomes with ductile with gradients long-read sequencing(see the figure). Traditional Sanger sequencing is the Microstructures that increase metal crystallite size from gold standard for the analysis and assem- nanoscale with surface depth are both strong and ductile bly of complete plasmid sequences from antibiotic-resistant strains of bacteria.This By K.Lu nisms of the extremely fine grains that in- approach may suffer from the need to iso- duces cracking.By applying surface plastic late or subclone individual high molecular teels can be made stronger,tougher deformation onto a bulk coarse-grained weight plasmids before sequencing (7). or more resistant to corrosion either metal,a distinctive microstructure is gener which is often technically difficult,time- by changing composition (adding in ated from the strain gradient:a nanograined consuming,and costly,and may be intrac- more carbon or other elements)or layer (several tens of micrometers thick) table for multiple plasmids.Short-read by modifying their microstructures. covers the coarse-grained substrate with a sequencing technologies can affordably An extreme microstructural route graded variation of grain size in between produce an assembly of a bacterial genome for strengthening materials is to reduce (see the first figure). that contains nonrepetitive sequences typi- the crystallite size from the micrometer Tensile tests of the heterogeneously cally in hundreds of "contigs"separated by scale ("coarse-grained")to the nanoscale. structured Cu cylinder (pulling the sample “collapsed repeats”indicative of multiple Nanograined aluminum or copper (Cu) along the long axis)showed that the top copies of the same sequence located in sev- may become even harder than high- eral different locations within the genome. strength steels,but these materials 0(m) These repeats are often mobile elements can be very brittle and crack when such as insertion sequences that may be pulled (deformed in tension),ap- found in multiple copies on plasmids,thus parently because strain becomes making it difficult to assemble plasmid localized and resists deformation. sequences. However,nanograined metals can Cataloging the collection of antibiotic- be plastically deformed under resistance genes in any particular bac- compression or rolling at ambient terium is relatively straightforward,but temperature,implying that moder- determining how these genes fit together ate deformation can occur if the within plasmids,which is critical for un- cracking process is suppressed.Tre- derstanding how these elements spread mendous efforts have been made to in clinical settings,can be more difficult. explore how to suppress strain lo- By contrast,the genome sequences pro- calization in tensioned nanomateri- duced through long-read sequencing offer als and make them ductile.Gradient a complete picture of the plasmid content microstructures,in which the grain of a bacterium,including the number,posi- size increases from nanoscale at tion,and context within mobile elements of the surface to coarse-grained in the every acquired antibiotic-resistance gene. core,were recently discovered to be Long-read genome assembly offers clear an effective approach to improving advantages for the resolution of complete ductility (-) plasmid sequences that can discriminate One advantage of metals in struc- plasmid diversity,antimicrobial-resistance tural applications is that they "sig- Gradient nanograined structure.After a surface mechanical gene context,and multiplicity.Such infor- nal"their impending failure-they grinding treatment to copper.grain sizes are about 20 nm in the mation will enhance our understanding of can deform and crack to some ex- topmost treated surface(outlined by dashed line)and increase plasmid carriage,transfer,epidemiology, tent before they completely fail. gradually to the microscale with depth. and evolution. However,when a piece of fully nanograined copper is pulled,catastrophic nanograined layer and the coarse-grained REFERENCES failure occurs immediately when the load core can be elongated coherently by as much 1.WHO.Antimicrobial resistance:Global report on surveil- exceeds its yield strength (the point at as ~60%before failure-comparable to that lance2014(2014). which permanent deformation begins),just in conventional Cu,but the sample's yield 2.S.Conlanetal..Sci.Transl.Med.254.254ral26(2014). 3.H.Ochman,J.G.Lawrence,E.A.Groisman,Nature 405. like most ceramics and other normal frag- strength is doubled (D).Almost no tensile 299(2000). ile materials.Such tensile brittleness is an elongation was observed in the nanograined 4.E.S.Snitkin etal.Sci.Transl.Med.4.148ral16(2012) Achilles'heel of nanomaterials that hinders layer as it was removed from the substrate 5.R.M.Hall,C.M.Collis.Mol.Microbiol.15.593(1995). their technological applications;for exam- Evidently,the observed extraordinary tensile 6.J.Mahillon,M.Chandler,Microbiol.Mol.Biol.Rev.62.725 (1998) ple,they cannot be strengthened by work 7.C.Venturini.S.A.Beatson,S.P.Djordjevic.M.J.Walker. hardening.The microscopic origin appears Shenyang National Laboratory for Materials Science.Institute FASEB124.1160(2010) to be early necking (decrease in cross sec- of Metal Research,Chinese Academy of Sciences,Shenyang 110016.China,and Herbert Gleiter Institute of Nanoscience tion)induced by strain localization prior Nanjing University of Science Technology.Nanjing 210094. 101126/science.1260471 to activation of plastic deformation mecha- China.E-mail:lu@imr.ac.cn SCIENCE sciencemag.org 19 SEPTEMBER 2014.VOL 345 ISSUE 6203 1455 Published by AAASSCIENCE sciencemag.org 19 SEPTEMBER 2014 • VOL 345 ISSUE 6203 1455 large-scale genomic studies conspicuously avoiding the complexities of plasmid struc￾ture. Genomic comparisons such as that described by Conlan et al. reveal how the dynamism in the structure and arrange￾ment of resistance elements can only be realized by “closing” plasmid genomes with long-read sequencing (see the figure). Traditional Sanger sequencing is the gold standard for the analysis and assem￾bly of complete plasmid sequences from antibiotic-resistant strains of bacteria. This approach may suffer from the need to iso￾late or subclone individual high molecular weight plasmids before sequencing ( 7), which is often technically difficult, time￾consuming, and costly, and may be intrac￾table for multiple plasmids. Short-read sequencing technologies can affordably produce an assembly of a bacterial genome that contains nonrepetitive sequences typi￾cally in hundreds of “contigs” separated by “collapsed repeats” indicative of multiple copies of the same sequence located in sev￾eral different locations within the genome. These repeats are often mobile elements such as insertion sequences that may be found in multiple copies on plasmids, thus making it difficult to assemble plasmid sequences. Cataloging the collection of antibiotic￾resistance genes in any particular bac￾terium is relatively straightforward, but determining how these genes fit together within plasmids, which is critical for un￾derstanding how these elements spread in clinical settings, can be more difficult. By contrast, the genome sequences pro￾duced through long-read sequencing offer a complete picture of the plasmid content of a bacterium, including the number, posi￾tion, and context within mobile elements of every acquired antibiotic-resistance gene. Long-read genome assembly offers clear advantages for the resolution of complete plasmid sequences that can discriminate plasmid diversity, antimicrobial-resistance gene context, and multiplicity. Such infor￾mation will enhance our understanding of plasmid carriage, transfer, epidemiology, and evolution. REFERENCES 1. WHO, Antimicrobial resistance: Global report on surveil￾lance 2014 (2014). 2. S. Conlan et al., Sci. Transl. Med. 254, 254ra126 (2014). 3. H. Ochman, J. G. Lawrence, E. A. Groisman, Nature 405, 299 (2000). 4. E. S. Snitkin et al. Sci. Transl. Med. 4, 148ra116 (2012). 5. R. M. Hall, C. M. Collis, Mol. Microbiol.15, 593 (1995). 6. J. Mahillon, M. Chandler, Microbiol. Mol. Biol. Rev. 62, 725 (1998). 7. C. Venturini, S. A. Beatson, S. P. Djordjevic, M. J. Walker, FASEB J. 24, 1160 (2010). S teels can be made stronger, tougher, or more resistant to corrosion either by changing composition (adding in more carbon or other elements) or by modifying their microstructures. An extreme microstructural route for strengthening materials is to reduce the crystallite size from the micrometer scale (“coarse-grained”) to the nanoscale. Nanograined aluminum or copper (Cu) may become even harder than high￾strength steels, but these materials can be very brittle and crack when pulled (deformed in tension), ap￾parently because strain becomes localized and resists deformation. However, nanograined metals can be plastically deformed under compression or rolling at ambient temperature, implying that moder￾ate deformation can occur if the cracking process is suppressed. Tre￾mendous efforts have been made to explore how to suppress strain lo￾calization in tensioned nanomateri￾als and make them ductile. Gradient microstructures, in which the grain size increases from nanoscale at the surface to coarse-grained in the core, were recently discovered to be an effective approach to improving ductility ( 1– 4). One advantage of metals in struc￾tural applications is that they “sig￾nal” their impending failure—they can deform and crack to some ex￾tent before they completely fail. However, when a piece of fully nanograined copper is pulled, catastrophic failure occurs immediately when the load exceeds its yield strength (the point at which permanent deformation begins), just like most ceramics and other normal frag￾ile materials. Such tensile brittleness is an Achilles’ heel of nanomaterials that hinders their technological applications; for exam￾ple, they cannot be strengthened by work hardening. The microscopic origin appears to be early necking (decrease in cross sec￾tion) induced by strain localization prior to activation of plastic deformation mecha￾By K. Lu Microstructures that increase metal crystallite size from nanoscale with surface depth are both strong and ductile NANOMATERIALS Making strong nanomaterials ductile with gradients nisms of the extremely fine grains that in￾duces cracking. By applying surface plastic deformation onto a bulk coarse-grained metal, a distinctive microstructure is gener￾ated from the strain gradient: a nanograined layer (several tens of micrometers thick) covers the coarse-grained substrate with a graded variation of grain size in between (see the first figure). Tensile tests of the heterogeneously structured Cu cylinder (pulling the sample along the long axis) showed that the top nanograined layer and the coarse-grained core can be elongated coherently by as much as ~60% before failure—comparable to that in conventional Cu, but the sample’s yield strength is doubled (1). Almost no tensile elongation was observed in the nanograined layer as it was removed from the substrate. Evidently, the observed extraordinary tensile 0 (￾m) 50 100 150 Gradient nanograined structure. After a surface mechanical grinding treatment to copper, grain sizes are about 20 nm in the topmost treated surface (outlined by dashed line) and increase gradually to the microscale with depth. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China, and Herbert Gleiter Institute of Nanoscience, Nanjing University of Science & Technology, Nanjing 210094, 10.1126/science.1260471 China. E-mail: lu@imr.ac.cn Published byAAAS