because the direction of both the nuclear andthe orbital spins, as well as the phase of thewavefunction, can make a 2np changearound the axis, then many different vortexstructures are possible and the energyrestriction on all but singly quantized vor-tices is lifted.

In their experiment, Blaauwgeers andcolleagues identify a doubly quantized vor-tex (that is, n42) in superfluid 3He from itsNMR signature. This vortex is shown inFig. 1. The cones point in the direction of theorbital angular momentum of the Cooperpairs. The circulation depends on the angleof rotation of the coloured bands around thecone axis. Following around the perimeter ofFig. 1, the cones make two full turns, corre-sponding to a 4p rotation.

In 3He, several quantities are changingsimultaneously, so there is no singularityalong the axis (the air-filled core in the plug-hole version) and all quantities vary smooth-ly across the axis. The vortex is also ratherlarge, of order 10 micrometres in diameter,or not far off the size of a small snowflake.Indeed, this is a new snowflake, a natural

structure having its own beautiful symmetryand the added bonus of stable symmetricalflow. Because superfluid 3He can only existbelow 2.6 mK above absolute zero, these new‘snowflakes’ are even more fugitive thanthose of ice.

Vortices in this superfluid are not simplyaesthetic structures in an arcane fluid. Thesymmetry of the wavefunction of superfluid3He mimics surprisingly closely the metric ofthe Universe. Vortices are the superfluid ana-logue of cosmic strings. In the Universe, cos-mic strings, if they indeed exist, are the relicsof cosmic turbulence frozen into space–timeby rapid phase transitions in the Universeshortly after the big bang. In the experimentof Blaauwgeers et al. the vortices are understrict control and can be injected one by oneinto the liquid as the rotation rate isincreased. Indeed, this is how the experimentis done. Until we find real cosmic strings tostudy, vortices in superfluid 3He are the bestsubstitute. ■

George Pickett is in the Department of Physics,University of Lancaster, Lancaster LA1 4YB, UK.e-mail: g.pickett@lancaster.ac.uk

Science2. These papers represent a milestonein meningococcal research. As well as pro-viding the basis for future studies of the biol-ogy and the disease-causing mechanisms ofthis important pathogen, they highlight thepotential of bacterial genomics to providemankind with new solutions to intractableinfectious diseases. However, the data alsoshow us that a headlong rush to evaluate newvaccine candidates should be tempered withcaution. Both genomes are replete with anunprecedented number of genetic elementsthat contribute to both genome fluidity andphysical variability, attesting to the adapt-ability of N. meningitidis and its potential toevade the human immune response.

Neisseria meningitidis is a major cause ofbacterial meningitis and septicaemia, dis-eases that all too frequently result in death ordisability. Serogroup A organisms are res-ponsible for large epidemics in sub-SaharanAfrica, whereas serogroup B and C organ-isms account for sporadic cases and numer-ous localized outbreaks worldwide3.

Despite the reputation it has earned as anaggressive pathogen, N. meningitidis is morefrequently found as a harmless commensal— an organism that lives with, but does notharm, the host — in the human nose andpharynx. It has been estimated that, at anygiven time, between 5% and 10% of peoplecarry the organism. Sporadic cases of diseaseoccur at the much lower annual rates of 1 to 5per 100,000. The bacterium lives only inhumans, passing from one person to anotherin aerosols; there are no other animal or envi-ronmental niches that it can colonize. It istherefore highly adapted to living in humansand has evolved a variety of mechanisms toevade our defences, including the ability toproduce polysaccharide capsules that pro-voke a poor immune response, and veryvariable cell-surface proteins that repre-sent an ever-changing target for the immunesystem.

The genetic basis for the adaptability ofN. meningitidis is evident from the analysesof the genome sequence data by Parkhill etal.1 and Tettelin et al.2. These data reveal anabundance of diverse repeat sequences ineach meningococcal strain. These sequencesare likely to have a key role in mediating vari-ation within each genome and hence in pro-moting the adaptability of the organism. Alarge proportion of repeat sequences is asso-ciated with genes encoding cell-surfacestructures that are involved in the interactionof the organism with the host and thus repre-sent possible targets for vaccine develop-ment. Tettelin et al.2 estimate that the expres-sion of as many as 65 genes in the serogroup Bstrain MC58 might be altered by the gain orloss of nucleotides within iterative sequencemotifs as a result of inaccurate DNA replica-tion. The effect of such sequence changes onthe expression of these genes and, ultimately,on the synthesis of the proteins they encode,

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NATURE | VOL 404 | 30 MARCH 2000 | www.nature.com 451

Figure 1 The doubly quantizedvortex observed by Blaauwgeersand colleagues. The cones indicatethe direction of the Cooper-pairangular momentum. The angularmomentum makes a full turn indirection on going round thecircular and hyperbolic cores. Thecirculation is given by the rotationof the cones about their axes. Thismakes a full 4p, double turn,following round the perimeter.

Meningococcal disease is dreaded byparents and medical practitionersalike for its rapid onset and the diffi-

culty of obtaining a timely and accurate diag-nosis. Although the causative bacterium,Neisseria meningitidis (Fig. 1, overleaf), isresponsive to antibiotic therapy, these fea-tures of the disease pose a particular problemfor treatment. Prevention of the diseasethrough effective immunization is thus amore attractive alternative than therapy. Forexample, during the past five months new

vaccines have been introduced into the UKvaccination programme that, for the firsttime, offer protection to young infantsagainst disease caused by N. meningitidisserogroup C. The success of this programmeemphasizes the need for the development ofvaccines against all pathogenic strains of N.meningitidis.

The complete genome sequences of theserogroup A and B organisms are nowreported by, respectively, Parkhill et al. onpage 502 of this issue1 and Tettelin et al. in

Bacterial genomics

ABC of meningococcal diversityIan M. Feavers

© 2000 Macmillan Magazines Ltd

provides the bacterium with the ability toswitch rapidly and reversibly between a vastnumber of possible combinations of physicalattributes.

Other types of repetitive DNA allow fur-ther diversity, mediating genomic recom-bination both within and between differentmeningococcal strains. Recombinationbetween strains makes use of the naturalability of the meningococcus to acquire DNAfrom its environment and is undoubtedlyhelped by the prevalence of DNA-uptakesequences — specific nucleotide sequencesthat enable the acquisition of DNA by othermeningococci — in the genomes. All of thesemechanisms provide the bacterium withthe means to overcome the host’s immuneresponse to both infection and vaccination.Such concerns have been raised recently withregard particularly to the ability of meningo-coccal strains to exchange the genes respon-sible for biosynthesis of their capsules4.

Fortunately, as well as highlighting theextent of the problem, meningococcalgenomics also provides possible solutions.Pizza et al.5 have screened the serotype Bgenome for genes encoding cell-surface anti-gens that are highly conserved in allmeningococcal strains. Their aim is to makeuse of these antigens as vaccine components.This approach, although giving encouragingresults in an animal model5, must now beproved in clinical studies. The comparisonof different meningococcal genomes mightalso reveal genes that distinguish the relative-ly small number of hypervirulent strainsfrom all other meningococci, and providethe basis for a strategy that specifically targetsthe properties encoded by such genes.

We do not yet know which strategy, orcombination of strategies, will provide an

All atoms follow the same body plan.They consist of a small, massive nucleusof positive charge surrounded by a

cloud of negative electrons, bound togetherby the electromagnetic force. One might saythat all atoms belong to a single phylum.Hadrons, elementary particles that are gov-erned by the strong nuclear force, are knownto populate three extremely well document-ed phyla. Baryons (such as protons and neu-trons) are based on three quarks having dif-ferent colour charges, one each of red, whiteand blue. Antibaryons are similarly con-structed from triads of antiquarks, andmesons are based on bound quark–anti-quark pairs, carrying equal and oppositecolour charges. Each of these hadron phylasupports dozens of species. For many yearsnow, physicists have been on the lookout forexotic new species, requiring more hadronphyla. We might now have the first hint froma theoretical analysis by Alford and Jaffe1 ofsomething suitably exotic.

What are these new particles? Among therange of possibilities dreamt up by theoristsare various types of glueball, made fromeither two or three colour gluons (the parti-cles that bind quarks together). Or there arecentaurons (also called hybrids) made froma quark, an antiquark and a gluon. Finally, wehave a hypothetical phylum that does not yethave an accepted name, based on two quarksand two antiquarks. It seems irresistible tocall these quarktets. Alford and Jaffe useingenious computer experiments to arguethat some long familiar, but hitherto enig-matic, particles are best interpreted asquarktets1.

Hundreds of different strongly interact-ing particles have been identified. Most arevery short-lived (less than 10120 seconds),and are observed only as enhancements inparticle production rates, known as reso-nances. The observed properties of many ofthese particles match up with the predictedproperties of different arrangements ofquarks and antiquarks conforming to one ofthe baryon, antibaryon or meson body plans.Within each plan, one has the freedom tochoose different properties for the quarks,

which can result in states with different totalenergies (or masses, according to E4mc2).By this reasoning, the pattern of resonancescan be seen as a ‘spectrum’, wholly analogousto the pattern of stationary states of an atom(as observed from its spectral lines).

The logical development of this line ofthought is the quark model2. The quarkmodel was cultivated during the 1960s andearly 1970s, finally reaching its most matureform in the so-called bag model3. Accordingto the bag model, hadrons consist of rela-tivistic, nearly free quarks (and/or anti-quarks) confined to a finite region of spaceby a universal, constant vacuum pressure.The quark/bag model accounts, semi-quantitatively, for an enormous range ofdata. But it is not a satisfactory fundamentaltheory. For example, simply trying to com-pute what happens when bag-hadrons moveresults in mathematical ambiguities.

The fundamental theory of the strongforce, quantum chromodynamics or QCD,emerged from the study of extremely high-energy collisions. QCD accepts quarks andantiquarks as fundamental ingredients, butalso requires colour gluons. Indeed, theexchange of colour gluons is fully responsi-ble for the strong interactions betweenquarks, much as the exchange of photonsis responsible for electromagnetic inter-actions. Unlike the quark/bag model, QCDis a proper, precisely defined fundamentaltheory.

If correct, QCD should be able to predictthe major properties and strong interactionsof hadrons. Unfortunately, however, theequations of QCD are very difficult to solve.Computations that strain at the limits ofmodern, massively parallel computers suc-ceed in producing, within a few per centaccuracy, the masses of a dozen or so of thelowest states of the hadron spectrum4. This isa remarkable achievement and confirms thecorrectness of a beautiful, parameter-freetheory5. Yet to understand the rest of thehadron spectrum one will need, for the fore-seeable future, to build upon older, but moremanageable, concepts and models. Withinthis humbler approach, QCD does not dic-

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452 NATURE | VOL 404 | 30 MARCH 2000 | www.nature.com

Particle physics

Backyard exoticaFrank Wilczek

Figure 1 The meningococcus — a bacterium withunprecedented potential for diversity1,2.

effective solution to the problem ofmeningococcal disease. But the public avail-ability of the two genome sequences is boundto hasten the development of comprehensivevaccines that offer protection against all vir-ulent meningococci. ■

Ian M. Feavers is in the Division of Bacteriology,National Institute for Biological Standards and

Control, Blanche Lane, South Mimms, Potters BarEN6 3QG, UK.e-mail: ifeavers@nibsc.ac.uk

1. Parkhill, J. et al. Nature 404, 502–506 (2000).

2. Tettelin, H. et al. Science 287, 1809–1815 (2000).

3. Schwartz, B., Moore, P. S. & Broome, C. V. Clin. Microbiol. Rev.

2 (Suppl.), 118–124 (1989).

4. Maiden, M. C. & Spratt, B. G. Lancet 354, 615–616 (1999).

5. Pizza, M. et al. Science 287, 1816–1820 (2000).

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