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dynamic phase diagram of possible behav- 1. Schal er, V., Weber, C., Semmrich, C., Frey, E. & Bausch, A. identical E. coli cells arise over time under the iours of the filaments. Just how universal this Nature 467, 73–77 (2010).
selective pressure imposed by the presence of diagram will be — whether it describes the 2. Joanny, J. F. & Prost, J. HFSP J. 3, 94–104 (2009).
3. Ramaswamy, S. Annu. Rev. Condens. Matter Phys. 1,
norfloxacin in the growth media. When kept behaviour of everything from flocks of birds on media containing a moderate, not fully to catalytic col oids — can be tested only by 4. Szabó, B. et al. Phys. Rev. E 74, 061908 (2006).
lethal, level of norfloxacin, the bacterial cel s comparison to experiments on the many 5. Dombrowski, C. et al. Phys. Rev. Lett. 93, 098103 (2004).
6. Narayan, V., Ramaswamy, S. & Menon, N. Science 317,
initially suffered from stunted growth. But within a couple of days, the overall popula- Jean-François Joanny is in the Physical Chemistry 7. Deseigne, J., Dauchot, O. & Chaté, H. Preprint at http://arxiv.org/abs/1004.1499 (2010).
tion growth rate increased because the pool of Unit (UMR 168, CNRS UPMC), Institut Curie, 8. Kudrol i, A., Lumay, G., Volfson, D. & Tsimring, L. S. Phys. cel s had together become more resistant to the Rev. Lett. 100, 058001 (2008).
Ramaswamy is at the Centre for Condensed- 9. Mal ouk, T. E. & Sen, A. Sci. Am. 300, 72–77 (2009).
10. Vicsek, T. et al. Phys. Rev. Lett. 75, 1226–1229 (1995).
The authors then increased the amount of 11. Toner, J. & Tu, Y. Phys. Rev. Lett. 75, 4326–4329 (1995).
norfloxacin, which slowed the overall popula- Institute of Science, Bangalore 560 012, India.
12. Chaté, H., Ginel i, F., Grégoire, G. & Raynaud, F. Phys. Rev. E tion growth again. But within a couple of days 77, 046113 (2008).
13. Mishra, S., Baskaran, A. & Marchetti, M. C. Phys. Rev. E 81,
the population developed resistance once more, and the growth rate recovered. This tug-of-war, presenting an ever-increasing chal enge to the cel s, was continued for ten days, at the end of which the population could MICROBIOLOGY
withstand some five times higher levels of nor- Altruistic defence
floxacin than it could initially. But that did not mean that every bacterial cel in the population The novelty of Lee and colleagues’ study2 lies in their temporal characterization of the A charitable deed by a few cel s in a bacterial culture can help the rest of
mechanisms responsible for resistance to nor- that population survive in the presence of antibiotics. This finding can aid
floxacin in individual cel s as they developed. further research into a major problem in public health.
The authors found that almost all of the resist- ant mutants were less effective at staving off the growth defect imposed by norfloxacin than they were isolated. In other words, when few cells in a bacterial popu lation ena- almost al of the isolated mutants were GGERLI/S ble them to fend off the harmful effects subjected to norfloxacin, their growth OE of the antibiotic. This gives such cel s rate was lower than that of the overall M. resistant and non-resistant cel s. But a advantage to their daughter cel s and to Lee et al. show that this is due to these readily diffuses into neighbouring cells tive. This, then, is the cause of the global Escherichia coli — a test case
directly cause this altruistic behaviour. for antibiotic resistance.
Indole is a molecule that is usually pro- tive action on the part of bacteria exposed to Lee et al.2 describe just such a charitable is not present, but is shut down when it is. The an antibiotic threat. The work highlights the deed carried out by individual bacteria in a authors demonstrate that the highly resistant importance of quantitatively understand- population of Escherichia coli subjected to an mutants developed mutations that could help ing microbial population dynamics in devel- antibiotic. The authors show that mutations them survive in the presence of norfloxacin oping the correct strategies for prescribing that develop in a few cel s enable them to assist in the first few days as well as allow indole neighbouring cells that have not undergone production to continue. This enabled them Imagine that a cel that developed resistance resistance-conferring mutations. The actions to assist their much less resistant neighbours can help its non-resistant neighbours by, say, of these few cells lead to a population-wide in counteracting the antibiotic. Because the secreting some substance that assists those resistance. But they do so in a roundabout highly resistant mutants invest energy to pro- neighbours in fighting off an antibiotic. This way that il ustrates how antibiotic resistance duce indole, which is not required for their type of behaviour might make it easier for the can emerge from seemingly harmless and own resistance, their help for their kin can whole bacterial population to avoid extinction. unrelated processes. be considered a form of altruism. Lee et al. From the point of view of the bacterial species Lee et al. used the antibiotic norfloxacin, repeated their experiments with another anti- as a whole, it would avoid the need to wait for which targets a protein that is essential for biotic, gentamicin, and found that the bacterial the rare resistant mutant to come to dominate DNA replication, and so for bacterial cell divi- response was the same (that is, it was not drug the population. It would also ensure that popu- sion and population growth. They studied specific).
lation-level genetic diversity is maintained.
how mutant cells within initial y genetical y Research such as this — tracking how and 10 Macmil an Publishers Limited. Al rights reserved which mutations develop in the presence in a biofilm4, and now in antibiotic resistance is a rich source of complex molecules. More of an antibiotic by isolating individual cells — shows that a pool of microbes can act in con- than 70 molecular species, which are charac- from a bacterial population — is essential for cert. Apart from its implications for research teristic of a carbon-rich chemistry, had already optimizing a dynamic strategy for prescribing in tackling antibiotic resistance, the new work2 been detected there before the Herschel era7. antibiotics. Such information can help in assess- adds to previous studies in challenging the In fact, about 50% of all molecules observed in ing the need for changes in the dosage and dura- conventional definition of what constitutes a astronomy have been detected in this object8.
tion of treatments, for example. The approach multicellular organism. Data from the IRAS infrared satellite have also highlights the fact that inter action between Hyun Youk and Alexander van Oudenaarden shown9,10 that about 4% of all Galactic car- different communities of resistant mutants that are in the Departments of Physics and of bon stars have signs of silicate grains in their form in a bacterial population can enable them Biology, Massachusetts Institute of Technology, expanding circumstellar envelopes. This sug- to mount a more formidable defence against Cambridge, Massachusetts 02139, USA. gested — contrary to what one would expect antibiotics. Single-cel behaviour that is mark- e-mail: [email protected] under thermal-equilibrium conditions and edly different from that at the population level for well-mixed gas mixtures — that the gas has been a subject of intense investigation in 1. Martínez, J. L., Banquero, F. & Andersson, D. I. Nature Rev. envelopes of these carbon stars contain both systems biology. Lee et al. provide another Microbiol. 5, 958–965 (2007).
oxygen-rich and carbon-rich material, thereby 2. Lee, H. H., Mol a, M. N., Cantor, C. R. & Col ins, J. J. Nature posing a challenge to the conventional under- Furthermore, the collective behaviour of 467, 82–85 (2010).
3. Bassler, B. L. & Losick, R. Cel 125, 237–246 (2006).
standing of the chemistry and evolution of single-cel ed organisms — as seen in the phe- 4. O’Toole, G., Kaplan, H. B. & Kolter, R. Annu. Rev. Microbiol. ageing stars. One possible explanation for this nomena of quorum sensing3 and metabolism 54, 49–79 (2000).
astonishing observation is that the stars’ evo- lutionary transition from an oxygen-rich to a carbon-rich phase occurred quite recently, so that the remains of previous epochs can stil be ASTROPHYSICS
traced in the stars’ outer envelopes. The sub- Unexpected warm water
sequent detection11, with the SWAS submil i- metre satel ite, of circumstel ar water vapour in IRC +10216 — a characteristic of oxygen-rich stars — has caused further astonishment.
This observation11 was based on the iden- The detection of water vapour in a carbon star has chal enged the
tification of a single water-vapour spectral understanding of ageing stars. The discovery that such water can be
line of low excitation, which corresponds to a warm shows that our knowledge of these objects is stil rudimentary.
transition between two energy levels that are populated even at low temperatures. The fact that the line is a low-excitation one may suggest In the short time since its launch on 14 May more carbon than oxygen, oxygen is mostly that the water vapour originates from the outer, 2009, the European Space Agency’s Herschel bound to carbon in the form of carbon monox- cold regions of the stel ar gas envelope. Another Space Observatory has delivered several ide (CO) because the molecule has a high bind- possibility is that the vapour arises from the astronomical discoveries in the infrared and ing energy (11 electronvolts). As a result, little vaporization of icy bodies, such as comets submil imetre regions of the electromagnetic oxygen is left free to form other oxides in such or minor planets, in orbit around the star11,12.
spectrum. On page 64 of this issue, Decin stel ar atmospheres, whereas carbon atoms are In their study, Decin et al.1 identify not just et al.1 report yet another of Herschel’s exciting available to form other carbon compounds. By one but numerous water-vapour lines in the findings: the detection of warm water vapour contrast, in normal stars such as the Sun, the spectra of IRC +10216 (see Fig. 1 on page 65). in the circumstel ar envelope of the carbon star atmosphere contains more oxygen than carbon However, many of these are high-excitation and the opposite occurs: carbon-containing lines, which — if the water molecules are ther- Carbon stars were first recognized in the molecules other than CO become rare.
mal y excited — means that the temperature of 1860s by William Huggins and Angelo Secchi. During the 1950s, investigators showed that the gas in which the lines are formed is of the On the basis of his purely visual inspection of a peculiar class of ageing red giant stars known order of 1,000 kelvin. These results point to the spectroscopic observations, Secchi defined this as asymptotic giant branch (AGB) stars — to existence of warm water vapour in the inner new group of objects as class IV in his classifica- which carbon stars belong — have important regions of the stel ar envelope, and seem to rule tion system of stellar spectra. These extremely roles in nucleosynthesis processes. For example, out models — including the vaporization-of- red stars, he noted2, were remarkably different the heavy ‘s-elements’ found in the Milky Way icy-bodies hypothesis — that posit that water from the orange stars of class III: “we cannot (so cal ed because they are created by relatively vapour originates only from the stel ar enve- identify precisely the sources of the lines and slow, hence the ‘s’, neutron capture by heavy lope’s cooler intermediate and outer regions.
the bands. We can say however that there is a atomic nuclei), as wel as nitrogen and carbon5, The strength of the newly discovered lines marked analogy with the reversed spectrum are believed to be produced in AGB stars and also goes against another hypothesis: that the of carbon.” Further observations by Hermann later expel ed into the inter stel ar medium. But existence of water vapour in the envelope’s Carl Vogel and Nils Dunér later strengthened the details of these processes have remained inner regions is due to shock waves that are unclear: we stil lack a complete understanding induced by the star’s pulsation and generate the The distinctive spectral features that char- of key mechanisms, not least of those that drive non-thermal-equilibrium chemistry needed to acterize carbon stars — notably the dominant the intensive winds from such stars. form water in a carbon-rich gas. The authors1 spectral bands of carbon compounds, such as There is therefore good reason to study car- suggest, instead, that the non-thermal-equilib- the C2 Swan bands in the green part of the spec- bon stars — not least, as we shal see, the Milky rium chemistry is the result of the penetration trum, and the lack of bands from oxides such as Way’s pulsating star IRC +10216. This visually of ultraviolet photons into the inner regions of TiO and H2O, which are characteristic of other faint, extended object is, as seen by an observer the envelope, possibly from the star but more types of cool star — is due to their atmospheres on Earth, the brightest source outside the Solar likely from interstellar space. But for these being richer in carbon than in oxygen, as was System in the 5-micrometre waveband6. Radio hypotheses to work, a highly clumpy circum- suggested by Charles Donald Shane3 and dem- observations have demonstrated that the opti- stellar envelope is required, so that enough onstrated by Henry Norris Russel 4. If there is cal y thick, dusty shel that surrounds the star of the ultraviolet radiation penetrates into its 10 Macmil an Publishers Limited. Al rights reserved

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