Cell Host & Microbe
Previews dysbiotic microbiota with high levels of NI1060 at the site of ligature placement in both wild-type and Nod1 / mice irrespective of the fact that the inflammatory indices in the wild-type animals are significantly elevated compared to their unresponsive Nod1 / counterparts. For the chicken protagonists who argue that dysbiosis is simply a consequence of disease, the egg men (and women) now have some data to argue otherwise.
REFERENCES Burns, E., Bachrach, G., Shapira, L., and Nussbaum, G. (2006). J. Immunol. 177, 8296–8300.
Darveau, R.P. (2010). Nat. Rev. Microbiol. 8, 481–490.
Hajishengallis, G., Liang, S., Payne, M.A., Hashim, A., Jotwani, R., Eskan, M.A., McIntosh, M.L., Alsam, A., Kirkwood, K.L., Lambris, J.D., et al. (2011). Cell Host Microbe 10, 497–506.
Haubek, D., Ennibi, O.K., Poulsen, K., Vaeth, M., Poulsen, S., and Kilian, M. (2008). Lancet 371, 237–242.
Jiao, Y., Darzi, Y., Tawaratsumida, K., Marchesan, J.T., Hasegawa, M., Moon, H., Chen, G.Y., Nu´n˜ez, G., Giannobile, W.V., Raes, J., and Inohara, N. (2013). Cell Host Microbe 13, this issue, 595–601.
Wade, W.G. (2011). J. Clin. Periodontol. 38(Suppl 11 ), 7–16.
Uracil Debases Pathogenic but Not Commensal Bacteria Susanna Valanne1 and Mika Ra¨met1,2,* 1Institute
of Biomedical Technology and BioMediTech, University of Tampere, Tampere 33014, Finland of Pediatrics, Tampere University Hospital, Tampere 33520, Finland *Correspondence: [email protected]
The metazoan gut harbors microbial communities on its mucosal surfaces, yet the mechanisms by which gut immunity tolerates symbiotic and commensal bacteria while eliminating pathogens is insufficiently understood. In a recent Cell paper, Lee et al. (2013) show that bacterial uracil, not secreted by commensal bacteria, triggers dual oxidase-dependent immunity. It has long been known that humans carry ten times as many microbial cells as their own cells, mostly in the gut (Erkosar et al., 2013). Studying the gut microbial community or its effects on our health can be arduous due to the complexity of the system. This can be overcome through the use of experimental model organisms such as Drosophila melanogaster, which has simpler microbial communities (Erkosar et al., 2013) and comes with many sophisticated methods for genetic manipulation of the host. Recently, for several reasons, Drosophila has emerged as an advantageous model for studying host-microbe interactions in the gut. First, only four to eight microbial species are normally found in a given fly population. Second, these bacteria are aerobes or at least aerotolerant and are therefore easy to grow and study in the laboratory. Finally, most Drosophila gut bacteria are also commensal bacteria in mammals, including humans (Erkosar et al., 2013).
In the Drosophila gut, two parallel immune systems control host-microbe homeostasis: namely, the Imd pathway and the dual oxidase (DUOX) pathway(s) (Ha et al., 2005). Upon binding of bacterial peptidoglycan (Leulier et al., 2003) to the Imd pathway receptor Peptidoglycan recognition protein LC (PGRP-LC) (Choe et al., 2002, Gottar et al., 2002, Ra¨met et al., 2002), the pathway is activated, leading to nuclear translocation of the NF-kB protein Relish. Although Relish is activated and nuclear, antimicrobial peptide expression is actively repressed in the (healthy) gut by several Imd pathway negative regulator molecules, such as Caudal and Pirk (Ryu et al., 2008, Kleino et al., 2008). This repression is needed to protect the beneficial commensal bacterial community. In contrast, the molecular mechanism of which microbial components activate the DUOX-regulatory pathway(s) had not been previously known. In a recent Cell paper, Lee et al. (2013) show that opportunistic pathogens, but
not beneficial commensal bacteria, activate DUOX-dependent gut immunity in Drosophila via a mechanism independent of peptidoglycan recognition. Pathogenic and commensal bacteria were shown to secrete similar amounts of peptidoglycan (and therefore induce the Imd pathway), but only pathogens activate the DUOXdependent pathway, leading to induction of reactive oxygen species (ROS) production. The authors set to identify the bacterial ligand responsible for inducing ROS production. First, they compared the ability of live, lyzed, and formalin-fixed opportunistic pathogen Erwinia carotovora to induce intestinal ROS production. Formalin-fixed dead E. carotovora failed to initiate ROS production, suggesting that the ligand for the DUOX pathway is a molecule that is secreted from the bacteria. Accordingly, culture supernatant of E. carotovora (but not of symbiotic gut bacteria Commensalibacter intestini) enhanced ROS generation in a DUOXdependent manner. To identify the
Cell Host & Microbe 13, May 15, 2013 ª2013 Elsevier Inc. 505
Cell Host & Microbe
Previews molecule(s) responsible for DUOX-activation, the authors used reverse-phase high-performance liquid chromatography (HPLC) to purify E. carotovora culture supernatant and thereafter analyzed the ability of different HPLC-purified fractions to induce in vivo ROS generation in the gut epithelia. In this way, the authors identified a fraction from E. carotovora culture supernatant that caused DUOX activation, whereas a product resulting from the similar purification process of the culture medium of symbiotic C. intestini did not show any activity. Using mass spectrometry and nuclear magnetic resonance, the fraction with high ROS production ability was shown to contain uracil. It was further determined that uracil is secreted only by pathogenic bacteria, since the six pathogens studied secreted significant amounts of uracil, whereas the commensal C. intestini did not. Moreover, pure uracil was able to induce ROS production in the Drosophila gut, whereas a uracil mutant strain of E. carotovora, generated by transposon-mediated random mutagenesis, caused significantly lowered ROS production in the Drosophila gut than did wild-type E. carotovora. It was also tested whether other nucleobases or uracil-related molecules also activate DUOX, and it was shown that bacterial uracil, and not other nucleobases, is the specific pathogenderived ligand that activates the DUOXdependent gut immunity in Drosophila. Epithelial maintenance and homeostasis in the gut is also a question of high importance, since bacterial infection causes damage to epithelial tissue
(Buchon et al., 2009). Bacterial infection accelerates the renewal program of enterocytes by stimulating intestinal stem cells, and Lee et al. (2013) demonstrated that the key molecule for this renewal program is bacterial uracil. The authors compared a wild-type and a uracil mutant E. carotovora line and showed that the wild-type line, but not the mutant, was able to induce an increase in enteroblast numbers. It was also shown that activation of JAK-STAT signaling, essential for enterocyte differentiation, was normal in the wild-type line but impaired in the uracil mutant. However, constitutive exposure to bacterial uracil is harmful to host physiology; long-term feeding of germ-free flies with uracil led to extensive apoptosis of gut cells and, ultimately, lethality. The cause for this pathogenesis in vivo in Drosophila was shown to be the chronic activation of the DUOX-dependent gut immunity and subsequent excess ROS generation (Lee et al., 2013). Immune tolerance to gut-colonizing commensal bacteria is essential for homeostasis between host and microbial cells. On the one hand, excess immune activation is harmful to the host since it kills commensal bacteria and damages the gut epithelium, but on the other hand, an immune response is needed against pathogenic bacteria. The study by Lee et al. (2013) suggests that uracil release could be a defining characteristic of pathogenic versus commensal bacteria, at least in the Drosophila gut. Although the authors demonstrated that uracil activates DUOX-dependent ROS generation also in Caenorhabditis elegans and hu-
506 Cell Host & Microbe 13, May 15, 2013 ª2013 Elsevier Inc.
man cell culture, it remains to be investigated whether similar mechanisms to those found in Drosophila are involved in more complex systems in vivo. In addition, it is still unclear why pathogenic bacteria in the (Drosophila) gut—and perhaps also in other mucosal surfaces—secrete uracil, while commensals do not. REFERENCES Buchon, N., Broderick, N.A., Poidevin, M., Pradervand, S., and Lemaitre, B. (2009). Cell Host Microbe 5, 200–211. Choe, K.M., Werner, T., Sto¨ven, S., Hultmark, D., and Anderson, K.V. (2002). Science 296, 877–886. Erkosar, B., Storelli, G., Defaye, A., and Leulier, F. (2013). Cell Host Microbe 13, 8–14. Gottar, M., Gobert, V., Michel, T., Belvin, M., Duyk, G., Hoffmann, J.A., Ferrandon, D., and Royet, J. (2002). Nature 416, 640–644. Ha, E.M., Oh, C.T., Bae, Y.S., and Lee, W.J. (2005). Science 310, 847–850. Kleino, A., Myllyma¨ki, H., Kallio, J., Vanha-aho, L.M., Oksanen, K., Ulvila, J., Hultmark, D., Valanne, S., and Ra¨met, M. (2008). J. Immunol. 180, 5413– 5422. Lee, K.A., Kim, S.H., Kim, E.K., Ha, E.M., You, H., Kim, B., Kim, M.J., Kwon, Y., Ryu, J.H., and Lee, W.J. (2013). Cell 153. Published online May 15, 2013. http://dx.doi.org/10.1016/j.cell. 2013.04.009. Leulier, F., Parquet, C., Pili-Floury, S., Ryu, J.H., Caroff, M., Lee, W.J., Mengin-Lecreulx, D., and Lemaitre, B. (2003). Nat. Immunol. 4, 478–484. Ra¨met, M., Manfruelli, P., Pearson, A., MatheyPrevot, B., and Ezekowitz, R.A. (2002). Nature 416, 644–648. Ryu, J.H., Kim, S.H., Lee, H.Y., Bai, J.Y., Nam, Y.D., Bae, J.W., Lee, D.G., Shin, S.C., Ha, E.M., and Lee, W.J. (2008). Science 319, 777–782.