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A Precious Metal Heist

2009; Cell Press; Volume: 5; Issue: 5 Linguagem: Inglês

10.1016/j.chom.2009.05.005

1934-6069

Eric P. Skaar,

Bacterial biofilms and quorum sensing

Nearly all bacterial pathogens require iron to successfully infect their vertebrate hosts. The host molecule lipocalin-2 exploits this by sequestering bacterial siderophores as a mechanism of protection against infection. Raffatellu et al., 2009Raffatellu M. George M.D. Akiyama Y. Hornsby M.J. Nuccio S.-P. Paixao T.A. Butler B.P. Chu H. Santos R.L. Berger T. et al.Cell Host Microbe. 2009; 5 (this issue): 476-486Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar show that Salmonella enterica serotype Typhimurium circumvents this through the production of the modified siderophore salmochelin. Nearly all bacterial pathogens require iron to successfully infect their vertebrate hosts. The host molecule lipocalin-2 exploits this by sequestering bacterial siderophores as a mechanism of protection against infection. Raffatellu et al., 2009Raffatellu M. George M.D. Akiyama Y. Hornsby M.J. Nuccio S.-P. Paixao T.A. Butler B.P. Chu H. Santos R.L. Berger T. et al.Cell Host Microbe. 2009; 5 (this issue): 476-486Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar show that Salmonella enterica serotype Typhimurium circumvents this through the production of the modified siderophore salmochelin. With a few exceptions, all bacterial pathogens require iron in order to colonize and infect their vertebrate hosts. This requirement is based on the obligatory role iron plays in a variety of cellular processes from DNA replication to protection against reactive oxygen species. The bacterial necessity for nutrient metals has been exploited by vertebrates that have evolved elaborate sequestration mechanisms to prevent access to metal in a process termed "nutritional immunity" (Weinberg, 1974Weinberg E.D. Science. 1974; 184: 952-956Crossref PubMed Scopus (347) Google Scholar). This nutrient sequestration has resulted in a molecular arms race in the battle for metal between host and pathogen. The importance of this battle was summarized by Eugene Weinberg, who wrote "In the resolution of the contest between invader and host, iron may be the critical determinant" (Weinberg, 1974Weinberg E.D. Science. 1974; 184: 952-956Crossref PubMed Scopus (347) Google Scholar). Salmonella enterica serotype Typhimurium causes nonsystemic enterocolitis, which represents the second-most frequent cause of food-borne illness in the United States (Mead et al., 1999Mead P.S. Slutsker L. Dietz V. McCaig L.F. Bresee J.S. Shapiro C. Griffin P.M. Tauxe R.V. FEmerg. Infect. Dis. 1999; 5: 607-625Crossref PubMed Scopus (5475) Google Scholar). Upon ingestion and intestinal colonization, S. Typhimurium elaborates two distinct type III secretion systems (T3SS-1 and T3SS-2) that target host cells and translocate a series of effector molecules that manipulate normal cellular responses. A primary result of this bacterial assault is the robust recruitment of neutrophils to the site of infection. The extravasation of neutrophils into the intestinal mucosa is associated with increased vascular permeability and significant damage to the uppermost ileal mucosa, further perpetuating neutrophil transmigration. The net result of this inflammatory response is the rapid movement of fluid from the blood into the intestinal lumen, leading to diarrhea. Host-mediated iron sequestration and absorption is particularly important within the intestinal lumen under both normal and pathological conditions. The large population of normal flora in the healthy gut necessitates the presence of growth control measures to prevent the rapid proliferation of typically harmless commensals. Since dietary iron is an abundant potential nutrient source to intestinal bacteria it must be rapidly sequestered following ingestion. In pathological conditions like enterocolitis, the severe tissue necrosis and osmotic dysregulation of the ileal mucosa results in the local accumulation of host iron-binding proteins such as plasma transferrin, ferritin, and hemoglobin. These iron-containing proteins represent a valuable nutrient source to invading microbes. To compensate for this inflammatory redistribution of iron sources in the intestine, neutrophils and epithelial cells release lipocalin-2, which binds enterobactin-type bacterial siderophores and renders them inaccessible to intestinal pathogens (Flo et al., 2004Flo T.H. Smith K.D. Sato S. Rodriguez D.J. Holmes M.A. Strong R.K. Akira S. Aderem A. Nature. 2004; 432: 917-921Crossref PubMed Scopus (1207) Google Scholar). In this issue of Cell Host & Microbe, Raffatellu et al., 2009Raffatellu M. George M.D. Akiyama Y. Hornsby M.J. Nuccio S.-P. Paixao T.A. Butler B.P. Chu H. Santos R.L. Berger T. et al.Cell Host Microbe. 2009; 5 (this issue): 476-486Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar report that S. Typhimurium is resistant to lipocalin-2, and this resistance is critical to the outcome of enterocolitis. The authors initiated this work by investigating the contribution of IL-17 and IL-22 to the host-pathogen interaction, as these cytokines are produced upon S. Typhimurium colonization and contribute to the recruitment of neutrophils to the site of infection. To identify antimicrobial factors produced upon IL-17 and IL-22 exposure, human colonic cancer epithelial cells were stimulated with these cytokines and microarray analysis was performed. Genes induced by IL-22 exposure in cultured cells were compared for overlap with genes induced in the ileal mucosa of rhesus macaques infected with S. Typhimurium. Although the authors noted significant overlap between these two conditions, they were most interested in genes involved in defense against infection. The group set their focus on the antisiderophore protein lipocalin-2 based on the fact that the gene encoding this protein (LCN2) exhibited a pronounced increase both in vitro after IL-22 stimulation and in vivo during S. Typhimurium infection. Studies into the regulation of lipocalin-2 revealed that, although IL-17 by itself does not induce lipocalin-2 expression, IL-17 does appear to synergize with IL-22 to control production of lipocalin-2 in the intestinal epithelium. A primary bacterial defense against lipocalin-2 involves the production of stealth siderophores. These molecules represent structurally modified enterobactin-type siderophores that are resistant to lipocalin-2 binding (Abergel et al., 2006Abergel R.J. Wilson M.K. Arceneaux J.E. Hoette T.M. Strong R.K. Byers B.R. Raymond K.N. Proc. Natl. Acad. Sci. USA. 2006; 103: 18499-18503Crossref PubMed Scopus (147) Google Scholar, Fischbach et al., 2006Fischbach M.A. Lin H. Zhou L. Yu Y. Abergel R.J. Liu D.R. Raymond K.N. Wanner B.L. Strong R.K. Walsh C.T. et al.Proc. Natl. Acad. Sci. USA. 2006; 103: 16502-16507Crossref PubMed Scopus (215) Google Scholar). In this regard, S. Typhimurium produces salmochelin, a glycoslyated derivative of enterochelin that is not targeted by lipocalin-2 (Baumler et al., 1996Baumler A.J. Tsolis R.M. van der Velden A.W. Stojiljkovic I. Anic S. Heffron F. Gene. 1996; 183: 207-213Crossref PubMed Scopus (133) Google Scholar, Fischbach et al., 2006Fischbach M.A. Lin H. Zhou L. Yu Y. Abergel R.J. Liu D.R. Raymond K.N. Wanner B.L. Strong R.K. Walsh C.T. et al.Proc. Natl. Acad. Sci. USA. 2006; 103: 16502-16507Crossref PubMed Scopus (215) Google Scholar, Hantke et al., 2003Hantke K. Nicholson G. Rabsch W. Winkelmann G. Proc. Natl. Acad. Sci. USA. 2003; 100: 3677-3682Crossref PubMed Scopus (252) Google Scholar). Salmochelin is produced by the iroBCDEiroN gene cluster in S. Typhimurium, and mutants in this system are sensitive to the inhibitory effects of lipocalin-2. In keeping with this, the authors found that S. Typhimurium strains lacking salmochelin (ΔiroBC) displayed reduced growth in cell culture medium in which lipocalin-2 expression had been induced by IL-17 and IL-22 (Raffatellu et al., 2009Raffatellu M. George M.D. Akiyama Y. Hornsby M.J. Nuccio S.-P. Paixao T.A. Butler B.P. Chu H. Santos R.L. Berger T. et al.Cell Host Microbe. 2009; 5 (this issue): 476-486Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar). Presumably this reduced growth is due to the inability of ΔiroBC to combat lipocalin-2-dependent siderophore sequestration. Consistent with this, the growth defect of ΔiroBC in lipocalin-2-containing medium was reversed upon addition of an iron source that is not targeted by lipocalin-2. Previous work has shown that the iroA gene cluster confers virulence to a nonpathogenic strain of Escherichia coli in a lipocalin-2-dependent manner (Fischbach et al., 2006Fischbach M.A. Lin H. Zhou L. Yu Y. Abergel R.J. Liu D.R. Raymond K.N. Wanner B.L. Strong R.K. Walsh C.T. et al.Proc. Natl. Acad. Sci. USA. 2006; 103: 16502-16507Crossref PubMed Scopus (215) Google Scholar). These results suggested that salmochelin-mediated lipocalin-2 evasion is critical to the outcome of S. Typhimurium infection. This supposition was confirmed in the study by Raffatellu et al., 2009Raffatellu M. George M.D. Akiyama Y. Hornsby M.J. Nuccio S.-P. Paixao T.A. Butler B.P. Chu H. Santos R.L. Berger T. et al.Cell Host Microbe. 2009; 5 (this issue): 476-486Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar, which was made possible by the advent of the streptomycin pretreated mouse model of S. Typhimurium infection (Barthel et al., 2003Barthel M. Hapfelmeier S. Quintanilla-Martinez L. Kremer M. Rohde M. Hogardt M. Pfeffer K. Russmann H. Hardt W.D. Infect. Immun. 2003; 71: 2839-2858Crossref PubMed Scopus (630) Google Scholar). Prior to the introduction of this model, studies into the pathogenesis of S. Typhimurium enterocolitis were complicated by the fact that oral infection of mice with S. Typhimurium leads to bacteremia and organ lesions characteristic of typhoidal salmonellosis. The streptomycin pretreated mouse model of S. Typhimurium infection provides a model that more closely resembles human enterocolitis. Although this model is imperfect in that the streptomycin-pretreated mice develop colitis and mouse typhoid in parallel (Barthel et al., 2003Barthel M. Hapfelmeier S. Quintanilla-Martinez L. Kremer M. Rohde M. Hogardt M. Pfeffer K. Russmann H. Hardt W.D. Infect. Immun. 2003; 71: 2839-2858Crossref PubMed Scopus (630) Google Scholar), it allows investigators to genetically manipulate both host and pathogen to identify the factors critical to the pathogenesis of enterocolitis. This latter quality is nicely demonstrated in Raffatellu et al. To demonstrate the in vivo contribution of salmochelin to the struggle for metal between host and pathogen, the authors infected streptomycin pretreated mice with S. Typhimurium (Raffatellu et al., 2009Raffatellu M. George M.D. Akiyama Y. Hornsby M.J. Nuccio S.-P. Paixao T.A. Butler B.P. Chu H. Santos R.L. Berger T. et al.Cell Host Microbe. 2009; 5 (this issue): 476-486Abstract Full Text Full Text PDF PubMed Scopus (352) Google Scholar). These studies revealed that the transcript levels of Lcn2 were markedly increased upon S. Typhimurium infection and salmochelin is required to protect against lipocalin-2-dependent siderophore sequestration. Further, the value of salmochelin to S. Typhimurium is only realized during an active inflammatory response. In the absence of inflammation, salmochelin does not afford a growth advantage to S. Typhimurium, presumably because salmochelin is most valuable in the presence of high quantities of neutrophil derived lipocalin-2 (Figure 1). Resistance to lipocalin-2 is a conserved strategy across multiple pathogenic microbes. The Gram-positive pathogen Bacillus anthracis produces the siderophore petrobactin, which incorporates a 3,4-dihydroxybenzoyl chelating subunit that prevents lipocalin-2 binding and renders this organism resistant to lipocalin-2 (Abergel et al., 2006Abergel R.J. Wilson M.K. Arceneaux J.E. Hoette T.M. Strong R.K. Byers B.R. Raymond K.N. Proc. Natl. Acad. Sci. USA. 2006; 103: 18499-18503Crossref PubMed Scopus (147) Google Scholar). The conservation of lipocalin-2 evasion as a virulence strategy across such diverse pathogens raises the exciting possibility that additional lipocalin-2-resistant siderophores have yet to be identified. This possibility is supported by the different mechanisms by which petrobactin and salmochelin evade lipocalin-2 binding. Structural alteration of bacterial siderophores to evade lipocalin-2 binding appears to be an example of convergent evolution of pathogens to evade a potent vertebrate defense response. Lipocalin-2-resistant siderophores are the most recently identified members of the arsenal in the arms race for nutrient metal. These findings lead one to wonder which components of the host and bacterial arsenals are yet to be discovered. It is possible that vertebrates produce additional antisiderophore molecules that protect against microbial infection. It is also conceivable that vertebrates produce lipocalin-2 analogs to target bacterial secreted proteins known as hemophores, which capture nutrient iron from host hemoproteins (Wandersman and Delepelaire, 2004Wandersman C. Delepelaire P. Annu. Rev. Microbiol. 2004; 58: 611-647Crossref PubMed Scopus (696) Google Scholar). Conversely, bacterial pathogens may directly target lipocalin-2-siderophore complexes as a strategy to regain control of secreted siderophores and access the sequestered iron. Finally, one questions how far the struggle for nutrient metal between host and pathogen extends past iron. The recent identification of neutrophil calprotectin as a molecule that prevents against bacterial infection through the chelation of nutrient manganese and zinc establishes metal chelation as a potent host defense that extends beyond iron alone (Corbin et al., 2008Corbin B.D. Seeley E.H. Raab A. Feldmann J. Miller M.R. Torres V.J. Anderson K.L. Dattilo B.M. Dunman P.M. Gerads R. et al.Science. 2008; 319: 962-965Crossref PubMed Scopus (581) Google Scholar). The work by Raffatellu et al. adds to the ever growing body of evidence supporting the idea that nutrient metal acquisition is vital to bacterial pathogenesis and that targeting bacterial metal acquisition systems has considerable therapeutic potential.