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Many bacteria, including numerous human pathogens, synthesize small molecules known as siderophores to scavenge iron. Enterobactin, a siderophore produced by enteric bacteria, is surprisingly ineffective as an iron-scavenging agent for bacteria growing in animals because of its hydrophobicity and its sequestration by the mammalian protein siderocalin, a component of the innate immune system. However, pathogenic strains of Escherichia coli and Salmonella use enzymes encoded by the iroA gene cluster to tailor enterobactin by glycosylation and linearization. The resulting modified forms of enterobactin, known as salmochelins, can evade siderocalin and are less hydrophobic than enterobactin, restoring this siderophore's iron-scavenging ability in mammals.