Klebsiella pneumoniae Yersiniabactin Promotes Respiratory Tract Infection through Evasion of Lipocalin 2

Klebsiella pneumoniae is a pathogen of increasing concern because of multidrug resistance, especially due to K. pneumoniae carbapenemases (KPCs). K. pneumoniae must acquire iron to replicate, and it utilizes iron-scavenging siderophores, such as enterobactin (Ent). The innate immune protein lipocalin 2 (Lcn2) is able to specifically bind Ent and disrupt iron acquisition. To determine whether K. pneumoniae must produce Lcn2-resistant siderophores to cause disease, we examined siderophore production by clinical isolates (n = 129) from respiratory, urine, blood, and stool samples and by defined siderophore mutants through genotyping and liquid chromatography-mass spectrometry. Three categories of K. pneumoniae isolates were identified: enterobactin positive (Ent(+)) (81%), enterobactin and yersiniabactin positive (Ent(+) Ybt(+)) (17%), and enterobactin and salmochelin (glycosylated Ent) positive (Ent(+) gly-Ent(+)) with or without Ybt (2%). Ent(+) Ybt(+) strains were significantly overrepresented among respiratory tract isolates (P = 0.0068) and β-lactam-resistant isolates (P = 0.0019), including the epidemic KPC-producing clone multilocus sequence type 258 (ST258). In ex vivo growth assays, gly-Ent but not Ybt allowed evasion of Lcn2 in human serum, whereas siderophores were dispensable for growth in human urine. In a murine pneumonia model, an Ent(+) strain was an opportunistic pathogen that was completely inhibited by Lcn2 but caused severe, disseminated disease in Lcn2(-/-) mice. In contrast, an Ent(+) Ybt(+) strain was a frank respiratory pathogen, causing pneumonia despite Lcn2. However, Lcn2 retained partial protection against disseminated disease. In summary, Ybt is a virulence factor that is prevalent among KPC-producing K. pneumoniae isolates and promotes respiratory tract infections through evasion of Lcn2.