Acinetobacter baumannii is a Gram-negative ‘superbug’ which could cause high-incidence life-threatening infections in hospitals, particularly pneumonia. Polymyxins, a group of lipopeptide antibiotics, are often used as a last resort to treat serous A. baumannii infections. However, little is known on the interplay among A. baumannii, human lung epithelial cells and polymyxins at the molecular level, which has substantially hindered the optimisation of polymyxin use in patients.
In this study, A. baumannii ATCC 19606 was co-cultured with A549 cells (multiplicity of infection 1:100) for 2 h with or without 2 mg/L polymyxin B (PMB). Transcriptional profiling of ATCC 19606 and A549 was conducted with RNAseq and microarray, respectively. Differentially expressed genes were identified with edgeR for the RNAseq and ANOVA for microarray with p<0.01.
Genes encoding siderophores from A. baumannii were decreased under PMB treatment alone and during infection with and without PMB. Correspondingly, the infection alone caused increased nutritional immunity in A549 cells. The iron importer TFR1, iron-storage protein ferritin and iron releasing regulator hepcidin were all upregulated in A549 cells during infection without PMB, indicating an increase of iron chelation. The zinc importer ZIPs and exporter ZnTs were downregulated in infection without PMB, while the metal binding protein metallothioneins were upregulated. The metallothionein genes were also upregulated by PMB itself. Endoplasmic reticulum (ER) stress sensors PERK and CREBH were upregulated, and their downstream genes GADD34, XBP1, IL-8 and hepcidin were also significantly increased in infection without PMB.
This systems approach provides a better understand of host-pathogen-drug interactions. Host responded to the infection by nutritional immunity, metal homeostasis regulation and ER stress responses. Interestingly, A. baumannii decreased iron chelation while the host increased iron sequestration in infection. Specifically, besides directly killing bacteria, PMB upregulated zinc-binding protein metallothioneins. Our results provide key mechanistic insights into optimising polymyxin use in patients.