AmpI functions as an iron exporter to alleviate {beta}-lactam-mediated ROS stress in Stenotrophomonas maltophilia [Mechanisms of Resistance]

Stenotrophomonas maltophilia is an organism with a remarkable capacity for drug resistance with several antibiotic resistance determinants in its genome. S. maltophilia genome codes for L1 and L2 responsible for intrinsic β-lactam resistance. The Smlt3721 gene (denoted ampI), located downstream of the L2 gene, encodes an inner membrane protein. The existence of L2-ampI operon was verified by RT-PCR. For aerobically grown S. maltophilia KJ, inactivation of ampI downregulated siderophore synthesis and iron acquisition systems and upregulated the iron storage system, as demonstrated by a transcriptome assay, suggesting that AmpI is involved in iron homeostasis. Compared with the wild-type KJ, ampI mutant had an elevated intracellular iron level, as revealed by ICP-MS analysis, and an increased sensitivity to H₂O₂, verifying the role of AmpI as an iron exporter. The β-lactam stress increased the intracellular reactive oxygen species (ROS) level and induced the expression of L1 gene and L2-ampI operon. Compared to its own parental strain, ampI mutant had a growth inferiority in β-lactam-containing media, and the ampI mutant viability was improved after complemented with plasmid pAmpI, either in β-lactamase-positive or in β-lactamase-negative genetically background. Collectively, upon the challenge of β-lactam, the inducibly expressed L1 and L2 β-lactamases contribute to β-lactam resistance by hydrolyzing β-lactam. AmpI functions as an iron exporter participating in rapidly weakening β-lactam-mediated ROS toxicity. The L1 gene and L2-ampI operon enables S. maltophilia to effectively cope with β-lactam-induced stress.

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