The Gram-negative anaerobe Fusobacterium nucleatum is an opportunistic human pathogen, most frequently associated with periodontal disease through dental biofilm formation and, increasingly, with colorectal cancer development and progression. F. nucleatum infections are routinely treated by broad-spectrum β-lactam antibiotics and metronidazole. However, these antibiotics can negatively impact the normal microflora. Therefore, the development of novel narrow-spectrum antimicrobials active against anaerobic pathogens is of great interest. Here, we examined the antimicrobial Zn ionophore PBT2, an 8-hydroxyquinoline analogue with metal chelating properties, against a single type isolate F. nucleatum ATCC 25586. PBT2-Zn was a potent inhibitor of growth and exhibited synergistic bactericidal (>3-log10 killing) activity at 5× MIC in planktonic cells, and at the MIC in biofilms grown in vitro. Physiological and transcriptional analyses uncovered a strong cellular response relating to Zn and Fe homeostasis in PBT2-Zn treated cells across subinhibitory and inhibitory concentrations. At 1× MIC, PBT2 alone induced a 3.75-fold increase in intracellular Zn, whereas PBT2-Zn challenge induced a 19-fold accumulation of intracellular Zn after 2 h. A corresponding 2.1-fold loss of Fe was observed at 1× MIC. Transcriptional analyses after subinhibitory PBT2-Zn challenge (0.125 μg/mL and 200 μM ZnSO4) revealed significant differential expression of 15 genes at 0.5 h, and 12 genes at 1 h. Upregulated genes included those with roles in Zn homeostasis (e.g., a Zn-transporting ATPase and the Zn-sensing transcriptional regulator, smtB) and hemin transport (hmuTUV) to re-establish Fe homeostasis. A concentration-dependent protective effect was observed for cells pretreated with hemin (50 μg/mL) prior to PBT2-Zn challenge. The data presented here supports our proposal that targeting the disruption of metallostasis by Zn-translocating ionophores is a strategy worth investigating further for the treatment of Gram-negative anaerobic pathogens.
Keywords: Fusobacterium nucleatum; PBT2; anaerobe; biofilm; metal homeostasis; zinc/iron.