Bismuth drug as antibiotic adjuvant to inhibit biofilm formation via a dual mechanism

Abstract

Burkholderia cepacia (B. cepacia) exhibits intrinsic resistance to many conventional antibiotics. A key factor contributing to this resistance is its ability to form biofilms, which hinder antibiotic penetration and make this bacterial infection very difficult to treat. Developing novel biofilm inhibitors to restore antibiotic efficacy is a promising strategy to combat antimicrobial resistance (AMR). Here, we show the combination of a bismuth drug, e.g., colloidal bismuth subcitrate (CBS), with antibiotics potently inhibits biofilm formation in B. cepacia. Mechanistic studies reveal that CBS promotes the degradation of cyclic diguanylate (c-di-GMP) by inducing accumulation of the biofilm signalling molecule, nitric oxide (NO), driven by bindings of bismuth(III) to cytochrome bo3 ubiquinol oxidase (CyoC) and cytochrome bd-I ubiquinol oxidase subunit 1 (CydA). Furthermore, CBS reduces cAMP level, downregulates crp, (encoding the cAMP receptor protein), and disrupts the formation of cAMP-CRP complex, leading to upregulation of rpoS, which further suppresses biofilm formation. These two bismuth-mediated pathways, i.e., NO-mediated c-di-GMP degradation and cAMP-CRP-regulated rpoS expression, work cooperatively to inhibit biofilm formation. Additionally, bismuth-antibiotic combinations effectively inhibit biofilm formation across a diverse range of bacteria. These findings emphasise the great potential of bismuth drugs as antibiotic adjuvants to tackle antimicrobial resistance.