Characterization of Fusarium circinatum biofilm and its matrix’s environmental response role

Abstract The aggregation of fungal cells embedded in a matrix of extracellular matrix (ECM) results in a biofilm— a microbial community of sessile cells attached to biotic and/or abiotic surfaces. In this study, we demonstrate for the first time that the fatal pine pitch canker agent, Fusarium circinatum , can lead a biofilm-like lifestyle with aggregated hyphal bundles wrapped in ECM. We measured the biofilm ECM of F. circinatum in response to some key environmental factors. Our study suggests that F. circinatum biofilms respond to a changing environment, demonstrated by poor and optimal biofilm development under particular abiotic conditions, including temperature and pH. Further analysis revealed that while planktonic cells produced small amounts of ECM per unit of the biomass, azole-exposed biofilms produced significantly more ECM than non-exposed biofilms. The increased synthesis of ECM in biofilms due to azole exposure explains why F. circinatum biofilms required greater drug dosages (Imazalil: 0.74 mg/L; Tebuconazole: 0.46 mg/L) to kill 50% of biofilm-derived cells than planktonic cells (Imazalil: 0.26 mg/L; Tebuconazole: 0.04 mg/L). Interestingly, azole exposure based on these dosages also led to biofilms that were resistant to DNase, which typically uncouples biofilms by penetrating and degrading biofilm extracellular DNA; we propose that DNases were likely hindered from reaching target cells by the ECM barricade, a phenomenon prevalent in most biofilm-forming pathogens of humans. Therefore, our results show how an important fungal phytopathogen’s sessile (biofilm) lifestyle could form a physical barrier against the surrounding environment.