Protoporphyrin IX derived from dual-species anaerobic biofilms of Fusobacterium necrophorum and Porphyromonas levii attenuates bovine neutrophil function

Abstract

Host immune cells and clinical interventions often fail to eradicate biofilm-mediated infections, resulting in chronic inflammation. The role of the biofilm three-dimensional structure in this tolerant phenotype has been studied extensively; however, the impact of small molecules released from biofilm-bacteria in modulating host immune function is less well understood. A model of mixed-species biofilms composed of Fusobacterium necrophorum and Porphyromonas levii was developed to evaluate bovine neutrophil responses to bioactive molecules released from either biofilm or planktonic bacteria. We hypothesized that different soluble extracellular factors (ECFs) would be released from planktonic and biofilm bacteria, resulting in altered neutrophil function. Neutrophils exposed to ECFs from planktonic bacteria showed significantly elevated levels of reactive oxygen species (ROS). In contrast, biofilm components from these same species of bacteria failed to induce such a response. Size-exclusion filtration of ECFs revealed that the bioactive molecule causing neutrophil ROS responses was below 3 kDa. Intensive heat, nuclease, lipase, or protease treatments of the <3 kDa fractions did not alter neutrophil functional responses. Protoporphyrin IX (PPIX) is an important heme precursor and growth requirement for many anaerobes. Porphyromonas species can accumulate environmental PPIX at the cell surface as a strategy to protect the bacteria from oxidative stress and we investigated the direct interaction of bovine neutrophils with PPIX. In the present study, evidence suggests that the accumulation of protoporphyrin in these dual-species biofilm ECFs attenuates neutrophil ROS production and chemotaxis. The diminished neutrophil response to biofilm ECFs via the action of PPIX may represent a biofilm immune-evasion strategy that could assist in explaining the ineffectual host clearance of biofilm-mediated infections involving these bacteria.