Identification and characterization of a haem biosynthesis locus in Veillonella

Haemin/haem is one of the essential nutrients required by periodontopathogens such as Porphyromonas gingivalis to grow in vitro. In the oral cavity, this nutrient is believed to be provided by the crevicular fluid, a serum-like exudate produced during gum inflammation. However, P. gingivalis is also present in the healthy dental biofilm where inflammation is absent. This study was designed to answer the question: what organism(s) in the healthy dental biofilm provides haemin/haem to those periodontal pathogens? We report here that veillonellae, a group of bridging species in dental biofilm development, harbour a complete gene cluster for haem biosynthesis. Haemin production was detected from cell lysate, suggesting that the haem biosynthesis pathway is functional in veillonellae. Using the only transformable strain Veillonella atypica OK5, we inactivated specific key genes in the haem biosynthesis pathway. Inactivation of hemE, encoding the enzyme uroporphyrinogen decarboxylase, not only abolished haemin production but also significantly decreased OK5-supported growth of P. gingivalis. A luciferase gene reporter to the hemEHG operon demonstrated up-regulation of operon expression by P. gingivalis. Analysis of all sequenced genomes of oral bacteria in the HOMD database identified three genera (Veillonella, Propionibacterium and Aggregatibacter) that have a complete haem biosynthesis gene cluster, suggesting that they all could be potential haemin/haem providers in the dental biofilm.