What type of bacteria predominate in primary endodontic infections of necrotic root canals?

Primary endodontic infections (those in previously untreated, necrotic root canals) are predominantly caused by obligate anaerobic bacteria. The low-oxygen, nutrient-rich environment of the necrotic pulp space selectively favors obligate anaerobes including: Prevotella spp., Porphyromonas spp., Fusobacterium nucleatum, Peptostreptococcus spp., and Treponema spp. These obligate anaerobes are polymicrobial in nature, with the infection typically involving 4–6 species. The composition of the microbiota in primary vs. secondary (post-treatment) infections differs significantly — secondary infections often contain facultative species more resistant to endodontic treatment (e.g., Enterococcus faecalis).