Endofungal bacteria: Emerging paradigms and future directions

The colonization of fungal hyphae and spores by bacteria represents a widespread phenomenon with significant ecological and biotechnological implications across all surveyed fungal phyla. First thought to be restricted to Mollicutes and Burkholderiaceae-Related Endobacteria, these endofungal associations exhibit remarkable diversity, from simple uniform populations to complex communities, contradicting earlier assumptions of uniform populations. Acquisition dynamics demonstrate both ancient co-evolutionary relationships and recent horizontal transfer events, with environmental factors driving strain-level variation in symbiont presence even within the same fungal species. Fungi can harbor either uniform or diverse bacterial communities, sometimes within specialized structures, and exhibit varying degrees of dependence on their symbionts. These interactions can be mutualistic, commensal, or parasitic, influencing fungal physiology, metabolism, and ecological function. Yet the underlying mechanisms have been thoroughly characterized in only a few model systems in which endofungal bacteria have been shown to affect nutrient acquisition, stress tolerance, secondary metabolite production, and even fungal pathogenicity. In return, fungi offer a protective niche and promote dispersion. These concepts collectively illustrate the evolutionary flexibility and ecological importance of fungi-bacteria partnerships across terrestrial ecosystems. This review synthesizes emerging paradigms in endofungal bacteria research, integrating recent discoveries that challenge traditional assumptions about these symbioses. We examine host specificity patterns, acquisition mechanisms, and functional impacts while identifying critical knowledge gaps requiring investigation. A deeper understanding of these associations is essential to establish standardized frameworks for their applications in agriculture, medicine, and environmental sustainability.