A New Perspective of Pseudomonas—Host Interactions: Distribution and Potential Ecological Functions of the Genus Pseudomonas within the Bark Beetle Holobiont

SIMPLE SUMMARY: Microbes play essential roles in the health of animals and plants. Hence, the study of microbe–host interactions is of utmost importance to understand nature. In the present work, we aimed to understand the ecological distribution and functions of the bacterial genus Pseudomonas in bark beetles. These beetles are small insects that live under the bark of trees. Some bark beetle species cause mass attacks in woodlands, decimating tree populations worldwide. Thus, a better understanding of its associated microbes may aid in finding solutions for these forest pests. Our revision summarizes how members of the genus Pseudomonas are ubiquitous in all life stages of different bark beetle species. Moreover, we found that these bacteria may benefit these insects by providing them with nutrients, protecting them from tree chemical defenses and antagonizing entomopathogenic fungi. ABSTRACT: Symbiosis between microbes and insects has been raised as a promising area for understanding biological implications of microbe–host interactions. Among them, the association between fungi and bark beetles has been generally recognized as essential for the bark beetle ecology. However, many works investigating bark beetle bacterial communities and their functions usually meet in a common finding: Pseudomonas is a broadly represented genus within this holobiont and it may provide beneficial roles to its host. Thus, we aimed to review available research on this microbe–host interaction and point out the probable relevance of Pseudomonas strains for these insects, in order to guide future research toward a deeper analysis of the importance of these bacteria for the beetle’s life cycle.