A wide variety of bacteria are associated with plant roots and have the capacity to enhance plant growth or improve stress tolerance. Among them, Caulobacter species are thought to play a functional role in the plant microbiome, leading to their classification as a hub species due to their pivotal interactions with plants. Caulobacter is a genus of Gram-negative, aerobic, oligotrophic bacteria that is notably widely distributed in freshwater and soil. 

Bacteriophages, also known as phages, are viruses that infect bacteria. They are found in almost all environments, including soil, fresh, and marine water samples, and most recently, in the free atmosphere. Bacteriophages can have one of four different life cycles: lytic, lysogenic, chronic, and pseudolysogenic. Typically, lytic phages bind to bacterial receptors, inject their DNA or RNA into the bacteria, replicate their genome, package it into protein capsid structures, and then lyse the cell and release new phage particles. In contrast, during a lysogenic infection, the phage DNA genome integrates into the host chromosome and then replicates as part of the bacterial chromosome which is inherited by the resulting daughter cells. This chromosomal integration will continue until stress conditions, such as DNA damage, cause the lysogenic phage genome to excise from the bacterial chromosome and enter the lytic cycle. 

Most well-studied model bacteriophages have a narrow host range infecting only a single species of bacteria, whereas broad-host-range phages can infect multiple species or even bacteria from different genera. Although bacteriophages are abundant and widely distributed, there have been limited investigations into their diversity in natural ecosystems. In their new study titled "The Isolation and Characterization of Novel Caulobacter and Non-Caulobacter Lysogenic Bacteria from Soil and the Discovery of Broad-Host-Range Phages Infecting Multiple Genera", graduate student Tannaz Mohammadi and her mentor Dr. Bert Ely explored how microbial interactions within the rhizosphere influence the diversity and functional roles of bacterial communities. They isolated seven new Caulobacter strains and fourteen new non-Caulobacter strains from a soil sampling site next to Rocky Branch Creek on the USC campus. Their results show that a high percentage of both the Caulobacter and the non-Caulobacter soil strains they sampled are lysogenic bacteria. However, in contrast to the narrow host range prediction, they found that most of the bacteriophages purified from these lysogenic bacteria can infect both Caulobacter and non-Caulobacter strains. Their study enhances our understanding of the relationships between phages and their bacterial hosts in soil ecosystems, with implications for ecological balance and agricultural practices aimed at improving plant health through microbial management strategies.