Bradyrhizobium as a model system to explore the molecular basis of spontaneous phage production

• Emerson Sherwood, Marine Science, University of Delaware

• Jeffry Fuhrmann, Plant and Soil Sciences, University of Delaware

Bacteriophages, or phages, are viruses that infect bacteria. Phages alter host populations via cell lysis and contribute to bacterial evolution and associated traits via horizontal gene transfer (HGT) within microbial communities. Phages exhibit two different lifestyles, virulent and temperate. Virulent phages infect their host and immediately enter a lytic cycle to produce new virions and lyse the cell to release their progeny. Conversely, temperate phages incorporate their DNA into the host genome and lay dormant indefinitely until a trigger causes the phage to excise from the host chromosome and enter a lytic cycle in a process called induction. Induction can result from external factors such as altered pH, temperature, or other conditions that stress the host cell. However, there is a seldom reported and poorly understood phenomenon called spontaneous phage production (SPP) in which temperate phages are released in the absence of an identifiable external stimulus. There is evidence that SPP can influence traits such as virulence, antibiotic resistance, and pathogenicity via HGT. Our lab maintains the University of Delaware Bradyrhizobium Culture Collection (UDBCC) comprising 353 accessions. Remarkably, approximately 70% of the UDBCC strains examined exhibit high levels of SPP in routine lab culture. Our study used Bradyrhizobium as a model system to explore the basis for SPP.  Bacterial growth and SPP were monitored over a period of ~60 hours for three strains of Bradyrhizobium representing three species. Additionally, samples were collected for subsequent transcriptomic analysis. One strain, USDA 94, exhibited the highest level of SPP, especially 30 to 36 hours post inoculation. Although SPP was observed, we were unable to verify previously reported rates and timings of phage production for the remaining two strains. The findings of this study will be used to select samples for transcriptomic analysis and exploration of the molecular and physiological basis of SPP.