The Isolation of Streptomyces Bacteriophage Host Range Mutants

Abstract

Bacteriophages are viruses that infect bacteria. They are ubiquitous in nature; found in every environmental niche where there are bacteria. Generally, phages infect a narrow range of hosts due to the specificity of their receptor binding proteins for substrates on the bacterial cell surface. Specificity is also determined by the defense mechanisms of the host and the phage’s ability to bypass them, as well as compatibility with the host replication machinery. The mechanisms behind phage-host specificity are poorly characterized, especially for the individual virus. It has been shown that mutations gained through natural evolution to the receptor binding proteins results in an expanded host range, compared to their wild-type counterparts. Streptomyces is the most extensively studied genus of mycelial Actinobacteria. These bacteria are filamentous, spore forming, and produce a chemically diverse variety of secondary metabolites. There are few phages found to infect this genus, with a bias towards those infecting S. griseus. The relationship between phages and Streptomyces is influenced by the unique characteristics of this genus. The various morphologies of and metabolites released by this host have been shown to combat phage infection. To expand the reservoir of Streptomyces phages, 50 novel phages were isolated on either S. avermitilis, S. coelicolor, or S. venezuelae using direct isolations. Five phages, EnochSoames, Celery, Superstar, Rideau and Wilkos were sequenced and annotated, shown to belong to the BD1, BD2, BD3, BF, and BN clusters, respectively. Sequencing revealed high homology amongst the BD cluster phages, and none with the other clusters. The host range of 24 phages were tested across a variety of Streptomyces species. One phage, EnochSoames, demonstrates a broad host range, infecting all three species tested. All other phages demonstrate a narrow host range and/or a reduction in infection on alternative hosts. This reduction was characterized through isolation of phage Rosita host range mutants, which are representative of individual phages which had mutated during amplification on the native host. Sequencing efforts of Rosita revealed that this phage may have DNA modifications. Overall, this data suggests phages can naturally mutate to infect a broader range of hosts and remain viable. In addition, Streptomyces phages are prevalent in nature and genetically diverse.