Isolation and characterization of a novel bacteriophage, ASC10, that lyses Francisella tularensis

2014 Fall.Francisella tularensis is an extremely infectious intracellular bacterium and the etiological agent of tularemia. Inhalation of Francisella tularensis can cause pulmonary tularemia, which has a mortality rate of 35% in the absence of treatment. Studies investigating the biology and molecular pathogenesis of Francisella tularensis have increased in the last few years especially after the U.S. Centers for Disease Control and Prevention (CDC) classified Francisella tularensis as a Category A Select Agent. In this dissertation, the identification and characterization of a novel temperate bacteriophage specific for Francisella species is described. Initial experiments focused on developing a media that would allow optimum growth of Francisella and recovery of bacteriophages. The preferred growth media for Francisella researchers is Mueller Hinton broth supplemented with IsoVitaleX enrichment mix. The research presented in this dissertation included development of a simple and low-cost brain heart infusion and Mueller Hinton base media, designated (BMFC), for enhancing the growth rate of all Francisella strains. BMFC media was compared with brain heart infusion media supplemented with cysteine (BHIc) for growth of Francisella tularensis subspecies novicida U112, Francisella tularensis subspecies holarctica LVS, and Francisella tularensis subspecies tularensis NR-50. Results from these experiments demonstrated that Francisella strains grow more rapidly when inoculated into BMFC media than when grown in BHIc media. A bacteriophage, designated ASC10, was discovered to be active against the majority of Francisella strains including F. tularensis Schu S4. This is the first bacteriophage reported to infect and lyse several Francisella species including F. tularensis subspecies holarctica strain LVS, F. tularensis subspecies tularensis strain Schu S4, F. philomiragia and F. t. novicida. Bacteriophage ASC10 was found to possess an icosahedral head of approximately 114 nm in diameter and a non-contractile tail of 92 nm in length. These measurements place ASC10 into the family of Siphoviridae. Bacteriophage ASC10 was isolated by mitomycin C induction of F. t. novicida NR-575 and plating of the phage lysate on F. t. novicida NR-584. Since these two strains are both mutants of F. t. novicida U112, it was hypothesized that they had genomic alterations that allowed F. t. novicida NR-584 to serve as a host for phage production. NextGen sequencing of the F. t. novicida NR-575 and NR-584 genomes and comparison by alignment revealed that F. t. novicida NR-584 had a 11.3 kb deletion encoding an abortive phage infection protein and a restriction-modification system. The location of the ASC10 prophage was identified within the Francisella novicida U112 genome by searching for homologous phage proteins using BLASTP. Based on significant similarity of clusters of phage-like proteins, it was determined that the ASC10 prophage is located between nucleotides 369,143 and 779,775. This location was chosen since it contained the integrase gene and other proteins associated with phage and was flanked by tRNA-serine loci. In summary, this dissertation describes the optimization of growth media for Francisella and the discovery of a unique phage that lyses Francisella species. The availability of a Francisella-specific bacteriophage is essential to develop rapid, field-deployable diagnostic assays, and would provide another tool in the arsenal for genetic manipulation of the Francisella genome