Hypothesis: a Plastically Produced Phenotype Predicts Host Specialization and Can Precede Subsequent Mutations in Bacteriophage

The Virulence Index: A Metric for Quantitative Analysis of Phage Virulence

Background: One of the main challenges in developing phage therapy and manufacturing phage products is the reliable evaluation of their efficacy, performance, and quality. Since phage virulence is intrinsically difficult to fully capture, researchers have turned to rapid but partially inadequate methods for its evaluation. Materials and Methods: This study demonstrates a standardized quantitative method to assess phage virulence based on three parameters: the virulence index (V_P )-quantifying the virulence of a phage against a host, the local virulence (v_i )-assessing killing potential at given multiplicities of infection (MOIs), and MV₅₀ -the MOI at which the phage achieves 50% of its maximum theoretical virulence. This was shown through comparative analysis of the virulence of phages T4, T5, and T7. Results: Under the conditions tested, phage T7 displayed the highest virulence, followed by phage T4 and, finally, by phage T5. The impact of parameters such as temperature and medium composition on virulence was shown for each phage. The use of the method to evaluate the virulence of combinations of phages-for example, for cocktail formulation-is also shown with phages T5 and T7. Conclusions: The method presented provides a platform for high-throughput quantitative assessment of phage virulence and quality control of phage products. It can also be applied to phage screening, evaluation of phage strains, phage mutants, infection conditions and/or the susceptibility of host strains, and the formulation of phage cocktails.

Large Phenotypic and Genetic Diversity of Prophages Induced from the Fish Pathogen Vibrio anguillarum

Vibrio anguillarum is a marine pathogenic bacterium that causes vibriosis in fish and shellfish. Although prophage-like sequences have been predicted in V. anguillarum strains, many are not characterized, and it is not known if they retain the functional capacity to form infectious particles that can infect and lysogenize other bacterial hosts. In this study, the genome sequences of 28 V. anguillarum strains revealed 55 different prophage-related elements. Chemical and spontaneous induction allowed a collection of 42 phage isolates, which were classified in seven different groups according to a multiplex PCR assay. One shared prophage sequence, p41 (group III), was present in 17 V. anguillarum strains, suggesting that this specific element is very dynamically exchanged among V. anguillarum populations. Interestingly, the host range of genetically identical phages was highly dependent on the strains used for proliferation, indicating that phenotypic properties of phages were partly regulated by the host. Finally, experimental evidence displayed that the induced phage ɸVa_90-11-287_p41 was able to lysogenize V. anguillarum strain Ba35, and subsequently spontaneously become released from the lysogenized cells, demonstrating an efficient transfer of the phage among V. anguillarum strains. Altogether, the results showed large genetic and functional diversity and broad distribution of prophages in V. anguillarum, and demonstrated the potential of prophages as drivers of evolution in V. anguillarum strains.