Diversification of Vibrio anguillarum Driven by the Bacteriophage CHOED

Marcela León,Daniel Castillo,Constantina Kokkari,Roberto Bastías,Katherine García,Pantelis Katharios

doi:10.3389/fmicb.2019.01396

Abstract

Bacteriophages are an important factor in bacterial evolution. Some reports suggest that lytic bacteriophages can select for resistant mutant strains with reduced virulence. The present study explores the role of the CHOED bacteriophage in the diversification and virulence of its host Vibrio anguillarum. Nine phage-resistant strains were analyzed for their phenotype and different virulence factors, showing alterations in their fitness, motility, biofilm formation, lipopolysaccharide profiles and/or protease activity. Seven of the nine phage-resistant strains showed virulence reduction in a Sparus aurata larvae model. However, this is not generalized since two of the resistant strains show equal virulence compared with the parental strain. The genomic analysis of representative resistant strains displayed that the majority of the mutations are specific for each isolate, affecting genes related to lipopolysaccharide biosynthesis, quorum sensing, motility, toxin and membrane transport. The observed mutations were coherent with the phenotypic and virulence differences observed. These results suggest that the CHOED phage acts as a selective pressure on V. anguillarum, allowing proliferation of resistant strains with different genotypes, phenotypes and degrees of virulence, contributing to bacterial diversification.

Highlights

Vibrio anguillarum is a marine bacterium that causes vibriosis in more than 50 fish species worldwide, including some species of high economic importance
Mutant strains of V. anguillarum that were resistant to infection by the CHOED bacteriophage were selected to evaluate potential changes in their phenotype and virulence
As observed with the bacterial growth, this second analysis showed that the resistant mutants had a diverse range of biochemical profiles, the majority of the tests included in the BIOLOG assay were not altered compared to the parent strain (Figure 1B). 17 out of 94 tests used in the analysis showed differences in at least one of the resistant mutants, representing 18% of all tests

Summary

Introduction

Vibrio anguillarum is a marine bacterium that causes vibriosis in more than 50 fish species worldwide, including some species of high economic importance This bacterium uses several virulence factors to develop the infection, such as flagellum, hemolysins, proteases, iron-uptake systems and outer membrane proteins (OMPs) that participate in osmoregulation between the marine and host environments (Frans et al, 2011). Phages play an important role in Diversification of Phage-Resistant Strains of Vibrio anguillarum the regulation of bacterial populations due to their lytic effect on bacteria (Bouvier and del Giorgio, 2007; Shapiro et al, 2010). They can directly affect bacterial evolution through the processes of transduction and lysogenic conversion (Canchaya et al, 2003). It has been shown that lytic phages can exert a selective pressure over their host, facilitating bacterial diversification (Brockhurst et al, 2005; Middelboe et al, 2009) and even reducing virulence of phage-resistant mutants (León and Bastías, 2015)

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Results

Conclusion