The mucosal surfaces of animals are habitat for microbes, including viruses. Bacteriophages—viruses that infect bacteria—were shown to be able to bind to mucus. This may result in a symbiotic relationship in which phages find bacterial hosts to infect, protecting the mucus-producing animal from bacterial infections in the process. Here, we studied phage binding on mucus and the effect of mucin on phage-bacterium interactions. The significance of our research is in showing that phage adhesion to mucus results in preventive protection against bacterial infections, which will serve as basis for the development of prophylactic phage therapy approaches. Besides, we also reveal that exposure to mucus upregulates bacterial virulence and that this is exploited by phages for infection, adding one additional layer to the metazoan-bacterium-phage biological interactions and ecology. This phenomenon might be widespread in the biosphere and thus crucial for understanding mucosal diseases, their outcome and treatment.

Studies on species’ responses to climate change have focused largely on the direct effect of abiotic factors and in particular temperature, neglecting the effects of biotic interactions in determining the outcome of climate change projections. Many microbes rely on strong interference competition; hence the fitness of many pathogenic bacteria could be a function of both their growth properties and intraspecific competition. However, due to technical challenges in distinguishing and tracking individual strains, experimental evidence on intraspecific competition has been limited so far. Here, we developed a robust application of the high-resolution melting (HRM) assay to study head-to-head competition between mixed genotype co-cultures of a waterborne bacterial pathogen of fish, Flavobacterium columnare, at two different temperatures. We found that competition outcome in liquid cultures seemed to be well predicted by growth yield of isolated strains, but was mostly inconsistent with interference competition results measured in inhibition tests on solid agar, especially as no growth inhibition between strain pairs was detected at the higher temperature. These results suggest that, for a given temperature, the factors driving competition outcome differ between liquid and solid environments.

Columnaris disease caused by Flavobacterium columnare is a serious problem in aquaculture, annually causing large economic losses around the world. Despite considerable research, the molecular epidemiology of F. columnare remains poorly understood. We investigated the population structure and spatiotemporal changes in the genetic diversity of F. columnare population in Finland by using a multilocus sequence typing (MLST) and analysis (MLSA) based on DNA sequence variation within six housekeeping genes. A total of 83 strains of F. columnare were collected from eight different areas located across the country between 2003 and 2012. Partial sequencing of six housekeeping genes (trpB, tuf, atpA, rpoD, gyrB and dnaK) revealed eight sequence types and a moderate level of genetic diversity (H = 0.460). Phylogenetic analysis of the concatenated protein-encoding gene sequence data (ca. 3,509 nucleotides) formed two lineages, which could be further divided into five clusters. All analysed F. columnare strains appeared to have a genetic origin distinct from that of another important fish pathogen form the genus Flavobacterium, F. psychrophilum. Although the value of the index of association between alleles, 0.292 (P < 0.001), supports some degree of clonality for this species in Finland, recombination has introduced molecular diversity to the population almost three times more than mutation. The results suggest that Finnish F. columnare strains have an epidemic population structure followed by clonal expansion of successful genotypes. Our study with reproducible methodology and comparable results establishes a robust framework for the discrimination and phylogenetic analysis of F. columnare isolates, which will help to improve our understanding about geographic distribution and epidemiology of columnaris disease.

Predicting the effects of global increase in temperatures on disease virulence is challenging, especially for environmental opportunistic bacteria, because pathogen fitness may be differentially affected by temperature within and outside host environment. So far, there is very little empirical evidence on the connections between optimal temperature range and virulence in environmentally growing pathogens. Here, we explored whether the virulence of an environmentally growing opportunistic fish pathogen, Flavobacterium columnare, is malleable to evolutionary changes via correlated selection on thermal tolerance. To this end, we experimentally quantified the thermal performance curves (TPCs) for maximum biomass yield of 49 F. columnare isolates from eight different geographic locations in Finland over ten years (2003–2012). We also characterized virulence profiles of these strains in a zebra fish (Danio rerio) infection model. We show that virulence among the strains increased over the years, but thermal generalism, and in particular tolerance to higher temperatures, was negatively associated with virulence. Our data suggest that temperature has a strong effect on the pathogen genetic diversity and therefore presumably also on disease dynamics. However, the observed increase in frequency and severity of F. columnare epidemics over the last decade cannot be directly linked to bacterial evolution due to increased mean temperature, but is most likely associated with factors related to increased length of growing season, or other time‐dependent change in environment. Our study demonstrates that complex interactions between the host, the pathogen and the environment influence disease virulence of an environmentally growing opportunistic pathogen.

Viruses are key actors of ecosystems and have major impacts on global biogeochemical cycles. Prophages deserve particular attention as they are ubiquitous in bacterial genomes and can enter a lytic cycle when triggered by environmental conditions. We explored how temperature affects the interactions between prophages and other biological levels using an opportunistic pathogen, the bacteriumSerratia marcescens, which harbours several prophages and that had undergone an evolution experiment under several temperature regimes. We found that the release of one of the prophages was temperature‐sensitive and malleable to evolutionary changes. We further discovered that the virulence of the bacterium in an insect model also evolved and was positively correlated with phage release rates. We determined through analysis of genetic and epigenetic data that changes in the bacterial outer cell wall structure possibly explain this phenomenon. We hypothezise that the temperature‐dependent phage release rate acted as a selection pressure onS. marcescensand that it resulted in modified bacterial virulence in the insect host. Our study system illustrates how viruses can mediate the influence of abiotic environmental changes to other biological levels and thus be involved in ecosystem feedback loops.