Evolutionary Instability of Symbiotic Function in Bradyrhizobium japonicum

Bacterial mutualists are often acquired from the environment by eukaryotic hosts. However, both theory and empirical work suggest that this bacterial lifestyle is evolutionarily unstable. Bacterial evolution outside of the host is predicted to favor traits that promote an independent lifestyle in the environment at a cost to symbiotic function. Consistent with these predictions, environmentally-acquired bacterial mutualists often lose symbiotic function over evolutionary time. Here, we investigate the evolutionary erosion of symbiotic traits in Bradyrhizobium japonicum, a nodulating root symbiont of legumes. Building on a previous published phylogeny we infer loss events of nodulation capability in a natural population of Bradyrhizobium, potentially driven by mutation or deletion of symbiosis loci. Subsequently, we experimentally evolved representative strains from the symbiont population under host-free in vitro conditions to examine potential drivers of these loss events. Among Bradyrhizobium genotypes that evolved significant increases in fitness in vitro, two exhibited reduced symbiotic quality, but no experimentally evolved strain lost nodulation capability or evolved any fixed changes at six sequenced loci. Our results are consistent with trade-offs between symbiotic quality and fitness in a host free environment. However, the drivers of loss-of-nodulation events in natural Bradyrhizobium populations remain unknown

Predation response of Vibrio fischeri biofilms to bacterivorus protists.

Vibrio fischeri proliferates in a sessile, stable community known as a biofilm, which is one alternative survival strategy of its life cycle. Although this survival strategy provides adequate protection from abiotic factors, marine biofilms are still susceptible to grazing by bacteria-consuming protozoa. Subsequently, grazing pressure can be controlled by certain defense mechanisms that confer higher biofilm antipredator fitness. In the present work, we hypothesized that V. fischeri exhibits an antipredator fitness behavior while forming biofilms. Different predators representing commonly found species in aquatic populations were examined, including the flagellates Rhynchomonas nasuta and Neobodo designis (early biofilm feeders) and the ciliate Tetrahymena pyriformis (late biofilm grazer). V. fischeri biofilms included isolates from both seawater and squid hosts (Euprymna and Sepiola species). Our results demonstrate inhibition of predation by biofilms, specifically, isolates from seawater. Additionally, antiprotozoan behavior was observed to be higher in late biofilms, particularly toward the ciliate T. pyriformis; however, inhibitory effects were found to be widespread among all isolates tested. These results provide an alternative explanation for the adaptive advantage and persistence of V. fischeri biofilms and provide an important contribution to the understanding of defensive mechanisms that exist in the out-of-host environment

Comparative analysis of quantitative methodologies for Vibrionaceae biofilms

© 2016, Institute of Microbiology, Academy of Sciences of the Czech Republic, v.v.i. Multiple symbiotic and free-living Vibrio spp. grow as a form of microbial community known as a biofilm. In the laboratory, methods to quantify Vibrio biofilm mass include crystal violet staining, direct colony-forming unit (CFU) counting, dry biofilm cell mass measurement, and observation of development of wrinkled colonies. Another approach for bacterial biofilms also involves the use of tetrazolium (XTT) assays (used widely in studies of fungi) that are an appropriate measure of metabolic activity and vitality of cells within the biofilm matrix. This study systematically tested five techniques, among which the XTT assay and wrinkled colony measurement provided the most reproducible, accurate, and efficient methods for the quantitative estimation of Vibrionaceae biofilms

The epidemiology and evolution of symbionts with mixed-mode Transmission

Vertical and horizontal transmission are terms that describe the transfer of symbionts from parents to offspring and among unrelated hosts, respectively. Many symbionts, including parasites, pathogens, mutualists, and microbiota, use a combination of both strategies, known as mixed-mode transmission (MMT). Here I review what is known about the evolution, ecology, and epidemiology of symbionts with MMT and compare MMT with our expec- tations for single-mode strategies. Symbionts with MMT are common and, in comparison with single-mode symbionts, show many surprising features. MMT combines the best of two worlds with regard to the ecological condi- tions required for persistence and plays a role in the evolution of virulence and genome architecture. Even rare transmission by the minority type of these two transmission modes can make a big difference for the system. This review explores the conceptual issues surrounding the dynamics of mixed- mode symbionts by reviewing literature from the entire range of host and symbiont taxa