Pathogen-microbiome interactions and the virulence of an entomopathogenic fungus.

Bacteria shape interactions between hosts and fungal pathogens. In some cases, bacteria associated with fungi are essential for pathogen virulence. In other systems, host-associated microbiomes confer resistance against fungal pathogens. We studied an aphid-specific entomopathogenic fungus called Pandora neoaphidis in the context of both host and pathogen microbiomes. Aphids host several species of heritable bacteria, some of which confer resistance against Pandora. We first found that spores that emerged from aphids that harbored protective bacteria were less virulent against subsequent hosts and did not grow on plate media. We then used 16S amplicon sequencing to study the bacterial microbiome of fungal mycelia and spores during plate culturing and host infection. We found that the bacterial community is remarkably stable in culture despite dramatic changes in pathogen virulence. Last, we used an experimentally transformed symbiont of aphids to show that Pandora can acquire host-associated bacteria during infection. Our results uncover new roles for bacteria in the dynamics of aphid-pathogen interactions and illustrate the importance of the broader microbiological context in studies of fungal pathogenesis. IMPORTANCE: Entomopathogenic fungi play important roles in the population dynamics of many insect species. Understanding the factors shaping entomopathogen virulence is critical for agricultural management and for the use of fungi in pest biocontrol. We show that heritable bacteria in aphids, which confer protection to their hosts against fungal entomopathogens, influence virulence against subsequent hosts. Aphids reproduce asexually and are typically surrounded by genetically identical offspring, and thus these effects likely shape the dynamics of fungal disease in aphid populations. Furthermore, fungal entomopathogens are known to rapidly lose virulence in lab culture, complicating their laboratory use. We show that this phenomenon is not driven by changes in the associated bacterial microbiome. These results contribute to our broader understanding of the aphid model system and shed light on the biology of the Entomophthorales-an important but understudied group of fungi.

Intraspecific variation in immune gene expression and heritable symbiont density.

Host genetic variation plays an important role in the structure and function of heritable microbial communities. Recent studies have shown that insects use immune mechanisms to regulate heritable symbionts. Here we test the hypothesis that variation in symbiont density among hosts is linked to intraspecific differences in the immune response to harboring symbionts. We show that pea aphids (Acyrthosiphon pisum) harboring the bacterial endosymbiont Regiella insecticola (but not all other species of symbionts) downregulate expression of key immune genes. We then functionally link immune expression with symbiont density using RNAi. The pea aphid species complex is comprised of multiple reproductively-isolated host plant-adapted populations. These 'biotypes' have distinct patterns of symbiont infections: for example, aphids from the Trifolium biotype are strongly associated with Regiella. Using RNAseq, we compare patterns of gene expression in response to Regiella in aphid genotypes from multiple biotypes, and we show that Trifolium aphids experience no downregulation of immune gene expression while hosting Regiella and harbor symbionts at lower densities. Using F1 hybrids between two biotypes, we find that symbiont density and immune gene expression are both intermediate in hybrids. We propose that in this system, Regiella symbionts are suppressing aphid immune mechanisms to increase their density, but that some hosts have adapted to prevent immune suppression in order to control symbiont numbers. This work therefore suggests that antagonistic coevolution can play a role in host-microbe interactions even when symbionts are transmitted vertically and provide a clear benefit to their hosts. The specific immune mechanisms that we find are downregulated in the presence of Regiella have been previously shown to combat pathogens in aphids, and thus this work also highlights the immune system's complex dual role in interacting with both beneficial and harmful microbes.

Respiratory-gated noncontrast SPACE MR angiography sequence at 3T for evaluation of the central veins of the chest: a feasibility study.

PURPOSE: To evaluate the feasibility of a respiratory-gated noncontrast magnetic resonance angiography (MRA) sequence for imaging the central veins of the chest. MATERIALS AND METHODS: Eleven healthy subjects underwent MRA of the central veins of the chest with a respiratory-gated noncontrast (SPACE) sequence. Qualitative visualization and signal homogeneity of each central venous segment were scored by two radiologists on a scale of 1-4. Signal-to-noise and contrast-to-noise ratios (SNR and CNR) were also calculated. Retrospective review of our imaging database revealed 13 patients with suspected pathology of the central veins who underwent a clinical MRA examination using the SPACE sequence as well as reference standard central venous imaging with contrast-enhanced MRA or conventional venography. RESULTS: In healthy subjects, all central venous segments demonstrated good to excellent venous visualization and homogeneity scores with the noncontrast SPACE sequence. The mean SNR and CNR of the central venous system were 192.7 and 175.0, respectively. In the 13 clinical examinations, the sensitivity and specificity for detection of stenosis or occlusions was 100% and 100% for reader 1 and 95% and 91% for reader 2, respectively. CONCLUSION: The respiratory-gated noncontrast SPACE sequence provided excellent imaging characteristics of the central veins in healthy subjects with promising diagnostic accuracy in patients with central venous pathology.

Glucose confers protection to Escherichia coli against contact killing by Vibrio cholerae.

Evolutionary arms races are broadly prevalent among organisms including bacteria, which have evolved defensive strategies against various attackers. A common microbial aggression mechanism is the type VI secretion system (T6SS), a contact-dependent bacterial weapon used to deliver toxic effector proteins into adjacent target cells. Sibling cells constitutively express immunity proteins that neutralize effectors. However, less is known about factors that protect non-sibling bacteria from T6SS attacks independently of cognate immunity proteins. In this study, we observe that human Escherichia coli commensal strains sensitive to T6SS attacks from Vibrio cholerae are protected when co-cultured with glucose. We confirm that glucose does not impair V. cholerae T6SS activity. Instead, we find that cells lacking the cAMP receptor protein (CRP), which regulates expression of hundreds of genes in response to glucose, survive significantly better against V. cholerae T6SS attacks even in the absence of glucose. Finally, we show that the glucose-mediated T6SS protection varies with different targets and killers. Our findings highlight the first example of an extracellular small molecule modulating a genetically controlled response for protection against T6SS attacks. This discovery may have major implications for microbial interactions during pathogen-host colonization and survival of bacteria in environmental communities.