Upcoming PMB Seminars

For a schedule of all Plant & Microbial Biology events, seminars, and lectures visit our calendar.

Wed, 04/30/2025 - 12:15
Wed, 04/30/2025 - 12:15

Unraveling mechanisms behind Salmonella persistence in a mouse model
Wednesday April 30, 2025

Speaker: Denise M. Monack, Professor, Stanford University School of Medicine

Salmonella is a remarkably adaptable enteric pathogen capable of persisting in hosts for extended periods and spreading stealthily to new ones. Our work dissects the intricate interplay among the immune system, the gut microbiota, and Salmonella using a mouse model of chronic infection. We have identified a unique subpopulation of “superspreader” mice that harbor high pathogen loads, remain asymptomatic, and efficiently transmit Salmonella to naïve hosts. Through careful microbiome analysis, we show that Bacteroides spp. exert powerful colonization resistance by producing the short-chain fatty acid propionate, thereby limiting Salmonella expansion. Intriguingly, these superspreader mice exhibit an abundance of regulatory T cells in the colon that express type 1 inflammatory genes and the transcription factor T-bet. These specialized Tregs are essential for suppressing cytotoxic CD4+ T cells, enabling the pathogen to persist in asymptomatic hosts. Finally, we reveal that Salmonella relies on an alpha-N-arabinofuranosidase to harness dietary L-arabinose—a critical metabolic step that drives the emergence of superspreaders. Together, these findings illuminate how Salmonella exploits both the microbiome and immune-mediated disease tolerance to establish chronic infection, offering new insights into pathogen persistence and opportunities for targeted therapeutic intervention.

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Past PMB Seminars

Wed, 04/23/2025 - 12:15
Wed, 04/23/2025 - 12:15

Breaking down non-host resistance to better understand the plant immunity system

Wednesday April 23, 2025

Speaker: Shunyuan Xiao, Professor, University of Maryland

Plants have evolved effective immune mechanisms to fight off various (potential) pathogens. The complete resistance of all genotypes of a plant species against any non-adapted pathogens is defined as non-host resistance (NHR). A prime example of NHR is rice's immunity against all rust and powdery mildew fungi. Because NHR is robust and durable, its potential application in improving resistance against adapted pathogens in crops has long been envisioned. However, this potential has not been explored because the molecular basis of NHR remains poorly characterized due to its high genetic complexity and lack of intraspecific genetic variation. To genetically dissect NHR, a combinatory approach that includes stepwise forward and reverse genetic screens has been taken to break down NHR of Arabidopsis against non-adapted and evolutionarily distant powdery mildew species. Our recent efforts have led to the identification of several novel immunity genes that contribute to NHR at both penetration and post-penetration stages. A higher-order Arabidopsis mutant fully susceptible to barley and powdery mildew has been obtained and used for screening mutants susceptible to rust pathogens. Similar research is being conducted in rice and wheat through collaborations. A complete dissection of NHR may improve our understanding of the multi-layered plant immune system and inspire novel strategies to boost crop resistance against adapted pathogens.

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Wed, 04/16/2025 - 12:15
Wed, 04/16/2025 - 12:15

Genes on the move: Measuring the rate and evolutionary impact of horizontal gene transfer in simple bacterial communities

Wednesday April 16, 2025

Speaker: Ben Kerr, Professor, University of Washington

The passage of genes from parents to offspring is a fundamental rule of heredity. However, bacteria violate this rule of strict vertical inheritance by shuttling DNA between unrelated cells through horizontal gene transfer (HGT). Common vehicles for HGT are conjugative plasmids, extrachromosomal pieces of DNA encoding the machinery for their own transfer. In addition to standard vertical transmission, genes on such plasmids can move between different strains or even different species of bacteria. This partial “uncoupling” of the evolutionary trajectory of plasmid-borne genes from the evolution of their host has interesting consequences for the ecology and evolution of mobile genes, which will be the focus of this presentation. First, I will discuss some challenges in measuring the rate of transfer, and I will present a novel estimate derived from a branching process framework and implemented through an adjustment of the classic fluctuation analysis of Luria and Delbrück. In particular, I will discuss how this new approach opens up unbiased estimates of the rate of HGT between bacterial species. Second, I will discuss the impact of cross-species HGT on protein evolution, focusing on a gene encoding an enzyme that degrades a ubiquitous class of antibiotics. In the process, we will revisit Sewall Wright’s fitness landscape metaphor. The degree of topographical alignment of landscapes for a mobile gene across different bacterial hosts can influence the nature of genetic evolution in the presence of HGT. Furthermore, such alignment affects the potential for adaptation to be “crowdsourced” to different members of a microbial community. This work offers perspectives on the spread of antibiotic resistance within bacterial communities as well as why certain genes may embrace a “mobile lifestyle” via HGT.

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Wed, 04/09/2025 - 12:15
Wed, 04/09/2025 - 12:15

Eco-evolutionary processes of assembly in wild microbial communities

Wednesday April 09, 2025

Speaker: Jennifer B.H. Martiny, Professor, University of California Irvine

The assembly of microbial communities is likely influenced by both ecological and evolutionary processes. At one end of the spectrum, the composition of microbial communities (assessed at broad taxonomic levels) dramatically shifts in response to environmental changes. At the other end, laboratory studies demonstrate the potential for rapid evolution in response to the similar environmental changes. In between these two extremes, closely related microbial strains coexist in natural communities, and this standing variation results in allele frequency shifts within microbial taxa. New advances in microbial population genomics allow us to investigate these blurry eco-evo boundaries. I will present our recent approaches to investigate ecological and evolutionary processes simultaneously in soil microbial communities.

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