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Faculty & Staff

  • Image of David Hershey

    David Hershey

    Assistant Professor of Bacteriology

    4476 Microbial Sciences Building

Job Listings

notifications_active Research Specialist/Lab Manager

Start and Promotion Dates

  • Assistant Professor: 2021


B.S., Biochemistry, Iowa State University 2010
Ph.D., Microbiology, University of California – Berkeley 2016
Postdoctoral Research: Department of Biochemistry & Molecular Biology, University of Chicago

Areas of Study

Mechanisms of surface colonization in bacteria
Assembly of complex polysaccharides
Regulation of motility behaviors

Research Overview

The Hershey laboratory seeks to uncover fundamental principles of colonization. Bacteria can grow in association with solid substrates that range in complexity from simple abiotic materials to specific human tissues. We use the freshwater bacterium Caulobacter crescentus as a model to understand how bacteria interact with surfaces. We apply genetic, biochemical, cell biological and structural approaches to investigate:

(i) how cells sense mechanical, chemical and social cues in their environments 
(ii) how diverse environmental stimuli are integrated to control surface-behaviors 
(iii) how the cell surface is modified to promote favorable interactions with target substrates

Transitioning to a surface-associated lifestyle requires precise developmental programs that fundamentally restructure the cell and its physiology. Numerous internal and external cues influence the colonization sequence, and elaborate signaling networks process these diverse stimuli to orchestrate surface adaptation. We aim to illuminate how complex sets of environmental stimuli are integrated to coordinate surface-behaviors and how surface adaptation is actuated across a range of spatial and temporal scales.

Research Papers

  • Urtecho G, Campbell DE, Hershey DM, Hussain FA, Whitaker RJ, O'Toole GA (2020) Discovering the Molecular Determinants of Phaeobacter inhibens Susceptibility to Phaeobacter Phage MD18. mSphere 5((6)): PMC7643831 · Pubmed · DOI

    Bacteriophages have immense potential as antibiotic therapies and in genetic engineering. Understanding the mechanisms that bacteriophages implement to infect their hosts will allow researchers to manipulate these systems and adapt them to specific bacterial targets. In this study, we isolated a bacteriophage capable of infecting the marine alphaproteobacterium and determined its mechanism of infection, a novel species of bacteriophage isolated in Woods Hole, MA, exhibits potent lytic ability against and appears to be of the morphotype. The genomic sequence of MD18 displayed significant similarity to another siphophage, the recently discovered Roseobacter phage DSS3P8, but genomic and phylogenetic analyses, assessing host range and a search of available metagenomes are all consistent with the conclusion that Phaeobacter phage MD18 is a novel lytic phage. We incubated MD18 with a library of barcoded transposon insertion mutants and identified 22 genes that appear to be required for phage predation of this host. Network analysis of these genes using genomic position, Gene Ontology (GO) term enrichment, and protein associations revealed that these genes are enriched for roles in assembly of a type IV pilus (T4P) and regulators of cellular morphology. Our results suggest that T4P serve as receptors for a novel marine virus that targets Bacteriophages are useful nonantibiotic therapeutics for bacterial infections as well as threats to industries utilizing bacterial agents. This study identified , a phage antagonist of , a bacterium with promising use as a probiotic for aquatic farming industries. Genomic analysis suggested that Phaeobacter phage MD18 has evolved to enhance its replication in by adopting favorable tRNA genes as well as through genomic sequence adaptation to resemble host codon usage. Lastly, a high-throughput analysis of transposon insertion mutants identified genes that modulate host susceptibility to phage MD18 and implicated the type IV pilus as the likely receptor recognized for adsorption. This study marks the first characterization of the relationship between and an environmentally sampled phage, which informs our understanding of natural threats to the bacterium and may promote the development of novel phage technologies for genetic manipulation of this host.

  • Hershey DM, Porfírio S, Black I, Jaehrig B, Heiss C, Azadi P, Fiebig A, Crosson S (2019) Composition of the Holdfast Polysaccharide from . Journal of bacteriology 201((17)): PMC6689307 · Pubmed · DOI

    Surface colonization is central to the lifestyles of many bacteria. Exploiting surface niches requires sophisticated systems for sensing and attaching to solid materials. synthesizes a polysaccharide-based adhesin known as the holdfast at one of its cell poles, which enables tight attachment to exogenous surfaces. The genes required for holdfast biosynthesis have been analyzed in detail, but difficulties in isolating analytical quantities of the adhesin have limited efforts to characterize its chemical structure. In this report, we describe a method to extract the holdfast from cultures and present a survey of its carbohydrate content. Glucose, 3--methylglucose, mannose, -acetylglucosamine, and xylose were detected in our extracts. Our results provide evidence that the holdfast contains a 1,4-linked backbone of glucose, mannose, -acetylglucosamine, and xylose that is decorated with branches at the C-6 positions of glucose and mannose. By defining the monosaccharide components in the polysaccharide, our work establishes a framework for characterizing enzymes in the holdfast pathway and provides a broader understanding of how polysaccharide adhesins are built. To colonize solid substrates, bacteria often deploy dedicated adhesins that facilitate attachment to surfaces. initiates surface colonization by secreting a carbohydrate-based adhesin called the holdfast. Because little is known about the chemical makeup of the holdfast, the pathway for its biosynthesis and the physical basis for its unique adhesive properties are poorly understood. This study outlines a method to extract the holdfast and describes the monosaccharide components contained within the adhesive matrix. The composition analysis adds to our understanding of the chemical basis for holdfast attachment and provides missing information needed to characterize enzymes in the biosynthetic pathway.

  • Hershey DM, Fiebig A, Crosson S (2019) A Genome-Wide Analysis of Adhesion in Identifies New Regulatory and Biosynthetic Components for Holdfast Assembly. mBio 10((1)): PMC6372794 · Pubmed · DOI

    Due to their intimate physical interactions with the environment, surface polysaccharides are critical determinants of fitness for bacteria. produces a specialized structure at one of its cell poles called the holdfast that enables attachment to surfaces. Previous studies have shown that the holdfast is composed of carbohydrate-based material and identified a number of genes required for holdfast development. However, incomplete information about its chemical structure, biosynthetic genes, and regulatory principles has limited progress in understanding the mechanism of holdfast synthesis. We leveraged the adhesive properties of the holdfast to perform a saturating screen for genes affecting attachment to cheesecloth over a multiday time course. Using similarities in the temporal profiles of mutants in a transposon library, we defined discrete clusters of genes with related effects on cheesecloth colonization. Holdfast synthesis, flagellar motility, type IV pilus assembly, and smooth lipopolysaccharide (SLPS) production represented key classes of adhesion determinants. Examining these clusters in detail allowed us to predict and experimentally define the functions of multiple uncharacterized genes in both the holdfast and SLPS pathways. In addition, we showed that the pilus and the flagellum control holdfast synthesis separately by modulating the holdfast inhibitor This report defines a set of genes contributing to adhesion that includes newly discovered genes required for holdfast biosynthesis and attachment. Our data provide evidence that the holdfast contains a complex polysaccharide with at least four monosaccharides in the repeating unit and underscore the central role of cell polarity in mediating attachment of to surfaces. Bacteria routinely encounter biotic and abiotic materials in their surrounding environments, and they often enlist specific behavioral programs to colonize these materials. Adhesion is an early step in colonizing a surface. produces a structure called the holdfast which allows this organism to attach to and colonize surfaces. To understand how the holdfast is produced, we performed a genome-wide search for genes that contribute to adhesion by selecting for mutants that could not attach to cheesecloth. We discovered complex interactions between genes that mediate surface contact and genes that contribute to holdfast development. Our genetic selection identified what likely represents a comprehensive set of genes required to generate a holdfast, laying the groundwork for a detailed characterization of the enzymes that build this specialized adhesin.

  • Hershey DM, Browne PJ, Iavarone AT, Teyra J, Lee EH, Sidhu SS, Komeili A (2016) Magnetite Biomineralization in Magnetospirillum magneticum Is Regulated by a Switch-like Behavior in the HtrA Protease MamE. The Journal of biological chemistry 291((34)):17941-52 PMC5016182 · Pubmed · DOI

    Magnetotactic bacteria are aquatic organisms that produce subcellular magnetic particles in order to orient in the earth's geomagnetic field. MamE, a predicted HtrA protease required to produce magnetite crystals in the magnetotactic bacterium Magnetospirillum magneticum AMB-1, was recently shown to promote the proteolytic processing of itself and two other biomineralization factors in vivo Here, we have analyzed the in vivo processing patterns of three proteolytic targets and used this information to reconstitute proteolysis with a purified form of MamE. MamE cleaves a custom peptide substrate with positive cooperativity, and its autoproteolysis can be stimulated with exogenous substrates or peptides that bind to either of its PDZ domains. A misregulated form of the protease that circumvents specific genetic requirements for proteolysis causes biomineralization defects, showing that proper regulation of its activity is required during magnetite biosynthesis in vivo Our results represent the first reconstitution of the proteolytic activity of MamE and show that its behavior is consistent with the previously proposed checkpoint model for biomineralization.

  • Hershey DM, Ren X, Melnyk RA, Browne PJ, Ozyamak E, Jones SR, Chang MC, Hurley JH, Komeili A (2016) MamO Is a Repurposed Serine Protease that Promotes Magnetite Biomineralization through Direct Transition Metal Binding in Magnetotactic Bacteria. PLoS biology 14((3)):e1002402 PMC4794232 · Pubmed · DOI

    Many living organisms transform inorganic atoms into highly ordered crystalline materials. An elegant example of such biomineralization processes is the production of nano-scale magnetic crystals in magnetotactic bacteria. Previous studies implicated the involvement of two putative serine proteases, MamE and MamO, during the early stages of magnetite formation in Magnetospirillum magneticum AMB-1. Here, using genetic analysis and X-ray crystallography, we show that MamO has a degenerate active site, rendering it incapable of protease activity. Instead, MamO promotes magnetosome formation through two genetically distinct, noncatalytic activities: activation of MamE-dependent proteolysis of biomineralization factors and direct binding to transition metal ions. By solving the structure of the protease domain bound to a metal ion, we identify a surface-exposed di-histidine motif in MamO that contributes to metal binding and show that it is required to initiate biomineralization in vivo. Finally, we find that pseudoproteases are widespread in magnetotactic bacteria and that they have evolved independently in three separate taxa. Our results highlight the versatility of protein scaffolds in accommodating new biochemical activities and provide unprecedented insight into the earliest stages of biomineralization.

  • Lu X, Hershey DM, Wang L, Bogdanove AJ, Peters RJ (2014) An ent-kaurene-derived diterpenoid virulence factor from Xanthomonas oryzae pv. oryzicola. The New phytologist 206((1)):295-302 · Pubmed · DOI

    Both plants and fungi produce ent-kaurene as a precursor to the gibberellin plant hormones. A number of rhizobia contain functionally conserved, sequentially acting ent-copalyl diphosphate and ent-kaurene synthases (CPS and KS, respectively), which are found within a well-conserved operon that may lead to the production of gibberellins. Intriguingly, the rice bacterial leaf streak pathogen Xanthomonas oryzae pv. oryzicola (Xoc) contains a homologous operon. Here, we report biochemical characterization of the encoded CPS and KS, and the impact of insertional mutagenesis on virulence and the plant defense response for these genes, as well as that for one of the cytochromes P450 (CYP112) found in the operon. Activity of the CPS and KS found in this phytopathogen was verified - that is, Xoc is capable of producing ent-kaurene. Moreover, knocking out CPS, KS or CYP112 led to mutant Xoc that exhibited reduced virulence. Investigation of the effect on marker gene transcript levels suggests that the Xoc diterpenoid affects the plant defense response, most directly that mediated by jasmonic acid (JA). Xoc produces an ent-kaurene-derived diterpenoid as a virulence factor, potentially a gibberellin phytohormone, which is antagonistic to JA, consistent with the recent recognition of opposing effects for these phytohormones on the microbial defense response.

  • Vos SM, Lyubimov AY, Hershey DM, Schoeffler AJ, Sengupta S, Nagaraja V, Berger JM (2014) Direct control of type IIA topoisomerase activity by a chromosomally encoded regulatory protein. Genes & development 28((13)):1485-97 PMC4083091 · Pubmed · DOI

    Precise control of supercoiling homeostasis is critical to DNA-dependent processes such as gene expression, replication, and damage response. Topoisomerases are central regulators of DNA supercoiling commonly thought to act independently in the recognition and modulation of chromosome superstructure; however, recent evidence has indicated that cells tightly regulate topoisomerase activity to support chromosome dynamics, transcriptional response, and replicative events. How topoisomerase control is executed and linked to the internal status of a cell is poorly understood. To investigate these connections, we determined the structure of Escherichia coli gyrase, a type IIA topoisomerase bound to YacG, a recently identified chromosomally encoded inhibitor protein. Phylogenetic analyses indicate that YacG is frequently associated with coenzyme A (CoA) production enzymes, linking the protein to metabolism and stress. The structure, along with supporting solution studies, shows that YacG represses gyrase by sterically occluding the principal DNA-binding site of the enzyme. Unexpectedly, YacG acts by both engaging two spatially segregated regions associated with small-molecule inhibitor interactions (fluoroquinolone antibiotics and the newly reported antagonist GSK299423) and remodeling the gyrase holoenzyme into an inactive, ATP-trapped configuration. This study establishes a new mechanism for the protein-based control of topoisomerases, an approach that may be used to alter supercoiling levels for responding to changes in cellular state.

  • Hershey DM, Lu X, Zi J, Peters RJ (2013) Functional conservation of the capacity for ent-kaurene biosynthesis and an associated operon in certain rhizobia. Journal of bacteriology 196((1)):100-6 PMC3911121 · Pubmed · DOI

    Bacterial interactions with plants are accompanied by complex signal exchange processes. Previously, the nitrogen-fixing symbiotic (rhizo)bacterium Bradyrhizobium japonicum was found to carry adjacent genes encoding two sequentially acting diterpene cyclases that together transform geranylgeranyl diphosphate to ent-kaurene, the olefin precursor to the gibberellin plant hormones. Species from the three other major genera of rhizobia were found to have homologous terpene synthase genes. Cloning and functional characterization of a representative set of these enzymes confirmed the capacity of each genus to produce ent-kaurene. Moreover, comparison of their genomic context revealed that these diterpene synthases are found in a conserved operon which includes an adjacent isoprenyl diphosphate synthase, shown here to produce the geranylgeranyl diphosphate precursor, providing a critical link to central metabolism. In addition, the rest of the operon consists of enzymatic genes that presumably lead to a more elaborated diterpenoid, although the production of gibberellins was not observed. Nevertheless, it has previously been shown that the operon is selectively expressed during nodulation, and the scattered distribution of the operon via independent horizontal gene transfer within the symbiotic plasmid or genomic island shown here suggests that such diterpenoid production may modulate the interaction of these particular symbionts with their host plants.

  • Morrone D, Lowry L, Determan MK, Hershey DM, Xu M, Peters RJ (2009) Increasing diterpene yield with a modular metabolic engineering system in E. coli: comparison of MEV and MEP isoprenoid precursor pathway engineering. Applied microbiology and biotechnology 85((6)):1893-906 PMC2811251 · Pubmed · DOI

    Engineering biosynthetic pathways in heterologous microbial host organisms offers an elegant approach to pathway elucidation via the incorporation of putative biosynthetic enzymes and characterization of resulting novel metabolites. Our previous work in Escherichia coli demonstrated the feasibility of a facile modular approach to engineering the production of labdane-related diterpene (20 carbon) natural products. However, yield was limited (<0.1 mg/L), presumably due to reliance on endogenous production of the isoprenoid precursors dimethylallyl diphosphate and isopentenyl diphosphate. Here, we report incorporation of either a heterologous mevalonate pathway (MEV) or enhancement of the endogenous methyl erythritol phosphate pathway (MEP) with our modular metabolic engineering system. With MEP pathway enhancement, it was found that pyruvate supplementation of rich media and simultaneous overexpression of three genes (idi, dxs, and dxr) resulted in the greatest increase in diterpene yield, indicating distributed metabolic control within this pathway. Incorporation of a heterologous MEV pathway in bioreactor grown cultures resulted in significantly higher yields than MEP pathway enhancement. We have established suitable growth conditions for diterpene production levels ranging from 10 to >100 mg/L of E. coli culture. These amounts are sufficient for nuclear magnetic resonance analyses, enabling characterization of enzymatic products and hence, pathway elucidation. Furthermore, these results represent an up to >1,000-fold improvement in diterpene production from our facile, modular platform, with MEP pathway enhancement offering a cost effective alternative with reasonable yield. Finally, we reiterate here that this modular approach is expandable and should be easily adaptable to the production of any terpenoid natural product.