Biology subjects

Dahlstrom, K. M.

Publications and source records attributed to Dahlstrom, K. M..

4 recordsLinked to original sources

bioRxiv2026

A bacterial chaperone-usher pathway links interkingdom association to antimicrobial protection of a fungal partner

Antimicrobial compounds (AMCs) influence species-level composition of microbial communities, governing beneficial and pathogenic interactions with plant and mammalian hosts. Phenazine-1-carboxylic acid (PCA) is a bacterially produced, redox-active AMC abundant in the rhizosphere. A recent study discovered the PCA-susceptible fungus Aspergillus calidoustus can grow in PCA-abundant environments by associating with phenazine-tolerant bacteria, including the novel species Paraburkholderia edwinii. P. edwinii's protection response is characterized by adherence to its fungal partner, bacterial aggregation, and sequestration and reduction of PCA, rendering PCA less toxic. Deletion of the bacterial transcriptional repressor HrcA generates a constitutively active protection response and enhanced protection of A. calidoustus. Here, we use the {Delta}hrcA strain to determine whether fungal protection uses bacterial self-protection machinery or dedicated interkingdom mechanisms. We identified a divergent archaic chaperone-usher pathway (CUP) operon encoding a predicted periplasmic chaperone, outer membrane usher, putative major pilin-like subunit, and putative adhesin-like tip protein. While many archaic CUP operons are annotated in diverse bacteria, their functions remain largely unexplored. We show these fungal attachment complex ( fac ) genes promote bacterial-fungal attachment, are required for P. edwinii to sequester and detoxify PCA, and contribute to full fungal protection from PCA stress, despite not being required for P. edwinii's own tolerance of PCA. We further identified the transcription regulator, FacR, that influences expression of the fac system. Understanding the functions of divergent archaic CUP systems may illuminate protective interkingdom partnerships and inform how pathogenic and beneficial microbes gain entry to AMC-replete environments, shaping microbial communities in agricultural and clinical contexts.

microbiology↗

bioRxiv2025

Interrogating the genus Yersinia to define the rules of lipopolysaccharide lipid A structure associated with pathogenicity.

Pathogen recognition by the immune system relies on germline-encoded pathogen recognition receptors which identify conserved pathogen-associated molecular patterns (PAMPs) such as the lipid A section of the lipopolysaccharide (LPS). The assumption that pathogens and mammalian-associated bacteria remodel their lipid A PAMP because of host-microbe co-evolution is a long held-belief of microbial pathogenesis. We set out to test this fundamental principle by interrogating a Gram-negative genus presenting evidence of evolutionary events linked to the acquisition of essential virulence traits, resulting in pathogenic and non-pathogenic species. The genus Yersinia fulfil these requirements; the acquisition of the pYV virulence plasmid is one of the evolutionary events associated with virulence. At 37{degrees}C, only pathogenic Yersinia switch to deacylated lipid A, a modification that diminishes TLR4/MD-2 recognition and reduces inflammation. An engineered chimeric pathogenic Yersinia strain expressing the non-pathogenic lipid A profile efficiently engages with TLR4, demonstrating it is sufficient to switch the acylation pattern to modify the recognition by TLR4 and subsequent activation of inflammation. The lipid As of pathogenic and non-pathogenic species are modified with aminoarabinose and palmitate; therefore, only the reduced acylation of the lipid A PAMP is a trait associated with virulence. The decorations of lipid A do not alter TLR4 engagement but confer resistance to antimicrobial peptides. The chimeric pathogenic Yersinia strain expressing the non-pathogenic lipid A profile allows to ascertain whether the switch in the lipid A PAMP affects virulence. This strain showed enhanced motility due to an upregulation of the flhDC master regulator, and impaired cellular invasion through downregulation of rovA, a key invasion regulator. The expression and function of pYV-encoded virulence factors Yops and YadA were not affected. Nonetheless, the chimeric strain was attenuated in vivo, demonstrating that virulence factors cannot overcome a switch in the lipid A PAMP associated with pathogenicity.

microbiology↗

bioRxiv2021

Paraburkholderia edwinii protects Aspergillus sp. from phenazines by acting as a toxin sponge

Many environmentally and clinically important fungi are sensitive to toxic, bacterially-produced, redox-active molecules called phenazines. Despite being vulnerable to phenazine-assault, fungi inhabit microbial communities that contain phenazine producers. Because many fungi cannot withstand phenazine challenge, but some bacterial species can, we hypothesized that bacterial partners may protect fungi in phenazine-replete environments. In the first soil sample we collected, we co-isolated several such physically associated pairings. We discovered the novel species Paraburkholderia edwinii and demonstrated it can protect a co-isolated Aspergillus species from phenazine-1-carboxylic acid (PCA) by sequestering it, acting as a toxin sponge; in turn, it also gains protection. When challenged with PCA, P. edwinii changes its morphology, forming aggregates within the growing fungal colony. Further, the fungal partner triggers P. edwinii to sequester PCA and maintains conditions that limit PCA toxicity by promoting an anoxic and highly reducing environment. A mutagenic screen revealed this program depends on the stress-inducible transcriptional repressor HrcA. We show that one relevant stressor in response to PCA challenge is fungal acidification and that acid stress causes P. edwinii to behave as though the fungus were present. Finally, we reveal this phenomenon as widespread among Paraburkholderia with moderate specificity among bacterial and fungal partners, including plant and human pathogens. Our discovery suggests a common mechanism by which fungi can gain access to phenazine-replete environments, and provides a tractable model system for its study. These results have implications for how rhizosphere microbial communities as well as plant and human infection sites are policed for fungal membership.

microbiology↗

bioRxiv2020

MapA, a second large RTX adhesin, contributes to biofilm formation by Pseudomonas fluorescens

Mechanisms by which cells attach to a surface and form a biofilm are diverse and differ greatly between organisms. The Gram-negative, Gammaproteobacterium Pseudomonas fluorescens attaches to a surface through the localization of the large type 1-secreted RTX adhesin LapA to the outer surface of the cell. LapA localization to the cell surface is controlled by the activities of a periplasmic protease, LapG and an inner-membrane spanning cyclic di-GMP responsive effector protein, LapD. A previous study identified a second, LapA-like protein encoded in the P. fluorescens Pf0-1 genome: Pfl01_1463. However, deletion of this gene had no discernible phenotype under our standard laboratory growth conditions. Here, we identified specific growth conditions wherein, Pfl01_1463, hereafter called MapA (Medium Adhesion Protein A) is a functional adhesin contributing to biofilm formation. This adhesin, like LapA, appears to be secreted through a Lap-related type 1 secretion machinery. We show MapA involvement in biofilm formation is also controlled by LapD and LapG, and that the differing roles of LapA and MapA in biofilm formation are achieved, at least in part, through the differences in the sequences of the two adhesins and their differential, cyclic di-GMP-dependent transcriptional regulation. This differential regulation appears to lead to different distributions of the expression of lapA and mapA within a biofilm. Our data indicate that the mechanisms by which a cell forms a biofilm and the components of a biofilm matrix can differ depending on growth conditions in the biofilm. ImportanceAdhesins are critical for the formation and maturation of bacterial biofilms. We identify a second adhesin in P. fluorescens, called MapA, which appears to play a role in biofilm maturation and whose regulation is distinct from the previously reported LapA adhesin, which is critical for biofilm initiation. Analysis of bacterial adhesins show that LapA-like and MapA-like adhesins are found broadly in Pseudomonads and related organisms, indicating that the utilization of different suites of adhesins may be broadly important in the Gammaproteobacteria.

microbiology↗