Biology subjects

Christman, N. D.

Publications and source records attributed to Christman, N. D..

4 recordsLinked to original sources

bioRxiv2026

The stoichiometry of minor-to-major pilins regulates the dynamic activity of the type IVa competence pilus in Vibrio cholerae

Type IVa pili (T4aP) are bacterial surface appendages that perform various functions including twitching motility, reversible surface attachment, microcolony formation, surface sensing, and DNA uptake for natural transformation. Pivotal to each of these functions is the ability of T4aP to be dynamically extended and retracted from the cell surface. However, the factors that regulate this dynamic activity remain poorly understood in most systems. To address this question, we employ the competence T4aP from Vibrio cholerae as a model system. T4aP are composed of major and minor pilin subunits, named based on their relative abundance in the pilus filament. Prior work has established that minor pilins form a complex that initiates T4aP assembly. This allows for the subsequent addition of major pilins to the filament, which promotes T4aP extension. Here, we uncover that the stoichiometry of minor-to-major pilins is a crucial determinant of T4aP dynamic activity. Specifically, we show that either 1) overexpressing minor pilins or 2) underexpressing the major pilin results in a dramatic increase in the frequency of T4aP dynamics. These results indicate that the stoichiometry of minor-to-major pilins, not their absolute abundance, is one mechanism that regulates T4aP dynamic activity. AUTHOR SUMMARYType IVa pili (T4aP) are a broadly conserved family of filamentous bacterial appendages that help bacteria colonize surfaces, move towards or away from stimuli, and gain new traits through a mechanism of horizontal gene transfer called natural transformation. T4aP are primarily composed of protein subunits called major and minor pilins, named based on their relative abundance in the pilus filament. Bacteria can dynamically extend and retract pilus filaments from their surface through polymerization and depolymerization of these pilins. This dynamic activity is critical for the activities that T4aP carry out. However, the factors that regulate this dynamic activity remain incompletely understood. Here, we find that the ratio of minor-to-major pilins is one factor that regulates the frequency of dynamic activity. Minor pilins are a universally conserved feature of T4aP in diderms. So, it is tempting to speculate that the minor-to-major pilin ratio represents a broadly conserved mechanism for controlling dynamic T4aP activity in diverse bacterial species.

microbiology↗

bioRxiv2025

The molecular basis for DNA-binding by competence T4P is distinct in Gram-positive and Gram-negative species

Competence type IV pili (T4P) are bacterial surface appendages that facilitate DNA uptake during horizontal gene transfer by natural transformation. These dynamic structures actively extend from the cell surface, bind to DNA in the environment, and then retract to import bound DNA into the cell. Competence T4P are found in diverse Gram-negative (diderm) and Gram-positive (monoderm) bacterial species. While the mechanism of DNA-binding by diderm competence T4P has been the recent focus of intensive study, relatively little is known about DNA-binding by monoderm competence T4P. Here, we use Streptococcus pneumoniae as a model system to address this question. Competence T4P likely bind to DNA via a tip-associated complex of proteins called minor pilins, and recent work highlights a high degree of structural conservation between the minor pilin tip complexes of monoderm and diderm competence T4P. In diderms, positively charged residues in one minor pilin, FimT, are critical for DNA-binding. We show that while these residues are conserved in ComGD, the FimT homolog of monoderms, they only play a minor role in DNA uptake for natural transformation. Instead, we find that two-positively charged residues in the neighboring minor pilin, ComGF (the PilW homolog of monoderms), play the dominant role in DNA uptake for natural transformation. Furthermore, we find that these residues are conserved in other monoderms, but not diderms. Together, these results suggest that the molecular basis for DNA-binding has either diverged or evolved independently in monoderm and diderm competence T4P. AUTHOR SUMMARYDiverse bacteria use extracellular structures called competence type IV pili (T4P) to take up DNA from their environment. The uptake of DNA by T4P is the first step of natural transformation, a mode of horizontal gene transfer that contributes to the spread of antibiotic resistance and virulence traits in diverse clinically relevant Gram-negative (diderm) and Gram-positive (monoderm) bacterial species. While the mechanism of DNA binding by competence T4P in diderms has been an area of recent study, relatively little is known about how monoderm competence T4P bind DNA. Here, we explore how monoderm competence T4P bind DNA using Streptococcus pneumoniae as a model system. Our results indicate that while monoderm T4P and diderm T4P likely have conserved structural features, the DNA-binding mechanism of each system is distinct.

microbiology↗

bioRxiv2021

Increased microbial diversity and decreased prevalence of common pathogens in the gut microbiomes of wild turkeys compared to domestic turkeys

Turkeys (Meleagris gallopavo) provide a globally important source of protein and constitute the second most important source of poultry meat in the world. Bacterial diseases are common in commercial poultry production causing significant production losses for farmers. Due to the increasingly recognized problems associated with large-scale/indiscriminant antibiotic use in agricultural settings, poultry producers need alternative methods to control common bacterial pathogens. In this study we compared the cecal microbiota of wild and domestic turkeys, hypothesizing that environmental pressures faced by wild birds may select for a disease-resistant microbial community. Sequence analysis of 16S rRNA genes amplified from cecal samples indicate that free-roaming wild turkeys carry a rich and variable microbiota compared to domestic turkeys raised on large-scale poultry farms. Wild turkeys also had very low levels of Staphylococcus, Salmonella and E. coli when compared to domestic turkeys. E. coli strains isolated from wild or domestic turkey cecal samples also belong to distinct phylogenetic backgrounds and differ in their propensity to carry virulence genes. E. coli strains isolated from factory-raised turkeys were far more likely to carry genes for capsule (kpsII, kpsIII) or siderophore (iroN, fyuA) synthesis than those isolated from wild turkeys. These results suggest that the microbiota of wild turkeys may provide colonization resistance against common poultry pathogens. ImportanceDue to the increasingly recognized problems associated with antibiotic use in agricultural settings, poultry producers need alternative methods to control common bacterial pathogens. In this study we compare the microbiota of wild and domestic turkeys. Results suggest that free ranging wild turkeys carry a distinct microbiome when compared to farm raised turkeys. The microbiome of wild birds contains very low levels of poultry pathogens compared to farm raised birds. The microbiomes of wild turkeys may be used to guide development of new ways to control disease in large scale poultry production.

microbiology↗

bioRxiv2020

A trimeric autotransporter enhances biofilm cohesiveness in Yersinia pseudotuberculosis but not in Yersinia pestis

Cohesion of biofilms made by Yersinia pestis and Yersinia pseudotuberculosis (Yptb) has been attributed solely to an extracellular polysaccharide matrix encoded by the hms genes (Hms-ECM). However, mutations in the Yptb BarA/UvrY/CsrB regulatory cascade enhance biofilm stability without dramatically increasing Hms-ECM production. We found that treatment with proteinase K enzyme effectively destabilized Yptb csrB mutant biofilms, suggesting that cell-cell interactions might be mediated by protein adhesins or extracellular matrix proteins. We identified an uncharacterized trimeric autotransporter lipoprotein (YPTB2394), repressed by csrB, which has been referred to as YadE. Biofilms made by a {Delta}yadE mutant strain were extremely sensitive to mechanical disruption. Overexpression of yadE in wild-type Yptb increased biofilm cohesion, similar to biofilms made by csrB or uvrY mutants. We found that the Rcs signaling cascade, which represses Hms-ECM production, activated expression of yadE. The yadE gene appears to be functional in Yptb but is a pseudogene in modern Y. pestis strains. Expression of functional yadE in Y. pestis KIM6+ altered the production of Hms-ECM and weakened biofilms made by these bacteria. This suggests that although the YadE autotransporter protein increases Yptb biofilm stability, it may be incompatible with Hms-ECM production that is essential for Y. pestis biofilm production in fleas. Inactivation of yadE in Y. pestis may be another instance of selective gene loss in the evolution of flea-borne transmission by this species. IMPORTANCEThe evolution of Yersinia pestis from its Y. pseudotuberculosis (Yptb) ancestor involved gene acquisition and gene losses, leading to differences in biofilm production. Characterizing the unique biofilm features of both species may provide better understanding of how each adapts to its specific niches. This study identifies a trimeric autotransporter YadE that promotes biofilm stability of Yptb but which has been inactivated in Y. pestis, likely because it is not compatible with Hms polysaccharide that is crucial for biofilms inside fleas. We also reveal that the Rcs signaling cascade, which represses Hms expression in Y. pestis, activates YadE in Yptb. The ability of Yptb to use polysaccharide or YadE protein for cell-cell adhesion may help it produce biofilms in different environments.

microbiology↗