The interaction of Escherichia coli O157 :H7 and Salmonella Typhimurium flagella with host cell membranes and cytoskeletal components

Uncovering bacterial-mammalian cell interactions via single-cell tracking

BACKGROUND

The interactions between bacterial pathogens and host cells are characterized by a multitude of complexities, leading to a wide range of heterogeneous outcomes. Despite extensive research, we still have a limited understanding of how bacterial motility in complex environments impacts their ability to tolerate antibiotics and adhere to mammalian cell surfaces. The challenge lies in unraveling the complexity of these interactions and developing quantitative microscopy approaches to predict the behavior of bacterial populations.

RESULTS

To address this challenge, we directed our efforts towards Pseudomonas aeruginosa, a pathogenic bacterium known for producing thick films in the lungs of cystic fibrosis patients, and Escherichia coli, used as a proof of concept to develop and demonstrate our single-cell tracking approaches. Our results revealed that P. aeruginosa exhibits diverse and complex interactions on mammalian cell surfaces, such as adhesion, rotational motion, and swimming, unlike the less interactive behavior of Escherichia coli. Our analysis indicated that P. aeruginosa demonstrated lower mean-squared displacement (MSD) values and greater adherence to mammalian cells compared to E. coli, which showed higher MSD slopes and less frequent adherence. Genetic mutations in membrane proteins of P. aeruginosa resulted in altered displacement patterns and reduced adhesion, with the ΔfliD mutant displaying a more Gaussian displacement distribution and significantly less adherence to mammalian cells. Adhesion and tolerance mechanisms are diverse and complex, potentially involving distinct pathways; however, our findings highlight the therapeutic potential of targeting the fliD gene (encoding a critical flagellum protein), as its deletion not only reduced adherence but also antibiotic tolerance.

CONCLUSIONS

Overall, our findings underscore the importance of single cell tracking in accurately assessing bacterial behavior over short time periods and highlight its significant potential in guiding effective intervention strategies.

Lipopolysaccharide Core Truncation in Invasive Escherichia coli O157:H7 ATCC 43895 Impairs Flagella and Curli Biosynthesis and Reduces Cell Invasion Ability

Escherichia coli O157:H7 (E. coli O157) is known for causing severe foodborne illnesses such as hemorrhagic colitis and hemolytic uremic syndrome. Although E. coli O157 is typically regarded as an extracellular pathogen and a weak biofilm producer, some E. coli O157 strains, including a clinical strain ATCC 43895, exhibit a notable ability to invade bovine crypt cells and other epithelial cells, as well as to form robust biofilm. This invasive strain persists in the bovine host significantly longer than non-invasive strains. Various surface-associated factors, including lipopolysaccharides (LPS), flagella, and other adhesins, likely contribute to this enhanced invasiveness and biofilm formation. In this study, we constructed a series of LPS-core deletion mutations (waaI, waaG, waaF, and waaC) in E. coli O157 ATCC 43895, resulting in stepwise truncations of the LPS. This approach enabled us to investigate the effects on the biosynthesis of key surface factors, such as flagella and curli, and the ability of this invasive strain to invade host cells. We confirmed the LPS structure and found that all LPS-core mutants failed to form biofilms, highlighting the crucial role of core oligosaccharides in biofilm formation. Additionally, the LPS inner-core mutants ΔwaaF and ΔwaaC lost the ability to produce flagella and curli. Furthermore, these inner-core mutants exhibited a dramatic reduction in adherence to and invasion of epithelial cells (MAC-T), showing an approximately 100-fold decrease in cell invasion compared with the outer-core mutants (waaI and waaG) and the wild type. These findings underscore the critical role of LPS-core truncation in impairing flagella and curli biosynthesis, thereby reducing the invasion capability of E. coli O157 ATCC 43895.

Helicobacter spp. are prevalent in wild mice and protect from lethal Citrobacter rodentium infection in the absence of adaptive immunity

Transfer of the gut microbiota from wild to laboratory mice alters the host's immune status and enhances resistance to infectious and metabolic diseases, but understanding of which microbes and how they promote host fitness is only emerging. Our analysis of metagenomic sequencing data reveals that Helicobacter spp. are enriched in wild compared with specific-pathogen-free (SPF) and conventionally housed mice, with multiple species commonly co-colonizing their hosts. We create laboratory mice harboring three non-SPF Helicobacter spp. to evaluate their effect on mucosal immunity and colonization resistance to the enteropathogen Citrobacter rodentium. Our experiments reveal that Helicobacter spp. interfere with C. rodentium colonization and attenuate C. rodentium-induced gut inflammation in wild-type (WT) mice, even preventing lethal infection in Rag2-/- SPF mice. Further analyses suggest that Helicobacter spp. interfere with tissue attachment of C. rodentium, putatively by reducing the availability of mucus-derived sugars. These results unveil pivotal protective functions of wild mouse microbiota constituents against intestinal infection.

SPI-1 virulence gene expression modulates motility of Salmonella Typhimurium in a proton motive force- and adhesins-dependent manner

Both the bacterial flagellum and the evolutionary related injectisome encoded on the Salmonella pathogenicity island 1 (SPI-1) play crucial roles during the infection cycle of Salmonella species. The interplay of both is highlighted by the complex cross-regulation that includes transcriptional control of the flagellar master regulatory operon flhDC by HilD, the master regulator of SPI-1 gene expression. Contrary to the HilD-dependent activation of flagellar gene expression, we report here that activation of HilD resulted in a dramatic loss of motility, which was dependent on the presence of SPI-1. Single cell analyses revealed that HilD-activation triggers a SPI-1-dependent induction of the stringent response and a substantial decrease in proton motive force (PMF), while flagellation remains unaffected. We further found that HilD activation enhances the adhesion of Salmonella to epithelial cells. A transcriptome analysis revealed a simultaneous upregulation of several adhesin systems, which, when overproduced, phenocopied the HilD-induced motility defect. We propose a model where the SPI-1-dependent depletion of the PMF and the upregulation of adhesins upon HilD-activation enable flagellated Salmonella to rapidly modulate their motility during infection, thereby enabling efficient adhesion to host cells and delivery of effector proteins.

Regulation of flagellar motility and biosynthesis in enterohemorrhagic Escherichia coli O157:H7

ABSTARCTEnterohemorrhagic Escherichia coli (EHEC) O157:H7 is a human pathogen that causes a variety of diseases, such as hemorrhagic colitis and lethal hemolytic uremic syndrome. Flagellum-dependent motility plays diverse roles in the pathogenesis of EHEC O157:H7, including its migration to an optimal host site, adherence and colonization, survival at the infection site, and post-infection dispersal. However, it is very expensive for cellular economy in terms of the number of genes and the energy required for flagellar biosynthesis and functioning. Furthermore, the flagellar filament bears strong antigenic properties that induce a strong host immune response. Consequently, the flagellar gene expression and biosynthesis are highly regulated to occur at the appropriate time and place by different regulatory influences. The present review focuses on the regulatory mechanisms of EHEC O157:H7 motility and flagellar biosynthesis, especially in terms of flagellar gene regulation by environmental factors, regulatory proteins, and small regulatory RNAs.

A Motile Doublet Form of Salmonella Typhimurium Diversifies Target Search Behaviour at the Epithelial Surface

The behaviors of infectious bacteria are commonly studied in bulk. This is effective to define the general properties of a given isolate, but insufficient to resolve subpopulations and unique single‐microbe behaviors within the bacterial pool. We here employ microscopy to study single‐bacterium characteristics among Salmonella enterica serovar Typhimurium (S.Tm), as they prepare for and launch invasion of epithelial host cells. We find that during the bacterial growth cycle, S.Tm populations switch gradually from fast planktonic growth to a host cell‐invasive phenotype, characterized by flagellar motility and expression of the Type‐three‐secretion‐system‐1. The indistinct nature of this shift leads to the establishment of a transient subpopulation of S.Tm “doublets”—waist‐bearing bacteria anticipating cell division—which simultaneously express host cell invasion machinery. In epithelial cell culture infections, these S.Tm doublets outperform their “singlet” brethren and represent a hyperinvasive subpopulation. Atop both glass and enteroid‐derived monolayers, doublets swim along markedly straighter trajectories than singlets, thereby diversifying search patterns and improving the surface exploration capacity of the total bacterial population. The straighter swimming, combined with an enhanced cell‐adhesion propensity, suffices to account for the hyperinvasive doublet phenotype. This work highlights bacterial cell length heterogeneity as a key determinant of target search patterns atop epithelia.

Flagellin outer domain dimerization modulates motility in pathogenic and soil bacteria from viscous environments

Flagellar filaments function as the propellers of the bacterial flagellum and their supercoiling is key to motility. The outer domains on the surface of the filament are non-critical for motility in many bacteria and their structures and functions are not conserved. Here, we show the atomic cryo-electron microscopy structures for flagellar filaments from enterohemorrhagic Escherichia coli O157:H7, enteropathogenic E. coli O127:H6, Achromobacter, and Sinorhizobium meliloti, where the outer domains dimerize or tetramerize to form either a sheath or a screw-like surface. These dimers are formed by 180° rotations of half of the outer domains. The outer domain sheath (ODS) plays a role in bacterial motility by stabilizing an intermediate waveform and prolonging the tumbling of E. coli cells. Bacteria with these ODS and screw-like flagellar filaments are commonly found in soil and human intestinal environments of relatively high viscosity suggesting a role for the dimerization in these environments.

It has been suggested that the outer domains of bacterial flagellins are not needed for motility. Here, the authors show that flagellar filament outer domains from some bacteria have unique structures which can alter the motility of the bacteria.

Strain specific motility patterns and surface adhesion of virulent and probiotic Escherichia coli

Bacterial motility provides the ability for bacterial dissemination and surface exploration, apart from a choice between surface colonisation and further motion. In this study, we characterised the movement trajectories of pathogenic and probiotic Escherichia coli strains (ATCC43890 and M17, respectively) at the landing stage (i.e., leaving the bulk and approaching the surface) and its correlation with adhesion patterns and efficiency. A poorly motile strain JM109 was used as a control. Using specially designed and manufactured microfluidic chambers, we found that the motion behaviour near surfaces drastically varied between the strains, correlating with adhesion patterns. We consider two bacterial strategies for effective surface colonisation: horizontal and vertical, based on the obtained results. The horizontal strategy demonstrated by the M17 strain is characterised by collective directed movements within the horizontal layer during a relatively long period and non-uniform adhesion patterns, suggesting co-dependence of bacteria in the course of adhesion. The vertical strategy demonstrated by the pathogenic ATCC43890 strain implies the individual movement of bacteria mainly in the vertical direction, a faster transition from bulk to near-surface swimming, and independent bacterial behaviour during adhesion, providing a uniform distribution over the surface.

Host-microbe metatranscriptome reveals differences between acute and chronic infections in diabetes-related foot ulcers

Virtually all diabetes-related foot ulcers (DRFUs) will become colonized by microorganisms that may increase the risk of developing an infection. The reasons why some ulcerations develop acute clinical infections (AI-DRFUs) whilst others develop chronic infection (CI-DRFUs) and the preceding host-microbe interactions in vivo remain largely unknown. Establishing that acute and chronic infections are distinct processes requires demonstrating that these are two different strategies employed by microbes when interacting with a host. In this study, dual-RNA seq was employed to differentiate the host-microbe metatranscriptome between DRFUs that had localized chronic infection or acute clinical infection. Comparison of the host metatranscriptome in AI-DRFUs relative to CI-DRFUs identified upregulated differentially expressed genes (DEGs) that functioned as regulators of vascular lymphatic inflammatory responses, T-cell signalling and olfactory receptors. Conversely, CI-DRFUs upregulated DEGs responsible for cellular homeostasis. Gene set enrichment analysis using Hallmark annotations revealed enrichment of immune and inflammatory profiles in CI-DRFUs relative to AI-DRFUs. Analysis of the microbial metatranscriptome identified the DEGs being enriched within AI-DRFUs relative to CI-DRFUs included several toxins, two-component systems, bacterial motility, secretion systems and genes encoding for energy metabolism. Functions relevant to DRFU pathology were further explored, including biofilm and bacterial pathogenesis. This identified that the expression of biofilm-associated genes was higher within CI-DRFUs compared to that of AI-DRFUs, with mucR being the most highly expressed gene. Collectively, these data provide insights into the host-microbe function in two clinically-distinct infective phenotypes that affect DRFUs. The data reveal that bacteria in acutely infected DRFUs prioritize motility over biofilm and demonstrate greater pathogenicity and mechanisms, which likely subvert host cellular and immune pathways to establish infection. Upregulation of genes for key vascular inflammatory mediators in acutely infected ulcers may contribute, in part, to the clinical picture of a red, hot, swollen foot, which differentiates an acutely infected ulcer from that of a chronic infection.

Mechanisms involved in the adaptation of Escherichia coli O157:H7 to the host intestinal microenvironment

Host adaptation of pathogens may increase intra- and interspecies transmission. We showed previously that the passage of a clinically isolated enterohemorrhagic Escherichia coli (EHEC) O157 strain (125/99) through the gastrointestinal tract of mice increases its pathogenicity in the same host. In this work, we aimed to elucidate the underlying mechanism(s) involved in the patho-adaptation of the stool-recovered (125RR) strain. We assessed the global transcription profile by microarray and found almost 100 differentially expressed genes in 125RR strain compared with 125/99 strain. We detected an overexpression of Type Three Secretion System (TTSS) proteins at the mRNA and protein levels and demonstrated increased adhesion to epithelial cell lines for the 125RR strain. Additional key attributes of the 125RR strain were: increased motility on semisolid agar, which correlated with an increased fliC mRNA level; reduced Stx2 production at the mRNA and protein levels; increased survival at pH 2.5, as determined by acid resistance assays. We tested whether the overexpression of the LEE-encoded regulator (ler) in trans in the 125/99 strain could recreate the increased pathogenicity observed in the 125RR strain. As anticipated ler overexpression led to increased expression of TTSS proteins and bacterial adhesion to epithelial cells in vitro but also increased mortality and intestinal colonization in vivo. We conclude that this host-adaptation process required changes in several mechanisms that improved EHEC O157 fitness in the new host. The research highlights some of the bacterial mechanisms required for horizontal transmission of these zoonotic pathogens between their animal and human populations.