Disruption of Escherichia coli Nissle 1917 K5 capsule biosynthesis, through loss of distinct kfi genes, modulates interaction with intestinal epithelial cells and impact on cell health

Coordinated optimization of the polymerization and transportation processes to enhance the yield of exopolysaccharide heparosan

Heparosan as the precursor for heparin biosynthesis has attracted intensive attention while Escherichia coli Nissle 1917 (EcN) has been applied as a chassis for heparosan biosynthesis. Here, after uncovering the pivotal role of KfiB in heparosan biosynthesis, we further demonstrate KfiB is involved in facilitating KpsT to translocate the nascent heparosan polysaccharide chain. As a result, an artificial expression cassette KfiACB was constructed with optimized RBS elements, resulting in 0.77 g/L heparosan in shake flask culture. Moreover, in view of the intracellular accumulation of heparosan, we further investigated the effects of overexpression of the ABC transport system proteins on heparosan biosynthesis. By co-overexpressing KfiACB with KpsTME, the heparosan production in flask cultures was increased to 1.03 g/L with an extracellular concentration of 0.96 g/L. Eventually, the engineered strain EcN/pET-kfiACB3-galU-kfiD-glmM/pCDF-kpsTME produced 12.2 g/L heparosan in 5-L fed-batch cultures while the extracellular heparosan was about 11.2 g/L. The results demonstrate the high-efficiency of the strategy for co-optimizing the polymerization and transportation for heparosan biosynthesis. Moreover, this strategy should be also available for enhancing the production of other polysaccharides.

Bacterial capsules: Occurrence, mechanism, and function

In environments characterized by extended multi-stress conditions, pathogens develop a variety of immune escape mechanisms to enhance their ability to infect the host. The capsules, polymers that bacteria secrete near their cell wall, participates in numerous bacterial life processes and plays a crucial role in resisting host immune attacks and adapting to their niche. Here, we discuss the relationship between capsules and bacterial virulence, summarizing the molecular mechanisms of capsular regulation and pathogenesis to provide new insights into the research on the pathogenesis of pathogenic bacteria.

Lipopolysaccharide structure modulates cationic biocide susceptibility and crystalline biofilm formation in Proteus mirabilis

Chlorhexidine (CHD) is a cationic biocide used ubiquitously in healthcare settings. Proteus mirabilis, an important pathogen of the catheterized urinary tract, and isolates of this species are often described as "resistant" to CHD-containing products used for catheter infection control. To identify the mechanisms underlying reduced CHD susceptibility in P. mirabilis, we subjected the CHD tolerant clinical isolate RS47 to random transposon mutagenesis and screened for mutants with reduced CHD minimum inhibitory concentrations (MICs). One mutant recovered from these screens (designated RS47-2) exhibited ~ 8-fold reduction in CHD MIC. Complete genome sequencing of RS47-2 showed a single mini-Tn5 insert in the waaC gene involved in lipopolysaccharide (LPS) inner core biosynthesis. Phenotypic screening of RS47-2 revealed a significant increase in cell surface hydrophobicity and serum susceptibility compared to the wildtype, and confirmed defects in LPS production congruent with waaC inactivation. Disruption of waaC was also associated with increased susceptibility to a range of other cationic biocides but did not affect susceptibility to antibiotics tested. Complementation studies showed that repression of smvA efflux activity in RS47-2 further increased susceptibility to CHD and other cationic biocides, reducing CHD MICs to values comparable with the most CHD susceptible isolates characterized. The formation of crystalline biofilms and blockage of urethral catheters was also significantly attenuated in RS47-2. Taken together, these data show that aspects of LPS structure and upregulation of the smvA efflux system function in synergy to modulate susceptibility to CHD and other cationic biocides, and that LPS structure is also an important factor in P. mirabilis crystalline biofilm formation.

Comprehensive Analysis of the Physiological Characterization of Escherichia coli Nissle 1917

Escherichia coli Nissle 1917 (EcN) is one of the probiotics that has drawn more attention from researchers in recent days as it extends many host beneficial effects. EcN is being used as a treatment regimen especially for gastrointestinal disorders for more than 100 years. Apart from its clinical applications in its original form, EcN is being genetically engineered to meet the therapeutic requirements which ultimately led to the gradual transformation of EcN from being a mere food supplement to a complex therapeutic agent. However, comprehensive analysis of physiological characterization of EcN is inadequate. In this study, we have systematically studied various physiological parameters and found that EcN grows very well at the normal as well as at stressful conditions such as temperature (30, 37 and 42 °C), nutritional (minimal and LB), pH (ranging from 3 to 7) and osmotic stress (0.4 M NaCl, 0.4 M KCl, 0.4 M Sucrose and salt conditions). However, EcN shows nearly onefold reduction in viability at extreme acidic conditions (pH 3 and 4). It produces biofilm and curlin very efficiently compared to the laboratory strain MG1655. Through genetic analysis we have also shown that EcN exhibits high level of transformation efficiency and greater ability to retain heterogenous plasmid. Very interestingly, we have found that EcN is highly resistant to P1 phage infection. Since, EcN is being exploited largely for its clinical and therapeutic applications, the results that we have reported here would add more value and further expand its scope in clinical and biotechnological research.

Advances in Escherichia coli Nissle 1917 as a customizable drug delivery system for disease treatment and diagnosis strategies

With the in-depth and comprehensive study of bacteria and their related ecosystems in the human body, bacterial-based drug delivery system has become an emerging biomimetic platform that can retain the innate biological functions. Benefiting from its good biocompatibility and ideal targeting ability as a biological carrier, Escherichia coli Nissle 1917 (ECN) has been focused on the treatment strategies of inflammatory bowel disease and tumor. The advantage of a bacterial carrier is that it can express exogenous protein while also acting as a natural capsule by releasing drug slowly as a result of its own colonization impact. In order to survive in harsh environments such as the digestive tract and tumor microenvironment, ECN can be modified or genetically engineered to enhance its function and host adaptability. The adoption of ECN carries or expresses drugs which are essential for accurate diagnosis and treatment. This review briefly describes the properties of ECN, the relationship between ECN and inflammation and tumor, and the strategy of using surface modification and genetic engineering to modify ECN as a delivery carrier for disease treatment.

Sequestration of gut pathobionts in intraluminal casts, a mechanism to avoid dysregulated T cell activation by pathobionts

T cells that express the transcription factor RORγ, regulatory (Treg), or conventional (Th17) are strongly influenced by intestinal symbionts. In a genetic approach to identify mechanisms underlying this influence, we performed a screen for microbial genes implicated, in germfree mice monocolonized with Escherichia coli Nissle. The loss of capsule-synthesis genes impaired clonal expansion and differentiation of intestinal RORγ⁺ T cells. Mechanistic exploration revealed that the capsule-less mutants remained able to induce species-specific immunoglobulin A (IgA) and were highly IgA-coated. They could still trigger myeloid cells, and more effectively damaged epithelial cells in vitro. Unlike wild-type microbes, capsule-less mutants were mostly engulfed in intraluminal casts, large agglomerates composed of myeloid cells extravasated into the gut lumen. We speculate that sequestration in luminal casts of potentially harmful microbes, favored by IgA binding, reduces the immune system's actual exposure, preserving host-microbe equilibrium. The variable immunostimulation by microbes that has been charted in recent years may not solely be conditioned by triggering molecules or metabolites but also by physical limits to immune system exposure.

Recent advances in bacterial therapeutics based on sense and response

Intelligent drug delivery is a promising strategy for cancer therapies. In recent years, with the rapid development of synthetic biology, some properties of bacteria, such as gene operability, excellent tumor colonization ability, and host-independent structure, make them ideal intelligent drug carriers and have attracted extensive attention. By implanting condition-responsive elements or gene circuits into bacteria, they can synthesize or release drugs by sensing stimuli. Therefore, compared with traditional drug delivery, the usage of bacteria for drug loading has better targeting ability and controllability, and can cope with the complex delivery environment of the body to achieve the intelligent delivery of drugs. This review mainly introduces the development of bacterial-based drug delivery carriers, including mechanisms of bacterial targeting to tumor colonization, gene deletions or mutations, environment-responsive elements, and gene circuits. Meanwhile, we summarize the challenges and prospects faced by bacteria in clinical research, and hope to provide ideas for clinical translation.

A programmable encapsulation system improves delivery of therapeutic bacteria in mice

Living bacteria therapies have been proposed as an alternative approach to treating a broad array of cancers. In this study, we developed a genetically encoded microbial encapsulation system with tunable and dynamic expression of surface capsular polysaccharides that enhances systemic delivery. Based on a small RNA screen of capsular biosynthesis pathways, we constructed inducible synthetic gene circuits that regulate bacterial encapsulation in Escherichia coli Nissle 1917. These bacteria are capable of temporarily evading immune attack, whereas subsequent loss of encapsulation results in effective clearance in vivo. This dynamic delivery strategy enabled a ten-fold increase in maximum tolerated dose of bacteria and improved anti-tumor efficacy in murine models of cancer. Furthermore, in situ encapsulation increased the fraction of microbial translocation among mouse tumors, leading to efficacy in distal tumors. The programmable encapsulation system promises to enhance the therapeutic utility of living engineered bacteria for cancer.

Transient capsule induction allows engineered bacteria to evade initial immune surveillance in a colorectal cancer model.

Integrative Genomics Sheds Light on Evolutionary Forces Shaping the Acidithiobacillia Class Acidophilic Lifestyle

Extreme acidophiles thrive in environments rich in protons (pH values <3) and often high levels of dissolved heavy metals. They are distributed across the three domains of the Tree of Life including members of the Proteobacteria. The Acidithiobacillia class is formed by the neutrophilic genus Thermithiobacillus along with the extremely acidophilic genera Fervidacidithiobacillus, Igneacidithiobacillus, Ambacidithiobacillus, and Acidithiobacillus. Phylogenomic reconstruction revealed a division in the Acidithiobacillia class correlating with the different pH optima that suggested that the acidophilic genera evolved from an ancestral neutrophile within the Acidithiobacillia. Genes and mechanisms denominated as "first line of defense" were key to explaining the Acidithiobacillia acidophilic lifestyle including preventing proton influx that allows the cell to maintain a near-neutral cytoplasmic pH and differ from the neutrophilic Acidithiobacillia ancestors that lacked these systems. Additional differences between the neutrophilic and acidophilic Acidithiobacillia included the higher number of gene copies in the acidophilic genera coding for "second line of defense" systems that neutralize and/or expel protons from cell. Gain of genes such as hopanoid biosynthesis involved in membrane stabilization at low pH and the functional redundancy for generating an internal positive membrane potential revealed the transition from neutrophilic properties to a new acidophilic lifestyle by shaping the Acidithiobacillaceae genomic structure. The presence of a pool of accessory genes with functional redundancy provides the opportunity to "hedge bet" in rapidly changing acidic environments. Although a core of mechanisms for acid resistance was inherited vertically from an inferred neutrophilic ancestor, the majority of mechanisms, especially those potentially involved in resistance to extremely low pH, were obtained from other extreme acidophiles by horizontal gene transfer (HGT) events.

Genome-wide identification of probiotic Escherichia coli Nissle 1917 (EcN) fitness genes during adhesion to the intestinal epithelial cells Caco-2

Escherichia coli Nissle 1917 (EcN) is an efficient probiotic strain extensively used worldwide because of its several health benefits. Adhesion to the intestinal cells is one of the prerequisites for a probiotic strain. To identify the genes essential for the adhesion of EcN on the intestinal cells, we utilized a quantitative genetic footprinting approach called transposon insertion sequencing (INSeq). A transposon insertion mutant library of EcN comprising of ~17,000 mutants was used to screen the adherence to the intestinal epithelial cells, Caco-2. The transposon insertion sites were identified from the input and output population by employing next-generation sequencing using the Ion torrent platform. Based on the relative abundance of reads in the input and output pools, we identified 113 candidate genes that are essential for the fitness of EcN during the adhesion and colonization on the Caco-2 cells. Functional categorization revealed that these fitness genes are associated with carbohydrate transport and metabolism, cell wall/membrane/envelope biogenesis, post-translational modification, stress response, motility and adhesion, and signal transduction. To further validate the genes identified in our INSeq analysis, we constructed individual knock-out mutants in five genes (cyclic di-GMP phosphodiesterase (gmp), hda, uidC, leuO, and hypothetical protein-coding gene). We investigated their ability to adhere to Caco-2 cells. Evaluation of these mutants showed reduced adhesion on Caco-2 cells, confirming their role in adhesion. Understanding the functions of these genes may provide novel insights into molecular regulation during colonization of probiotic bacteria to the intestinal cells, and useful to develop designer probiotic strains.

Transcriptional Profiling of the Probiotic Escherichia coli Nissle 1917 Strain under Simulated Microgravity

Long-term space missions affect the gut microbiome of astronauts, especially the viability of some pathogens. Probiotics may be an effective solution for the management of gut microbiomes, but there is a lack of studies regarding the physiology of probiotics in microgravity. Here, we investigated the effects of microgravity on the probiotic Escherichia coli Nissle 1917 (EcN) by comparing transcriptomic data during exponential and stationary growth phases under simulated microgravity and normal gravity. Microgravity conditions affected several physiological features of EcN, including its growth profile, biofilm formation, stress responses, metal ion transport/utilization, and response to carbon starvation. We found that some changes, such as decreased adhesion ability and acid resistance, may be disadvantageous to EcN relative to gut pathogens under microgravity, indicating the need to develop probiotics optimized for space flight.

Surface components and metabolites of probiotics for regulation of intestinal epithelial barrier

The gut microbiota can significantly affect the function of the intestinal barrier. Some intestinal probiotics (such as Lactobacillus, Bifidobacteria, a few Escherichia coli strains, and a new generation of probiotics including Bacteroides thetaiotaomicron and Akkermansia muciniphila) can maintain intestinal epithelial homeostasis and promote health. This review first summarizes probiotics' regulation of the intestinal epithelium via their surface compounds. Surface layer proteins, flagella, pili and capsular polysaccharides constitute microbial-associated molecular patterns and specifically bind to pattern recognition receptors, which can regulate signaling pathways to produce cytokines or inhibit apoptosis, thereby attenuating inflammation and enhancing the function of the gut epithelium. The review also explains the effects of metabolites (such as secreted proteins, organic acids, indole, extracellular vesicles and bacteriocins) of probiotics on host receptors and the mechanisms by which these metabolites regulate gut epithelial barrier function. Previous reviews summarized the role of the surface macromolecules or metabolites of gut microbes (including both probiotics and pathogens) in human health. However, these reviews were mostly focused on the interactions between these substances and the intestinal mucosal immune system. In the current review, we only focused on probiotics and discussed the molecular interaction between these bacteria and the gut epithelial barrier.

K5 Capsule and Lipopolysaccharide Are Important in Resistance to T4 Phage Attack in Probiotic E. coli Strain Nissle 1917

Rapidly growing antibiotic resistance among gastrointestinal pathogens, and the ability of antibiotics to induce the virulence of these pathogens makes it increasingly difficult to rely on antibiotics to treat gastrointestinal infections. The probiotic Escherichia coli strain Nissle 1917 (EcN) is the active component of the pharmaceutical preparation Mutaflor^® and has been successfully used in the treatment of gastrointestinal disorders. Gut bacteriophages are dominant players in maintaining the microbial homeostasis in the gut, however, their interaction with incoming probiotic bacteria remains to be at conception. The presence of bacteriophages in the gut makes it inevitable for any probiotic bacteria to be phage resistant, in order to survive and successfully colonize the gut. This study addresses the phage resistance of EcN, specifically against lytic T4 phage infection. From various experiments we could show that (i) EcN is resistant toward T4 phage infection, (ii) EcN's K5 polysaccharide capsule plays a crucial role in T4 phage resistance and (iii) EcN's lipopolysaccharide (LPS) inactivates T4 phages and notably, treatment with the antibiotic polymyxin B which neutralizes the LPS destroyed the phage inactivation ability of isolated LPS from EcN. Combination of these identified properties in EcN was not found in other tested commensal E. coli strains. Our results further indicated that N-acetylglucosamine at the distal end of O6 antigen in EcN's LPS could be the interacting partner with T4 phages. From our findings, we have reported for the first time, the role of EcN's K5 capsule and LPS in its defense against T4 phages. In addition, by inactivating the T4 phages, EcN also protects E. coli K-12 strains from phage infection in tri-culture experiments. Our research highlights phage resistance as an additional safety feature of EcN, a clinically successful probiotic E. coli strain.

Escherichia coli Nissle 1917 enhances bioavailability of serotonin in gut tissues through modulation of synthesis and clearance

Accumulating evidence shows indigenous gut microbes can interact with the human host through modulation of serotonin (5-HT) signaling. Here we investigate the impact of the probiotic Escherichia coli Nissle 1917 (EcN) on 5-HT signalling in gut tissues. Ex-vivo mouse ileal tissue sections were treated with either EcN or the human gut commensal MG1655, and effects on levels of 5-HT, precursors, and metabolites, were evaluated using amperometry and high performance liquid chromatography with electrochemical detection (HPLC-EC). Exposure of tissue to EcN cells, but not MG1655 cells, was found to increase levels of extra-cellular 5-HT. These effects were not observed when tissues were treated with cell-free supernatant from bacterial cultures. In contrast, when supernatant recovered from untreated ileal tissue was pre-incubated with EcN, the derivative cell-free supernatant was able to elevate 5-HT overflow when used to treat fresh ileal tissue. Measurement of 5-HT precursors and metabolites indicated EcN also increases intracellular 5-HTP and reduces 5-HIAA. The former pointed to modulation of tryptophan hydroxylase-1 to enhance 5-HT synthesis, while the latter indicates an impact on clearance into enterocytes through SERT. Taken together, these findings show EcN is able to enhance 5-HT bioavailability in ileal tissues through interaction with compounds secreted from host tissues.