Clostridium scindens exacerbates experimental necrotizing enterocolitis via upregulation of the apical sodium-dependent bile acid transporter

Necrotizing enterocolitis (NEC) is the most common gastrointestinal emergency in premature infants. Evidence indicates that bile acid homeostasis is disrupted during NEC: ileal bile acid levels are elevated in animals with experimental NEC, as is expression of the apical sodium-dependent bile acid transporter (Asbt). In addition, bile acids, which are synthesized in the liver, are extensively modified by the gut microbiome, including via the conversion of primary bile acids to more cytotoxic secondary forms. We hypothesized that the addition of bile acid-modifying bacteria would increase susceptibility to NEC in a neonatal rat model of the disease. The secondary bile acid-producing species Clostridium scindens exacerbated both incidence and severity of NEC. C. scindens upregulated the bile acid transporter Asbt and increased levels of intraenterocyte bile acids. Treatment with C. scindens also altered bile acid profiles and increased hydrophobicity of the ileal intracellular bile acid pool. The ability of C. scindens to enhance NEC requires bile acids, as pharmacological sequestration of ileal bile acids protects animals from developing disease. These findings indicate that bile acid-modifying bacteria can contribute to NEC pathology and provide additional evidence for the role of bile acids in the pathophysiology of experimental NEC.NEW & NOTEWORTHY Necrotizing enterocolitis (NEC), a life-threatening gastrointestinal emergency in premature infants, is characterized by dysregulation of bile acid homeostasis. We demonstrate that administering the secondary bile acid-producing bacterium Clostridium scindens enhances NEC in a neonatal rat model of the disease. C. scindens-enhanced NEC is dependent on bile acids and driven by upregulation of the ileal bile acid transporter Asbt. This is the first report of bile acid-modifying bacteria exacerbating experimental NEC pathology.

What is the role of CT-based Hounsfield unit assessment in the evaluation of bone mineral density in patients undergoing 1- or 2-level lumbar spinal fusion for degenerative spinal pathologies? A prospective study

BACKGROUND CONTEXT

Computed tomography-based vertebral attenuation values (CT-based HU) have been shown to correlate with T-scores on DEXA scan; and have been acknowledged as an independent factor for predicting fragility fractures. Most patients undergoing lumbar surgeries require CT as part of their preoperative evaluation.

PURPOSE

The current study was thus planned to evaluate the role of lumbar CT as an opportunistic investigation in determining BMD preoperatively in patients undergoing lumbar fusion.

STUDY DESIGN

Prospective cohort study.

PATIENT SAMPLE

Patients older than 45 years, who underwent one- to two-level lumbar (L3-S1 levels) fusions.

OUTCOME MEASURES

Comparison of the quantitative assessment of osteoporosis using Hounsfield Units (HU) on CT (L1-L5) and mean lumbar T-scores on DEXA (Dual Energy X-ray Absorptiometry).

HYPOTHESIS

HU on CT is comparable to T-score on DEXA as a suitable modality for the assessment of osteoporosis in patients undergoing one- to two-level lumbar fusion.

METHODS

A prospective cohort study was conducted between January and December 2021. Patients older than 45 years, who underwent one- to two-level lumbar (L3-S1 levels) fusions and had complete clinico-radiological records, were prospectively enrolled. A comparison was drawn between the HU (measured by placing an oval region of interest [ROI] over axial, sagittal and coronal images of lumbar vertebrae) on CT and T-scores on DEXA, and analyzed statistically. The HU values correlating best with normal (group A), osteopenia (B) and osteoporosis (C) categories (classified based on T-scores of lumbar spines) were determined statistically.

RESULTS

Overall, 87 patients (mean age of 60.56±11.63 years; 63 [72.4%] female patients) were prospectively studied. There was a statistically significant difference in the mean age (p=.01) and sex distribution (predominantly female patients; p=.03) of patients belonging to groups B (osteopenic) and C (osteoporotic patients), as compared with group A. The greatest correlation between T-score (on DEXA) and HU (on CT) for differentiating osteopenia (group B) from group A was observed at levels L1 (p<.001), L2 (p<.001) and L3 (p<.001). Based on receiver-operating characteristic (ROC) curve analysis, the cut-off values for HU for identifying osteopenia were 159 (at L1; sensitivity 81.6 and specificity 80) and 162 (at L2; sensitivity 80 and specificity 71.1). In addition, there was statistically significant correlation between T-score (on DEXA) and HU at all the lumbar levels for distinguishing osteoporosis (group C), although the difference was most evident at the upper lumbar (L1 and L2) levels (p<.001). Based on ROC analysis, cut-off HU values for defining osteoporosis were 127 (at L1; sensitivity 71.3 and specificity 70) and 117 (at L2; sensitivity 65.5 and specificity 90).

CONCLUSION

Based on our study, the measurement of HU on CT at upper lumbar levels can be considered as "surrogate marker" for BMD in the diagnosis of osteopenia (cut-off: 159 at L1, 162 at L2) and osteoporosis (cut-off: 127 at L1, 117 at L2) in patients undergoing lumbar fusion surgeries. The HU measurements on CT at the lower lumbar levels (L4 and L5) are less reliable in this preoperative scenario.

Low-Toxin Clostridioides difficile RT027 Strains Exhibit Robust Virulence

Clostridioides difficile is a leading cause of healthcare-associated infections worldwide. Currently, there is a lack of consensus for an optimal diagnostic method for C. difficile infection (CDI). Multi-step diagnostic algorithms use enzyme immunosorbent analysis (EIA)-based detection of C. difficile toxins TcdA/TcdB in stool, premised on the rationale that EIA toxin-negative (Tox⁻) patients have less severe disease and shorter diarrhoea duration. The aim of this study was to characterize toxigenic (i.e. tcdA/tcdB-positive) C. difficile strains isolated from diarrheic patient stool with an EIA Tox⁻ (i.e. “discrepant”) CDI diagnostic test result. Recovered strains were DNA fingerprinted (ribotyped), subjected to multiple toxin, genome and proteome evaluations, and assessed for virulence. Overall, of 1243 C. difficile-positive patient stool specimens from Southern Arizona hospitals, 31% were discrepant. For RT027 (the most prevalent ribotype)-containing specimens, 34% were discrepant; the corresponding RT027 isolates were cytotoxic to cultured fibroblasts, but their total toxin levels were comparable to, or lower than, the historic low-toxin-producing C. difficile strain CD630. Nevertheless, these low-toxin RT027 strains (LT-027) exhibited similar lethality to a clade-matched high-toxin RT027 strain in Golden Syrian hamsters, and heightened colonization and persistence in mice. Genomics and proteomics analyses of LT-027 strains identified unique genes and altered protein abundances, respectively, relative to high-toxin RT027 strains. Collectively, our data highlight the robust virulence of LT-027 C. difficile, provide a strong argument for reconsidering the clinical significance of a Tox⁻ EIA result, and underscore the potential limitations of current diagnostic protocols.

Phylogenomic analysis of Clostridioides difficile ribotype 106 strains reveals novel genetic islands and emergent phenotypes

Clostridioides difficile infection (CDI) is a major healthcare-associated diarrheal disease. Consistent with trends across the United States, C. difficile RT106 was the second-most prevalent molecular type in our surveillance in Arizona from 2015 to 2018. A representative RT106 strain displayed robust virulence and 100% lethality in the hamster model of acute CDI. We identified a unique 46 KB genomic island (GI1) in all RT106 strains sequenced to date, including those in public databases. GI1 was not found in its entirety in any other C. difficile clade, or indeed, in any other microbial genome; however, smaller segments were detected in Enterococcus faecium strains. Molecular clock analyses suggested that GI1 was horizontally acquired and sequentially assembled over time. GI1 encodes homologs of VanZ and a SrtB-anchored collagen-binding adhesin, and correspondingly, all tested RT106 strains had increased teicoplanin resistance, and a majority displayed collagen-dependent biofilm formation. Two additional genomic islands (GI2 and GI3) were also present in a subset of RT106 strains. All three islands are predicted to encode mobile genetic elements as well as virulence factors. Emergent phenotypes associated with these genetic islands may have contributed to the relatively rapid expansion of RT106 in US healthcare and community settings.

Sublaminar banding as an adjunct to pedicle screw-rod constructs: a review and technical note on novel hybrid constructs in spinal deformity surgery

Sublaminar implants that encircle cortical bone are well-established adjuncts to pedicle screw-rod constructs in pediatric deformity surgery. Sublaminar bands (SLBs) in particular carry the advantage of relatively greater bone contact surface area as compared to wires and pullout loads that are independent of bone mineral density, in contrast to pedicle screws. Whereas the relevant technical considerations have been reported for pediatric deformity correction, an understanding of the relative procedural specifics of these techniques is missing for adult spinal deformity (ASD), despite several case series that have used distinct posterior tethering techniques for proximal junctional kyphosis prevention. In this paper, the authors summarize the relevant literature and describe a novel technique wherein bilateral tensioned SLBs are introduced at the nonfused proximal junctional level of long-segment ASD constructs.

Biomechanical assessment of proximal junctional semi-rigid fixation in long-segment thoracolumbar constructs

OBJECTIVE

Proximal junctional kyphosis (PJK) and failure (PJF) are potentially catastrophic complications that result from abrupt changes in stress across rigid instrumented and mobile non-fused segments of the spine (transition zone) after adult spinal deformity surgery. Recently, data have indicated that extension (widening) of the transitional zone via use of proximal junctional (PJ) semi-rigid fixation can mitigate this complication. To assess the biomechanical effectiveness of 3 semi-rigid fixation constructs (compared to pedicle screw fixation alone), the authors performed cadaveric studies that measured the extent of PJ motion and intradiscal pressure changes (ΔIDP).

METHODS

To measure flexibility and ΔIDP at the PJ segments, moments in flexion, extension, lateral bending (LB), and torsion were conducted in 13 fresh-frozen human cadaveric specimens. Five testing cycles were conducted, including intact (INT), T10–L2 pedicle screw-rod fixation alone (PSF), supplemental hybrid T9 Mersilene tape insertion (MT), hybrid T9 sublaminar band insertion (SLB1), and hybrid T8/T9 sublaminar band insertion (SLB2).

RESULTS

Compared to PSF, SLB1 significantly reduced flexibility at the level rostral to the upper-instrumented vertebral level (UIV+1) under moments in 3 directions (flexion, LB, and torsion, p ≤ 0.01). SLB2 significantly reduced motion in all directions at UIV+1 (flexion, extension, LB, torsion, p < 0.05) and at UIV+2 (LB, torsion, p ≤ 0.03). MT only reduced flexibility in extension at UIV+1 (p = 0.02). All 3 constructs revealed significant reductions in ΔIDP at UIV+1 in flexion (MT, SLB1, SLB2, p ≤ 0.02) and torsion (MT, SLB1, SLB2, p ≤ 0.05), while SLB1 and SLB2 significantly reduced ΔIDP in extension (SLB1, SLB2, p ≤ 0.02) and SLB2 reduced ΔIDP in LB (p = 0.05). At UIV+2, SLB2 similarly significantly reduced ΔIDP in extension, LB, and torsion (p ≤ 0.05).

CONCLUSIONS

Compared to MT, the SLB1 and SLB2 constructs significantly reduced flexibility and ΔIDP in various directions through the application of robust anteroposterior force vectors at UIV+1 and UIV+2. These findings indicate that semi-rigid sublaminar banding can most effectively expand the transition zone and mitigate stresses at the PJ levels of long-segment thoracolumbar constructs.

Nonsteroidal Anti-inflammatory Drugs Alter the Microbiota and Exacerbate Clostridium difficile Colitis while Dysregulating the Inflammatory Response

Clostridium difficile infection (CDI) is a major public health threat worldwide. The use of nonsteroidal anti-inflammatory drugs (NSAIDs) is associated with enhanced susceptibility to and severity of CDI; however, the mechanisms driving this phenomenon have not been elucidated. NSAIDs alter prostaglandin (PG) metabolism by inhibiting cyclooxygenase (COX) enzymes. Here, we found that treatment with the NSAID indomethacin prior to infection altered the microbiota and dramatically increased mortality and the intestinal pathology associated with CDI in mice. We demonstrated that in C. difficile-infected animals, indomethacin treatment led to PG deregulation, an altered proinflammatory transcriptional and protein profile, and perturbed epithelial cell junctions. These effects were paralleled by increased recruitment of intestinal neutrophils and CD4⁺ cells and also by a perturbation of the gut microbiota. Together, these data implicate NSAIDs in the disruption of protective COX-mediated PG production during CDI, resulting in altered epithelial integrity and associated immune responses.IMPORTANCEClostridium difficile infection (CDI) is a spore-forming anaerobic bacterium and leading cause of antibiotic-associated colitis. Epidemiological data suggest that use of nonsteroidal anti-inflammatory drugs (NSAIDs) increases the risk for CDI in humans, a potentially important observation given the widespread use of NSAIDs. Prior studies in rodent models of CDI found that NSAID exposure following infection increases the severity of CDI, but mechanisms to explain this are lacking. Here we present new data from a mouse model of antibiotic-associated CDI suggesting that brief NSAID exposure prior to CDI increases the severity of the infectious colitis. These data shed new light on potential mechanisms linking NSAID use to worsened CDI, including drug-induced disturbances to the gut microbiome and colonic epithelial integrity. Studies were limited to a single NSAID (indomethacin), so future studies are needed to assess the generalizability of our findings and to establish a direct link to the human condition.

Prospective assessment of the safety and early outcomes of sublaminar band placement for the prevention of proximal junctional kyphosis

OBJECTIVE

Proximal junctional kyphosis (PJK) can progress to proximal junctional failure (PJF), a widely recognized early and serious complication of multisegment spinal instrumentation for the treatment of adult spinal deformity (ASD). Sublaminar band placement has been suggested as a possible technique to prevent PJK and PJF but carries the theoretical possibility of a paradoxical increase in these complications as a result of the required muscle dissection and posterior ligamentous disruption. In this study, the authors prospectively assess the safety as well as the early clinical and radiological outcomes of sublaminar band insertion at the upper instrumented vertebra (UIV) plus 1 level (UIV+1).

METHODS

Between August 2015 and February 2017, 40 consecutive patients underwent either upper (T2–4) or lower (T8–10) thoracic sublaminar band placement at the UIV+1 during long-segment thoracolumbar arthrodesis surgery. Outcome measures were prospectively collected and uploaded to a web-based REDCap database specifically designed to include demographic, clinical, and radiological data. All patients underwent clinical assessment, as well as radiological assessment with anteroposterior and lateral 36-inch whole-spine standing radiographs both pre- and postoperatively.

RESULTS

Forty patients (24 women and 16 men) were included in this study. Median age at surgery was 64.0 years with an IQR of 57.7–70.0 years. Median follow-up was 12 months (IQR 6–15 months). Three procedure-related complications were noted, including 2 intraoperative cerebrospinal spinal fluid leaks and 1 transient neurological deficit. Median visual analog scale (VAS) scores for back pain significantly improved after surgery (preoperatively: 8.0, IQR 6.0–10.0; 1-year follow-up: 2.0, IQR 0.0–6.0; p = 0.001). Median Oswestry Disability Index (version 2.1a) scores also significantly improved after surgery (preoperatively: 56.0, IQR 45.0–64.0; 1-year follow-up: 46.0, IQR 22.2–54.0; p < 0.001). Sagittal vertical axis (preoperatively: 9.0 cm, IQR 5.3–11.6 cm; final follow-up: 4.7 cm, IQR 2.0–6.6 cm; p < 0.001), pelvic incidence-lumbar lordosis mismatch (24.7°, IQR 11.2°–31.2°; 7.7°, IQR −1.2° to 19.5°; p < 0.001), and pelvic tilt (28.7°, IQR 20.4°–32.6°; 17.1°, IQR 10.8°–25.2°; p < 0.001) were all improved at the final follow-up. While proximal junctional (PJ) Cobb angles increased overall at the final follow-up (preoperatively: 4.2°, IQR 1.9°–7.4°; final follow-up: 8.0°, IQR 5.8°–10.3°; p = 0.002), the significant increase was primarily noted starting at the immediate postoperative time point (7.2°, IQR 4.4°–11.8°; p = 0.001) and not beyond. Three patients (7.5%) developed radiological PJK (mean ΔPJ Cobb 15.5°), while there were no instances of PJF in this cohort.

CONCLUSIONS

Sublaminar band placement at the UIV+1 during long-segment thoracolumbar instrumented arthrodesis is relatively safe and is not associated with an increased rate of PJK. Moreover, no subjects developed PJF. Prospective large-scale and long-term analysis is needed to define the potential benefit of sublaminar bands in reducing the incidence of PJK and PJF following surgery for ASD.

Clinical trial registration no.: NCT02411799 (clinicaltrials.gov)

Modulation of Intestinal Paracellular Transport by Bacterial Pathogens

The passive and regulated movement of ions, solutes, and water via spaces between cells of the epithelial monolayer plays a critical role in the normal intestinal functioning. This paracellular pathway displays a high level of structural and functional specialization, with the membrane-spanning complexes of the tight junctions, adherens junctions, and desmosomes ensuring its integrity. Tight junction proteins, like occludin, tricellulin, and the claudin family isoforms, play prominent roles as barriers to unrestricted paracellular transport. The past decade has witnessed major advances in our understanding of the architecture and function of epithelial tight junctions. While it has been long appreciated that microbes, notably bacterial and viral pathogens, target and disrupt junctional complexes and alter paracellular permeability, the precise mechanisms remain to be defined. Notably, renewed efforts will be required to interpret the available data on pathogen-mediated barrier disruption in the context of the most recent findings on tight junction structure and function. While much of the focus has been on pathogen-induced dysregulation of junctional complexes, commensal microbiota and their products may influence paracellular permeability and contribute to the normal physiology of the gut. Finally, microbes and their products have become important tools in exploring host systems, including the junctional properties of epithelial cells. © 2018 American Physiological Society. Compr Physiol 8:823-842, 2018.

Memories of a virulent past

Studies on bacterial virulence and host-pathogen interactions usually rely on type strains isolated from human or veterinary populations. For instance, the El Tor strain N16961 has been extensively used to characterize Vibrio cholerae virulence, while E2348/69 is a default choice for studies on enteropathogenic E. coli interactions with host cells. Subsequent to isolation, such strains are passaged under laboratory conditions in rich medium, and often genetically manipulated to induce specific mutations or other alterations. While the cumulative knowledge gained by focused studies on a limited number of bacterial isolates allows for rapid scientific progress, strain diversity resulting from prolonged propagation and manipulation in different laboratories may lead to a distorted perspective and, at times, confound attempts to replicate specific experiments. Exploring such aberrations is an inevitable and necessary, if not always welcome, part of scientific progress.

The trendy microbes

After dominating the earth in numbers and biomass for a good 3.5 billion years, microbes are now finally in the public consciousness. The vigorous surge in the study of microbes promises to yield novel approaches to treating diseases and maintaining health. In the middle of the deluge of scientific publications and lay articles, we pause to reflect on this trend.

Muramyl dipeptide: Not just another brick in the wall

The robust expression of microbial pattern recognition receptors such as TLR4 and Nod2 in intestinal stem cells reflects an active communication dynamic between the host and the gut microbiota. A new study reveals that muramyl dipeptide, the bacterial cell wall peptidoglycan motif, activates Nod2 within crypt base columnar Lgr5-positive stem cells and promotes their survival. Apart from the immediate relevance to the growth of organoids for in vitro experiments, the study raises new questions about the molecular mechanisms whereby gut microbes influence intestinal physiology.

Off-label abuse of antibiotics by bacteria

Antibiotics and antibiotic resistance made news on several fronts in the past year. Many public health organizations, including the CDC, used terms such as "crisis", "catastrophic consequences", and "nightmare scenario" to highlight the rapid emergence and spread of antibiotic resistance. A report from the Pew Commission on Industrial Farm Animal Production, on the fifth anniversary of the publication of its landmark 2008 report, noted that state and federal legislative efforts to limit non-therapeutic use of antibiotics in animal production were thwarted by drug and food animal industries. In its lobbying disclosures, the Farm Bureau stated that such efforts to limit use of animal antibiotics were "based on emotion and no credible peer reviewed science." Meanwhile, there have been inexorable advances in our understanding of the molecular mechanisms by which antibiotics induce diversity and resistance in bacteria. This article reviews one study that probed the role of the bacterial general stress response in sub-inhibitory antibiotic-induced mutagenesis and antibiotic resistance.

The meddling microbes midst our medicines

It is not surprising that the complex metabolic machinery of the gut microbiome has accidental, or directed, ability to alter our medicines and influence their efficacy. What is not known is the extent to which this has contributed to drug failures or contraindications, or to the derailment of clinical trials. Some studies are unraveling the mechanisms by which the microbiota alter specific drugs, such as digoxin, and contribute to variations in efficacies between patient populations. Microbiome profiling, therefore, may well become an inevitable arm of all clinical trials in the future.

Early, reliable, utilitarian predictive factors for fat embolism syndrome in polytrauma patients

BACKGROUND

Fat embolism is one of the apocalyptic pulmonary complications following high energy trauma situations. Since delay in diagnosis may have devastating consequences, early, easily accessible and relatively inexpensive investigations for risk stratification may prove useful, especially in developing nations.

MATERIALS AND METHODS

This prospective trial included a total of 67 young polytrauma patients, in whom the role of nine easily available, rapidly performable clinical or laboratory investigations (or observations noted at admission) in predicting the later occurrence of fat embolism syndrome were assessed. All the patients also underwent continuous monitoring of oxygen saturation with pulsoximetry.

RESULTS

The correlation between initial serum lactate (within 12 hours of injury) and hypoxia was statistically significant. There was a trend towards correlation with FES(by Gurd's criteria) (P=0.07), Sensitivity of 24-hour monitoring of oxygen saturation in predicting later pulmonary deterioration approached 100%.

CONCLUSIONS

The combination of three factors including polytrauma (with NISS >17), serum lactate >22 mmol/l at admission (within 12 hours of injury) fall in oxygen saturation (SaO2 below 90% in the initial 24 hours) predict the development of post-traumatic pulmonary complications, especially the fat embolism syndrome.

Spirochaetes and their twisted ways

Biological systematists have had a long tradition of encountering organisms that are not quite what they seem to be. Among the microbes, horizontal gene transfer and evolutionary pressures result in organisms that have distinguished themselves from their closest relatives. The recent analyses of several Spirochetes reveal members that are not spiral shaped, and ones that appear to have extensively acquired genetic material from phylogenetically distant, but environmentally proximate, organisms.

Sizing up microbes

The size range of life forms is dictated by basic principles of physics. Large microorganisms, with sizes approaching a millimeter, have compensating features that address the immutable laws of physics. For pathogens, size may impact a range of functions, such as adherence and immune evasion. We review several recent studies on factors impacting, and impacted by, the size of microorganisms.

Leptin signaling protects the gut from Entamoeba histolytica infection

The role of the adipose-derived hormone leptin, and leptin receptors, in signaling satiety to the central nervous system and regulating energy balance is well recognized. But leptin also acts on peripheral tissues such as skeletal muscles, adipose tissues, pancreas, liver, intestine and the immune system. The existence of different splice variants of leptin receptor and the numerous intracellular signaling pathways triggered by leptin make this a truly versatile system. Two recent studies explore the link between malnutrition, leptin signaling and susceptibility to amebic infection. These studies point to important and novel aspects of leptin signaling in maintaining gut homeostasis and warding off infections.

The major facilitator superfamily-type protein LbtC promotes the utilization of the legiobactin siderophore by Legionella pneumophila

The Gram-negative bacterium Legionella pneumophila elaborates the siderophore legiobactin. We previously showed that cytoplasmic LbtA helps mediate legiobactin synthesis, inner-membrane LbtB promotes export of legiobactin, and outer-membrane LbtU acts as the ferrisiderophore receptor. RT-PCR analyses now identified lbtC as an iron-repressed gene that is the final gene in an operon containing lbtA and lbtB. In silico analysis predicted that LbtC is an inner-membrane protein that belongs to the major facilitator superfamily (MFS). Although capable of normal growth in standard media, lbtC mutants were defective for growth on iron-depleted agar media. While producing normal levels of legiobactin, lbtC mutants were unable to utilize supplied legiobactin to stimulate growth on iron-depleted media and displayed an impaired ability to take up radiolabelled iron. All lbtC mutant phenotypes were complemented by reintroduction of an intact copy of lbtC. When a cloned copy of both lbtC and lbtU was introduced into a heterologous bacterium (Legionella longbeachae), the organism acquired the ability to utilize legiobactin to grow better on low-iron media. Together, these data indicate that LbtC is involved in the uptake of legiobactin, and based upon its predicted location is most likely the mediator of ferrilegiobactin transport across the inner membrane. The data are also a unique documentation of how an MFS protein can promote bacterial iron-siderophore import, standing in contrast to the vast majority of studies which have defined ABC-type permeases as the mediators of siderophore import across the Gram-negative inner membrane or the Gram-positive cytoplasmic membrane.

Enteropathogenic E. coli non-LEE encoded effectors NleH1 and NleH2 attenuate NF-κB activation

Enteric bacterial pathogens have evolved sophisticated strategies to evade host immune defences. Some pathogens deliver anti-inflammatory effector molecules into the host cell cytoplasm via a type III secretion system (T3SS). Enteropathogenic Escherichia coli (EPEC) inhibits inflammation by an undefined, T3SS-dependent mechanism. Two proteins encoded outside of the EPEC locus of enterocyte effacement (LEE) pathogenicity island, non-LEE-encoded effector H1 (NleH1) and H2 (NleH2), display sequence similarity to Shigella flexneri OspG, which inhibits activation of the pro-inflammatory transcription factor NF-κB. We hypothesized that the anti-inflammatory effects of EPEC were mediated by NleH1 and NleH2. In this study, we examined the effect of NleH1/H2 on the NF-κB pathway. We show that NleH1/H2 are secreted via the T3SS and that transfection of cells with plasmids harbouring nleH1 or nleH2 decreased IKK-β-induced NF-κB activity and attenuated TNF-α-induced degradation of phospho-IκBα by preventing ubiquitination. Serum KC levels were higher in mice infected with ΔnleH1H2 than those infected with WT EPEC, indicating that NleH1/H2 dampen pro-inflammatory cytokine expression. ΔnleH1H2 was cleared more rapidly than WT EPEC while complementation of ΔnleH1H2 with either NleH1 or NleH2 prolonged colonization. Together, these data show that NleH1 and NleH2 function to dampen host inflammation and facilitate EPEC colonization during pathogenesis.

Spore-forming Bacilli and Clostridia in human disease

Many Gram-positive spore-forming bacteria in the Firmicute phylum are important members of the human commensal microbiota, which, in rare cases, cause opportunistic infections. Other spore-formers, however, have evolved to become dedicated pathogens that can cause a striking variety of diseases. Despite variations in disease presentation, the etiologic agent is often the spore, with bacterially produced toxins playing a central role in the pathophysiology of infection. This review will focus on the specific diseases caused by spores of the Clostridia and Bacilli.

Self portraits. Gut microbiology

In dealing with gastric contents, faesces and urine, chemically and bacteriologically, no very complex apparatus is required, and with a brief preliminary training any practitioner possessing the instinct of research may in a very few years acquire the power of perceiving, in many of the chronic gastro-intestinal conditions coming before him daily, order—pathological order if you will—where before he saw nothing but chaos. David Sommerville, Lancet 1910.

Clostridium difficile infection: An overview of the disease and its pathogenesis, epidemiology and interventions

Clostridium difficile infection (CDI) is the primary cause of antibiotic-associated diarrhea and is a significant nosocomial disease. In the past ten years, variant toxin-producing strains of C. difficile have emerged, that have been associated with severe disease as well as outbreaks worldwide. This review summarizes current information on C. difficile pathogenesis and disease, and highlights interventions used to combat single and recurrent episodes of CDI.

Enteric infection meets intestinal function: how bacterial pathogens cause diarrhoea

Infectious diarrhoea is a significant contributor to morbidity and mortality worldwide. In bacterium-induced diarrhoea, rapid loss of fluids and electrolytes results from inhibition of the normal absorptive function of the intestine as well as the activation of secretory processes. Advances in the past 10 years in the fields of gastrointestinal physiology, innate immunity and enteric bacterial virulence mechanisms highlight the multifactorial nature of infectious diarrhoea. This review explores the various mechanisms that contribute to loss of fluids and electrolytes following bacterial infections, and attempts to link these events to specific virulence factors and toxins.

Enteropathogenic Escherichia coli-induced epidermal growth factor receptor activation contributes to physiological alterations in intestinal epithelial cells

The diarrheagenic pathogen enteropathogenic Escherichia coli (EPEC) is responsible for significant infant mortality and morbidity, particularly in developing countries. EPEC pathogenesis relies on a type III secretion system-mediated transfer of virulence effectors into host cells. EPEC modulates host cell survival and inflammation, although the proximal signaling pathways have not been well defined. We therefore examined the effect of EPEC on the epidermal growth factor receptor (EGFR), a known upstream activator of both the prosurvival phosphoinositide 3-kinase/Akt and proinflammatory mitogen-activated protein (MAP) kinase pathways. EPEC induced the autophosphorylation of EGFR in intestinal epithelial cells within 15 min postinfection, with maximal phosphorylation being observed at 4 h. Filter-sterilized supernatants of EPEC cultures also stimulated EGFR phosphorylation, suggesting that a secreted component(s) contributes to this activity. EPEC-induced EGFR phosphorylation was blocked by the pharmacological inhibitor tyrphostin AG1478, as well as by EGFR-neutralizing antibodies. Inhibition of EGFR phosphorylation by AG1478 had no effect on bacterial adherence, actin recruitment to sites of attachment, or EPEC-induced epithelial barrier function alteration. EPEC-mediated Akt phosphorylation, however, was inhibited by both AG1478 and EGFR-neutralizing antibodies. Correspondingly, inhibition of EGFR activation increased the apoptosis/necrosis of infected epithelial cells. Inhibition of EGFR phosphorylation also curtailed EPEC-induced ERK1/2 (MAP kinase) phosphorylation and, correspondingly, the production of the proinflammatory cytokine interleukin-8 by infected epithelial cells. Our studies suggest that EGFR is a key proximal signaling molecule during EPEC pathogenesis.

Balance of bacterial pro- and anti-inflammatory mediators dictates net effect of enteropathogenic Escherichia coli on intestinal epithelial cells

Enteropathogenic Escherichia coli (EPEC) virulence requires a type III secretion system (TTSS) to deliver effector molecules in host cells. Although the TTSS is crucial to EPEC pathogenesis, its function in EPEC-induced inflammation is not known. The aim of this study was to investigate the role of the TTSS in EPEC-induced inflammation. HT-29 intestinal epithelial cells were infected with wild-type (WT) EPEC or select mutant strains or exposed to corresponding filter-sterilized supernatants (SN), and interleukin-8 (IL-8) secretion was determined by ELISA. EPEC SN stimulated significantly greater IL-8 production than EPEC organisms. Flagellin, as well as a TTSS-independent >50-kDa nonflagellin protein, was found to significantly contribute to this response. Dose-response studies showed that increasing concentrations of WT SN proportionally increased IL-8, whereas increasing multiplicity of infection of EPEC inversely correlated with IL-8 secretion, suggesting that EPEC dampens this host response. Infection with DeltaescN (nonfunctional TTSS) markedly increased IL-8 compared with WT, indicating that a functional TTSS is required for this anti-inflammatory property; complementation of escN restored the attenuated response. Mutation of espB also enhanced the IL-8 response, and complementation returned IL-8 to near WT levels, suggesting involvement of this effector. The anti-inflammatory effect extends to both bacterial and host-derived proinflammatory stimuli, since prior infection with EPEC suppressed the IL-8 response to tumor necrosis factor-alpha, IL-1beta, and enterohemorrhagic E. coli flagellin. These findings indicate that EPEC-induced inflammation is a balance between pro- and anti-inflammatory proteins; extracellular factors, including flagellin and an unidentified TTSS-independent, >50-kDa protein, trigger inflammation while intracellular TTSS-dependent factors, including EspB, attenuate this response.

lbtA and lbtB are required for production of the Legionella pneumophila siderophore legiobactin

Under iron stress, Legionella pneumophila secretes legiobactin, a nonclassical siderophore that is reactive in the chrome azurol S (CAS) assay. Here, we have optimized conditions for legiobactin expression, shown its biological activity, and identified two genes, lbtA and lbtB, which are involved in legiobactin production. lbtA appears to be iron repressed and encodes a protein that has significant homology with siderophore synthetases, and FrgA, a previously described iron-regulated protein of L. pneumophila. lbtB encodes a protein homologous with members of the major facilitator superfamily of multidrug efflux pumps. Mutants lacking lbtA or lbtB were defective for legiobactin, producing 40 to 70% less CAS reactivity in deferrated chemically defined medium (CDM). In bioassays, mutant CDM culture supernatants, unlike those of the wild type, did not support growth of iron-limited wild-type bacteria in 2',2'-dipyridyl-containing buffered charcoal yeast extract (BCYE) agar and a ferrous iron transport mutant on BCYE agar without added iron. The lbtA mutant was modestly defective for growth in deferrated CDM containing the iron chelator citrate, indicating that legiobactin is required in conditions of severe iron limitation. Complementation of the lbt mutants restored both siderophore expression, as measured by the CAS assay and bioassays, and bacterial growth in deferrated, citrate-containing media. The lbtA mutant replicated as the wild type did in macrophages, amoebae, and the lungs of mice. However, L. pneumophila expresses lbtA in the macrophage, suggesting that legiobactin, though not required, may play a dispensable role in intracellular growth. The discovery of lbtAB represents the first identification of genes required for L. pneumophila siderophore expression.

Enteropathogenic Escherichia coli EspG disrupts microtubules and in conjunction with Orf3 enhances perturbation of the tight junction barrier

EspG, a secreted effector of enteropathogenic Escherichia coli (EPEC), as well as its homologue Orf3, has been shown to disrupt microtubules (MTs) in fibroblasts and non-polarized epithelial cells. The roles of MTs and the effects of MT disruption in these cell types differ significantly. The aim of this study was to investigate the effects of EspG on polarized, host target intestinal epithelial cells. Immunofluorescent labelling of tubulin showed that EPEC caused progressive fragmentation and loss of the MT network in cells harbouring attached organisms. Immunoblots of proteins extracted from EPEC-infected cells showed a corresponding loss of alpha-tubulin. Type III secretion system (TTSS)-deficient strains had no effect on MT suggesting TTSS dependence. Mutation of espG, but not espF or map, ablated EPEC's effects on MTs for up to 6 h. Ectopic expression of EspG in HeLa cells caused MT disruption. While deletion of espG alone had no effect on the EPEC-induced decrease in transepithelial electrical resistance (TER), mutation of both espG and orf3 significantly delayed the kinetics of this response. Complementation of the double mutant with espG alone restored the kinetics of TER drop to that of wild type. Herein, we describe a previously unrecognized phenotype for the EPEC effectors EspG and Orf3.

Cytokeratin 18 interacts with the enteropathogenic Escherichia coli secreted protein F (EspF) and is redistributed after infection

Enteropathogenic Escherichia coli (EPEC) pathogenesis requires the delivery of effector proteins into host cytosol by a type III secretion system. The effector protein EspF, while critical for disruption of epithelial barrier function through alteration of tight junctions, is not required for bacterial viability or attachment. Yeast two-hybrid analyses revealed host intermediate filament (IF) protein cytokeratin 18 (CK18) as an interacting partner of EspF. This was confirmed by co-immunoprecipitation of extracts from EPEC-infected epithelial cells. EPEC infection increased detergent-soluble CK18 amounts without significantly altering CK18 expression. The adaptor protein 14-3-3 binds to CK18 and modulates its solubility. EPEC infection promoted CK18/14-3-3 interactions, corresponding to the increase of CK18 in the soluble fractions. 14-3-3 also co-immunoprecipitated with EspF, suggesting that CK18, 14-3-3 and EspF may form a complex in infected cells. The 14-3-3zeta isoform was co-immunoprecipitated with CK18, suggesting the involvement of specific signalling pathways. Immunofluorescence studies revealed a dramatic alteration in the architecture of the IF network in EPEC-infected epithelial cells. IF fragmentation, evident at 2 h post infection, progressed to a collapse of this network at later time points. The secretion mutant (DeltaescN) failed to alter CK18 solubility and IF morphology, while deletion of espF partially impaired the ability of EPEC to induce CK18 modifications. These results suggest that modifications in 14-3-3 interactions and IF network, modulated by type III secreted proteins, may be an important step in EPEC pathogenesis.

Differing roles of protein kinase C-zeta in disruption of tight junction barrier by enteropathogenic and enterohemorrhagic Escherichia coli

BACKGROUND & AIMS

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli harbor highly homologous pathogenicity islands yet show key differences in their mechanisms of action. Both disrupt host intestinal epithelial tight junctions, but the effects of enteropathogenic E. coli are more profound than those of enterohemorrhagic E. coli. The basis for this is not understood. The atypical protein kinase C isoform, protein kinase C-zeta, associates with and regulates the tight junction complex. The aim of this study was to compare the role of protein kinase C-zeta in the disruption of tight junctions after infection with enteropathogenic E. coli and enterohemorrhagic E. coli.

METHODS

Model intestinal epithelial monolayers infected by enteropathogenic E. coli or enterohemorrhagic E. coli were used for these studies.

RESULTS

Neither bisindolylmaleimide nor Gö6976, which block several protein kinase C isoforms but not protein kinase C-zeta, protected against the decrease in transepithelial electrical resistance after enteropathogenic E. coli infection. Rottlerin at concentrations that block novel and atypical isoforms, including protein kinase C-zeta, significantly attenuated the decrease in transepithelial electrical resistance. The specific inhibitory peptide, myristoylated protein kinase C-zeta pseudosubstrate, also significantly decreased the enteropathogenic E. coli -associated decrease in transepithelial electrical resistance and redistribution of tight junction proteins. In contrast to enteropathogenic E. coli, the level of protein kinase C-zeta enzyme activity stimulated by enterohemorrhagic E. coli was transient and minor, and protein kinase C-zeta inhibition had no effect on the decrease in transepithelial electrical resistance or the redistribution of occludin.

CONCLUSIONS

The differential regulation of protein kinase C-zeta by enteropathogenic E. coli and enterohemorrhagic E. coli may in part explain the less profound effect of the latter on the barrier function of tight junctions.

Comparative analysis of EspF from enteropathogenic and enterohemorrhagic Escherichia coli in alteration of epithelial barrier function

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) are related intestinal pathogens that harbor highly similar pathogenicity islands known as the locus of enterocyte effacement (LEE). Despite their genetic similarity, these two pathogens disrupt epithelial tight junction barrier function with distinct kinetics. EHEC-induced reduction in transepithelial electrical resistance (TER), a measure of barrier function disruption, is significantly slower and more modest in comparison to that induced by EPEC. The variation in bacterial adherence only partially accounted for these differences. The LEE-encoded effector protein EspF has been shown to be critical for EPEC-induced alterations in TER. EspF from both EPEC and EHEC is expressed and secreted upon growth in tissue culture medium. The mutation of EHEC cesF suggested that the optimal expression and secretion of EHEC EspF required its chaperone CesF, as has been shown for EPEC. In contrast to EPEC espF and cesF, mutation of the corresponding EHEC homologs did not dramatically alter the decrease in TER. These differences could possibly be explained by the presence of additional espF-like sequences (designated U- and M-espF, where the letter designations refer to the specific cryptic prophage sequences on the EHEC chromosome closest to the respective genes) in EHEC. Reverse transcription-PCR analyses revealed coordinate regulation of EHEC U-espF and the LEE-encoded espF, with enhanced expression in bacteria grown in Dulbecco-Vogt modified Eagle's medium compared to bacteria grown in Luria broth. Both EHEC espF and U-espF complemented an EPEC espF deletion strain for barrier function alteration. The overexpression of U-espF, but not espF, in wild-type EHEC potentiated the TER response. These studies reveal further similarities and differences in the pathogenesis of EPEC and EHEC.

Microbes and their products--physiological effects upon mammalian mucosa

A dynamic array of interactions occurs between pathogens and host mucosal surfaces. The signature molecules unique to microbial pathogens allow the mammalian immune system to recognize them as non-self. This recognition, mediated by the toll-like receptor proteins, results in innate immune responses targeted against the invading organism. Pathogens also elaborate a variety of proteins that actively engage host signaling pathways and subvert them to facilitate their growth and dispersal. These interactions, developed over a long evolutionary period, have been specialized to exquisite detail. These proteins and toxins are either secreted into the medium or directly delivered into host cells by specialized secretion systems. An array of host function alterations is mediated by microbial pathogens including inflammatory responses, secretory responses, alteration of host cytoskeleton, disruption of epithelial tight junctions and apoptosis. The signaling axes involved in these interactions are potential targets for therapeutic strategies against infectious microbes.

Intestinal epithelial responses to enteric pathogens: effects on the tight junction barrier, ion transport, and inflammation

The effects of pathogenic organisms on host intestinal epithelial cells are vast. Innumerable signalling pathways are triggered leading ultimately to drastic changes in physiological functions. Here, the ways in which enteric bacterial pathogens utilise and impact on the three major physiological functions of the intestinal epithelium are discussed: alterations in the structure and function of the tight junction barrier, induction of fluid and electrolyte secretion, and activation of the inflammatory cascade. This field of investigation, which was virtually non-existent a decade ago, has now exploded, thus rapidly expanding our understanding of bacterial pathogenesis. Through increased delineation of the ways in which microbes alter host physiology, we simultaneous gain insight into the normal regulatory mechanisms of the intestinal epithelium.

The cytochrome c maturation locus of Legionella pneumophila promotes iron assimilation and intracellular infection and contains a strain-specific insertion sequence element

Previously, we obtained a Legionella pneumophila mutant, NU208, that is hypersensitive to iron chelators when grown on standard Legionella media. Here, we demonstrate that NU208 is also impaired for growth in media that simply lack their iron supplement. The mutant was not, however, impaired for the production of legiobactin, the only known L. pneumophila siderophore. Importantly, NU208 was also highly defective for intracellular growth in human U937 cell macrophages and Hartmannella and Acanthamoeba amoebae. The growth defect within macrophages was exacerbated by treatment of the host cells with an iron chelator. Sequence analysis demonstrated that the transposon disruption in NU208 lies within an open reading frame that is highly similar to the cytochrome c maturation gene, ccmC. CcmC is generally recognized for its role in the heme export step of cytochrome biogenesis. Indeed, NU208 lacked cytochrome c. Phenotypic analysis of two additional, independently derived ccmC mutants confirmed that the growth defect in low-iron medium and impaired infectivity were associated with the transposon insertion and not an entirely spontaneous second-site mutation. trans-complementation analysis of NU208 confirmed that L. pneumophila ccmC is required for cytochrome c production, growth under low-iron growth conditions, and at least some forms of intracellular infection. Although ccm genes have recently been implicated in iron assimilation, our data indicate, for the first time, that a ccm gene can be required for bacterial growth in an intracellular niche. Complete sequence analysis of the ccm locus from strain 130b identified the genes ccmA-H. Interestingly, however, we also observed that a 1.8-kb insertion sequence element was positioned between ccmB and ccmC. Southern hybridizations indicated that the open reading frame within this element (ISLp 1) was present in multiple copies in some strains of L. pneumophila but was absent from others. These findings represent the first evidence for a transposable element in Legionella and the first identification of an L. pneumophila strain-specific gene.

Innate immunity and the gut

The intestinal epithelium encounters a unique environment consisting of microbes, both commensals and pathogens, as well as dietary nutrients and antigens. This complex composition necessitates the presence of a dynamic system of defense to contain both pathogenic and commensal bacteria within the lumen yet allow for nutrient absorption. Tight junctions provide protection of the intercellular spaces while other surface molecules, such as intestinal trefoil factor, help to maintain the structural integrity of the epithelium. Other more active processes, including upregulated expression and activation of antimicrobial peptides and enhanced fluid secretion, provide a second level of innate defense. Despite providing the interface between an exuberant immune system and a highly antigenic lumenal environment, the intestinal epithelium must remain quiescent. As such, several novel antiinflammatory mechanisms were recently identified. Studies that elaborate the various aspects of these pathways are discussed in this review.

The Legionella pneumophila iraAB locus is required for iron assimilation, intracellular infection, and virulence

Legionella pneumophila, a facultative intracellular parasite of human alveolar macrophages and protozoa, causes Legionnaires' disease. Using mini-Tn10 mutagenesis, we previously isolated a L. pneumophila mutant that was hypersensitive to iron chelators. This mutant, NU216, and its allelic equivalent, NU216R, were also defective for intracellular infection, particularly in iron-deficient host cells. To determine whether NU216R was attenuated for virulence, we assessed its ability to cause disease in guinea pigs following intratracheal inoculation. NU216R-infected animals yielded 1,000-fold fewer bacteria from their lungs and spleen compared to wild-type-130b-infected animals that had received a 50-fold-lower dose. Moreover, NU216R-infected animals subsequently cleared the bacteria from these sites. While infection with 130b resulted in high fever, weight loss, and ruffled fur, inoculation with NU216R did not elicit any signs of disease. DNA sequence analysis revealed that the transposon insertion in NU216R lies in the first open reading frame of a two-gene operon. This open reading frame (iraA) encodes a 272-amino-acid protein that shows sequence similarity to methyltransferases. The second open reading frame (iraB) encodes a 501-amino-acid protein that is highly similar to di- and tripeptide transporters from both prokaryotes and eukaryotes. Southern hybridization analyses determined that the iraAB locus was largely limited to strains of L. pneumophila, the most pathogenic of the Legionella species. A newly derived mutant containing a targeted disruption of iraB showed reduced ability to grow under iron-depleted extracellular conditions, but it did not have an infectivity defect in the macrophage-like U937 cells. These data suggest that iraA is critical for virulence of L. pneumophila while iraB is involved in a novel method of iron acquisition which may utilize iron-loaded peptides.

Template secondary structure promotes polymerase jumping during PCR amplification

Pairs of primers flanking known miniTn10 transposon insertion sites were used to confirm the presence of the transposon in DNA isolated from Legionella pneumophila mutants. It was expected that the polymerase chain reaction products derived from the mutant template would be larger than those from the wild-type (WT) template due to the presence of the 1.8-kb transposon. Instead, it was observed that the mutant template yielded a product of almost the same size as that yielded by WT template. We present evidence to indicate that the aberrant product from the mutant template is a direct result of secondary structure of the template resulting from an inverted repeat sequence present in the miniTn10 transposon.

Identification and temperature regulation of Legionella pneumophila genes involved in type IV pilus biogenesis and type II protein secretion

Previously, we had isolated by transposon mutagenesis a Legionella pneumophila mutant that appeared defective for intracellular iron acquisition. While sequencing in the proximity of the mini-Tn10 insertion, we found a locus that had a predicted protein product with strong similarity to PilB from Pseudomonas aeruginosa. PilB is a component of the type II secretory pathway, which is required for the assembly of type IV pili. Consequently, the locus was cloned and sequenced. Within this 4-kb region were three genes that appeared to be organized in an operon and encoded homologs of P. aeruginosa PilB, PilC, and PilD, proteins essential for pilus production and type II protein secretion. Northern blot analysis identified a transcript large enough to include all three genes and showed a substantial increase in expression of this operon when L. pneumophila was grown at 30 degrees C as opposed to 37 degrees C. The latter observation was then correlated with an increase in piliation when bacteria were grown at the lower temperature. Southern hybridization analysis indicated that the pilB locus was conserved within L. pneumophila serogroups and other Legionella species. These data represent the first isolation of type II secretory genes from an intracellular parasite and indicate that the legionellae express temperature-regulated type IV pili.

Isolation of a gene encoding a novel spectinomycin phosphotransferase from Legionella pneumophila

A gene capable of conferring spectinomycin resistance was isolated from Legionella pneumophila, the agent of Legionnaires' disease. The gene (aph) encoded a 36-kDa protein which has similarity to aminoglycoside phosphotransferases. Biochemical analysis confirmed that aph encodes a phosphotransferase which modifies spectinomycin but not hygromycin, kanamycin, or streptomycin. The strain that was the source of aph demonstrated resistance to spectinomycin, and Southern hybridizations determined that aph also exists in other legionellae.

Kinetic characterization of 4-amino 4-deoxychorismate synthase from Escherichia coli

The metabolic fate of p-aminobenzoic acid (PABA) in Escherichia coli is its incorporation into the vitamin folic acid. PABA is derived from the aromatic branch point precursor chorismate in two steps. Aminodeoxychorismate (ADC) synthase converts chorismate and glutamine to ADC and glutamate and is composed of two subunits, PabA and PabB. ADC lyase removes pyruvate from ADC, aromatizes the ring, and generates PABA. While there is much interest in the mechanism of chorismate aminations, there has been little work done on the ADC synthase reaction. We report that PabA requires a preincubation with dithiothreitol for maximal activity as measured by its ability to support the glutamine-dependent amination of chorismate by PabB. PabB glutamine enhances the protective effect of PabA. Incubation with fresh dithiothreitol reverses the inactivation of PabB. We conclude that both PabA and PabB have cysteine residues which are essential for catalytic function and/or for subunit interaction. Using conditions established for maximal activity of the proteins, we measured the Km values for the glutamine-dependent and ammonia-dependent aminations of chorismate, catalyzed by PabB alone and by the ADC synthase complex. Kinetic studies with substrates and the inhibitor 6-diazo-5-oxo-L-norleucine were consistent with an ordered bi-bi mechanism in which chorismate binds first. No inhibition of ADC synthase activity was observed when p-aminobenzoate, sulfanilamide, sulfathiazole, and several compounds requiring folate for their biosynthesis were used.