Suppressor Mutations in Type II Secretion Mutants of Vibrio cholerae: Inactivation of the VesC Protease.

The type II secretion system (T2SS) is a conserved transport pathway responsible for the secretion of a range of virulence factors by many pathogens, including Vibrio cholerae Disruption of the T2SS genes in V. cholerae results in loss of secretion, changes in cell envelope function, and growth defects. While T2SS mutants are viable, high-throughput genomic analyses have listed these genes among essential genes. To investigate whether secondary mutations arise as a consequence of T2SS inactivation, we sequenced the genomes of six V. cholerae T2SS mutants with deletions or insertions in either the epsG, epsL, or epsM genes and identified secondary mutations in all mutants. Two of the six T2SS mutants contain distinct mutations in the gene encoding the T2SS-secreted protease VesC. Other mutations were found in genes coding for V. cholerae cell envelope proteins. Subsequent sequence analysis of the vesC gene in 92 additional T2SS mutant isolates identified another 19 unique mutations including insertions or deletions, sequence duplications, and single-nucleotide changes resulting in amino acid substitutions in the VesC protein. Analysis of VesC variants and the X-ray crystallographic structure of wild-type VesC suggested that all mutations lead to loss of VesC production and/or function. One possible mechanism by which V. cholerae T2SS mutagenesis can be tolerated is through selection of vesC-inactivating mutations, which may, in part, suppress cell envelope damage, establishing permissive conditions for the disruption of the T2SS. Other mutations may have been acquired in genes encoding essential cell envelope proteins to prevent proteolysis by VesC.IMPORTANCE Genome-wide transposon mutagenesis has identified the genes encoding the T2SS in Vibrio cholerae as essential for viability, but the reason for this is unclear. Mutants with deletions or insertions in these genes can be isolated, suggesting that they have acquired secondary mutations that suppress their growth defect. Through whole-genome sequencing and phenotypic analysis of T2SS mutants, we show that one means by which the growth defect can be suppressed is through mutations in the gene encoding the T2SS substrate VesC. VesC homologues are present in other Vibrio species and close relatives, and this may be why inactivation of the T2SS in species such as Vibrio vulnificus, Vibrio sp. strain 60, and Aeromonas hydrophila also results in a pleiotropic effect on their outer membrane assembly and integrity.

Understanding key drivers and barriers to implementation of the WHO recommendations for the case management of childhood pneumonia and possible serious bacterial infection with amoxicillin dispersible tablets (DT) in Bangladesh: a qualitative study.

BACKGROUND: Pneumonia and possible serious bacterial infection (PSBI) are leading causes of death among under-five children. The World Health Organization (WHO) issued global recommendations for the case management of childhood pneumonia and PSBI when referral is not feasible with oral amoxicillin. However, few governments to date have incorporated child-friendly amoxicillin dispersible tablets (DT) into their national treatment guidelines and policies. We aimed to understand the key drivers to the implementation of WHO recommendations for childhood pneumonia and PSBI using amoxicillin DT in Bangladesh. METHODS: A qualitative study was conducted from October 2017 to March 2018 in two districts of Bangladesh. Interviews were completed with 67 participants consisting of government officials and key stakeholders, international development agencies, health service providers (HSPs), and caregivers of young children diagnosed and treated with amoxicillin for pneumonia or PSBI. Data were analyzed thematically. RESULTS: Policies and operational planning emerged as paramount to ensuring access to essential medicines for childhood pneumonia and PSBI. Though amoxicillin DT is included for National Newborn Health Programme and Integrated Management of Childhood Illnesses in the Operational Plan of the Directorate General of Health Services, inclusion in Community-Based Healthcare Project and Directorate General of Family Planning policies is imperative to securing national supply, access, and uptake. At the sub-national level, training on the use of amoxicillin DT as a first line intervention is lacking, resulting in inadequate management of childhood pneumonia by HSPs. Advocacy activities are needed to create community-wide demand among key stakeholders, HSPs, and caregivers not yet convinced that amoxicillin DT is the preferred formulation for the management of childhood pneumonia and PSBI. CONCLUSION: Challenges in policy and supply at the national level and HSP preparedness at the sub-national levels contribute to the slow adoption of WHO recommendations for amoxicillin DT in Bangladesh. A consultation meeting to disseminate study findings was instrumental in driving the development of recommendations by key stakeholders to address these challenges. A comprehensive and inclusive evidence-based strategy involving all divisions of the Ministry of Health and Family Welfare will be required to achieve national adoption of WHO recommendations and country-wide introduction of amoxicillin DT in Bangladesh.

A model for oxygen conservation associated with titration during pediatric oxygen therapy.

BACKGROUND: Continuous oxygen treatment is essential for managing children with hypoxemia, but access to oxygen in low-resource countries remains problematic. Given the high burden of pneumonia in these countries and the fact that flow can be gradually reduced as therapy progresses, oxygen conservation through routine titration warrants exploration. AIM: To determine the amount of oxygen saved via titration during oxygen therapy for children with hypoxemic pneumonia. METHODS: Based on published clinical data, we developed a model of oxygen flow rates needed to manage hypoxemia, assuming recommended flow rate at start of therapy, and comparing total oxygen used with routine titration every 3 minutes or once every 24 hours versus no titration. RESULTS: Titration every 3 minutes or every 24 hours provided oxygen savings estimated at 11.7% ± 5.1% and 8.1% ± 5.1% (average ± standard error of the mean, n = 3), respectively. For every 100 patients, 44 or 30 kiloliters would be saved-equivalent to 733 or 500 hours at 1 liter per minute. CONCLUSIONS: Ongoing titration can conserve oxygen, even performed once-daily. While clinical validation is necessary, these findings could provide incentive for the routine use of pulse oximeters for patient management, as well as further development of automated systems.

Reservoir Cannulas for Pediatric Oxygen Therapy: A Proof-of-Concept Study.

Hypoxemia is a complication of pneumonia-the leading infectious cause of death in children worldwide. Treatment generally requires oxygen-enriched air, but access in low-resource settings is expensive and unreliable. We explored use of reservoir cannulas (RCs), which yield oxygen savings in adults but have not been examined in children. Toddler, small child, and adolescent breathing profiles were simulated with artificial lung and airway models. An oxygen concentrator provided flow rates of 0 to 5 L/min via a standard nasal cannula (NC) or RC, and delivered oxygen fraction (FdO2) was measured. The oxygen savings ratio (SR) and absolute flow savings (AFS) were calculated, comparing NC and RC. We demonstrated proof-of-concept that pendant RCs could conserve oxygen during pediatric therapy. SR mean and standard deviation were 1.1 ± 0.2 to 1.4 ± 0.4, 1.1 ± 0.1 to 1.7 ± 0.3, and 1.3 ± 0.1 to 2.4 ± 0.3 for toddler, small child, and adolescent models, respectively. Maximum AFS observed were 0.3 ± 0.3, 0.2 ± 0.1, and 1.4 ± 0.3 L/min for the same models. RCs have the potential to reduce oxygen consumption during treatment of hypoxemia in children; however, further evaluation of products is needed, followed by clinical analysis in patients.

mPneumonia, an Innovation for Diagnosing and Treating Childhood Pneumonia in Low-Resource Settings: A Feasibility, Usability and Acceptability Study in Ghana.

Pneumonia is the leading cause of infectious disease mortality in children. Currently, health care providers (HCPs) are trained to use World Health Organization Integrated Management of Childhood Illness (IMCI) paper-based protocols and manually assess respiratory rate to diagnose pneumonia in low-resource settings (LRS). However, this approach of relying on clinical signs alone has proven problematic. Hypoxemia, a diagnostic indicator of pneumonia severity associated with an increased risk of death, is not assessed because pulse oximetry is often not available in LRS. To improve HCPs' ability to diagnose, classify, and manage pneumonia and other childhood illnesses, "mPneumonia" was developed. mPneumonia is a mobile health application that integrates a digital version of the IMCI algorithm with a software-based breath counter and a pulse oximeter. A design-stage qualitative pilot study was conducted to assess feasibility, usability, and acceptability of mPneumonia in six health centers and five community-based health planning and services centers in Ghana. Nine health administrators, 30 HCPs, and 30 caregivers were interviewed. Transcribed interview audio recordings were coded and analyzed for common themes. Health administrators reported mPneumonia would be feasible to implement with approval and buy-in from national and regional decision makers. HCPs felt using the mPneumonia application would be feasible to integrate into their work with the potential to improve accurate patient care. They reported it was "easy to use" and provided confidence in diagnosis and treatment recommendations. HCPs and caregivers viewed the pulse oximeter and breath counter favorably. Challenges included electricity requirements for charging and the time needed to complete the application. Some caregivers saw mPneumonia as a sign of modernity, increasing their trust in the care received. Other caregivers were hesitant or confused about the new technology. Overall, this technology was valued by users and is a promising innovation for improving quality of care in frontline health facilities.

mPneumonia: Development of an Innovative mHealth Application for Diagnosing and Treating Childhood Pneumonia and Other Childhood Illnesses in Low-Resource Settings.

Pneumonia is the leading infectious cause of death in children worldwide. Each year, pneumonia kills an estimated 935,000 children under five years of age, with most of these deaths occurring in developing countries. The current approach for pneumonia diagnosis in low-resource settings--using the World Health Organization Integrated Management of Childhood Illness (IMCI) paper-based protocols and relying on a health care provider's ability to manually count respiratory rate--has proven inadequate. Furthermore, hypoxemia--a diagnostic indicator of the presence and severity of pneumonia often associated with an increased risk of death--is not assessed because pulse oximetry is frequently not available in low-resource settings. In an effort to address childhood pneumonia mortality and improve frontline health care providers' ability to diagnose, classify, and manage pneumonia and other childhood illnesses, PATH collaborated with the University of Washington to develop "mPneumonia," an innovative mobile health application using an Android tablet. mPneumonia integrates a digital version of the IMCI algorithm with a software-based breath counter and a pediatric pulse oximeter. We conducted a design-stage usability field test of mPneumonia in Ghana, with the goal of creating a user-friendly diagnostic and management tool for childhood pneumonia and other childhood illnesses that would improve diagnostic accuracy and facilitate adherence by health care providers to established guidelines in low-resource settings. The results of the field test provided valuable information for understanding the usability and acceptability of mPneumonia among health care providers, and identifying approaches to iterate and improve. This critical feedback helped ascertain the common failure modes related to the user interface design, navigation, and accessibility of mPneumonia and the modifications required to improve user experience and create a tool aimed at decreasing mortality from pneumonia and other childhood illnesses in low-resource settings.

Functional and structural characterization of Vibrio cholerae extracellular serine protease B, VesB.

The chymotrypsin subfamily A of serine proteases consists primarily of eukaryotic proteases, including only a few proteases of bacterial origin. VesB, a newly identified serine protease that is secreted by the type II secretion system in Vibrio cholerae, belongs to this subfamily. VesB is likely produced as a zymogen because sequence alignment with trypsinogen identified a putative cleavage site for activation and a catalytic triad, His-Asp-Ser. Using synthetic peptides, VesB efficiently cleaved a trypsin substrate, but not chymotrypsin and elastase substrates. The reversible serine protease inhibitor, benzamidine, inhibited VesB and served as an immobilized ligand for VesB affinity purification, further indicating its relationship with trypsin-like enzymes. Consistent with this family of serine proteases, N-terminal sequencing implied that the propeptide is removed in the secreted form of VesB. Separate mutagenesis of the activation site and catalytic serine rendered VesB inactive, confirming the importance of these features for activity, but not for secretion. Similar to trypsin but, in contrast to thrombin and other coagulation factors, Na(+) did not stimulate the activity of VesB, despite containing the Tyr(250) signature. The crystal structure of catalytically inactive pro-VesB revealed that the protease domain is structurally similar to trypsinogen. The C-terminal domain of VesB was found to adopt an immunoglobulin (Ig)-fold that is structurally homologous to Ig-folds of other extracellular Vibrio proteins. Possible roles of the Ig-fold domain in stability, substrate specificity, cell surface association, and type II secretion of VesB, the first bacterial multidomain trypsin-like protease with known structure, are discussed.

A dodecameric ring-like structure of the N0 domain of the type II secretin from enterotoxigenic Escherichia coli.

In many bacteria, secretins from the type II secretion system (T2SS) function as outer membrane gated channels that enable passage of folded proteins from the periplasm into the extracellular milieu. Cryo-electron microscopy of the T2SS secretin GspD revealed previously the dodecameric cylindrical architecture of secretins, and crystal structures of periplasmic secretin domains showed a modular domain organization. However, no high-resolution experimental data has as yet been provided about how the entire T2SS secretin or its domains are organized in a cylindrical fashion. Here we present a crystal structure of the N0 domain of the T2SS secretin GspD from enterotoxigenic Escherichia coli containing a helix with 12 subunits per turn. The helix has an outer diameter of ∼125Å and a pitch of only 24Å which suggests a model of a cylindrical dodecameric N0 ring whose dimensions correspond with the cryo-electron microscopy map of Vibrio cholerae GspD. The N0 domain is known to interact with the HR domain of the inner membrane T2SS protein GspC. When the new N0 ring model is combined with the known N0·HR crystal structure, a dodecameric double-ring of twelve N0-HR heterodimers is obtained. In contrast, the previously observed compact N0-N1 GspD module is not compatible with the N0 ring. Interestingly, a N0-N1 T3SS homolog is compatible with forming a N0-N1 dodecameric ring, due to a different N0-vs-N1 orientation. This suggests that the dodecameric N0 ring is an important feature of T2SS secretins with periplasmic domains undergoing considerable motions during exoprotein translocation.

Screening a fragment cocktail library using ultrafiltration.

Ultrafiltration provides a generic method to discover ligands for protein drug targets with millimolar to micromolar K(d), the typical range of fragment-based drug discovery. This method was tailored to a 96-well format, and cocktails of fragment-sized molecules, with molecular masses between 150 and 300 Da, were screened against medical structural genomics target proteins. The validity of the method was confirmed through competitive binding assays in the presence of ligands known to bind the target proteins.

The binding of cholera toxin to the periplasmic vestibule of the type II secretion channel.

The type II secretion system (T2SS) is a large macromolecular complex spanning the inner and outer membranes of many gram-negative bacteria. The T2SS is responsible for the secretion of virulence factors such as cholera toxin (CT) and heat-labile enterotoxin (LT) from Vibrio cholerae and enterotoxigenic Escherichia coli, respectively. CT and LT are closely related AB5 heterohexamers, composed of one A subunit and a B-pentamer. Both CT and LT are translocated, as folded protein complexes, from the periplasm across the outer membrane through the type II secretion channel, the secretin GspD. We recently published the 19 Å structure of the V. cholerae secretin (VcGspD) in its closed state and showed by SPR measurements that the periplasmic domain of GspD interacts with the B-pentamer complex. Here we extend these studies by characterizing the binding of the cholera toxin B-pentamer to VcGspD using electron microscopy of negatively stained preparations. Our studies indicate that the pentamer is captured within the large periplasmic vestibule of VcGspD. These new results agree well with our previously published studies and are in accord with a piston-driven type II secretion mechanism.