I am a physician-scientist trained clinically in pediatrics and scientifically as a molecular microbiologist. I leverage this duel training to develop new methods to improve the quality of care and research conducted during large-scale infectious disease outbreaks. Our lab studies diarrheal diseases which kill one in ten children globally between the ages of 1 month and 5 years of life. We target the disease cholera because of the devastating impact the disease has on poor and vulnerable populations worldwide. In addition, cholera is a model system for how water-borne diseases infect and transmit in resource-poor settings. Cholera is one of the fastest killing pathogens from the time of drinking contaminated water to the time of death (<20 hours). Thankfully, access to, and knowledgeable use, of oral fluids like pedialyte can drop case fatality rates from 20-40% to less than 1%. Our primary clinical research challenge is to ask how we might leverage mobile technology to improve both the access and knowledge to high-quality care. To do this we collaborate with the Stanford Computer Science Department, the Stanford Geospatial Center, and the Bangladesh Ministry of Health and Family Welfare. We have built and are actively testing a prototype android technology in Northern Bangladesh. These clinical endeavors are enhanced and informed by an ambitious basic science agenda to define how selective pressures like antibiotics and lytic phage exacerbate disease severity, disrupt the commensal intestinal microflora, and influence transmission. We test and validate field observations in animal models of cholera at Stanford. This co-dependence of clinical and basic science has made for exciting translational research that we hope benefits patients and improves our fundamental understanding of pathogenesis and disease transmission.
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Instructor, Pediatrics - Infectious Diseases
Honors & Awards
NIH Director's Early Independence Award, National Institutes of Health (US) (2014-2019)
Boards, Advisory Committees, Professional Organizations
Senior Fellow, Stanford Center for Innovation in Global Health (2015 - Present)
Board Certification, American Board of Pediatrics (2014)
Residency, Stanford University School of Medicine, Pediatrics (2013)
MD, Tufts University School of Medicine (2010)
PhD, Tufts University School of Medicine, Molecular Biology and Microbiology (2010)
MS, University of Hawaii, Microbiology (2001)
BA, Cornell University, Biology (1998)
S. Schild, E. Nelson, A. Camilli. "United States Patent W02009049013 A2 Cholera Vaccines (Outer Membrane Vesicle Vaccine)", Tufts University, Oct 9, 2009
- Is a Cholera Outbreak Preventable in Post-earthquake Nepal? PLoS neglected tropical diseases 2015; 9 (8)
- Antibiotics for Both Moderate and Severe Cholera. NEW ENGLAND JOURNAL OF MEDICINE 2011; 364 (1): 5-7
- Beyond cholera-the Zimbabwe health crisis LANCET INFECTIOUS DISEASES 2009; 9 (10): 587-588
Cholera transmission: the host, pathogen and bacteriophage dynamic
NATURE REVIEWS MICROBIOLOGY
2009; 7 (10): 693-702
Zimbabwe offers the most recent example of the tragedy that befalls a country and its people when cholera strikes. The 2008-2009 outbreak rapidly spread across every province and brought rates of mortality similar to those witnessed as a consequence of cholera infections a hundred years ago. In this Review we highlight the advances that will help to unravel how interactions between the host, the bacterial pathogen and the lytic bacteriophage might propel and quench cholera outbreaks in endemic settings and in emergent epidemic regions such as Zimbabwe.
View details for DOI 10.1038/nrmicro2204
View details for Web of Science ID 000269831000014
View details for PubMedID 19756008
High Prevalence of Spirochetosis in Cholera Patients, Bangladesh
EMERGING INFECTIOUS DISEASES
2009; 15 (4): 571-573
The microbes that accompany the etiologic agent of cholera, Vibrio cholerae, are only now being defined. In this study, spirochetes from the genus Brachyspira were identified at high titers in more than one third of cholera patients in Bangladesh. Spirochetosis should now be tracked in the setting of cholera outbreaks.
View details for DOI 10.3201/eid1504.081214
View details for Web of Science ID 000264644900009
View details for PubMedID 19331734
Characterization of Vibrio cholerae Outer Membrane Vesicles as a Candidate Vaccine for Cholera
INFECTION AND IMMUNITY
2009; 77 (1): 472-484
Outer membrane vesicles (OMVs) offer a new approach for an effective cholera vaccine. We recently demonstrated that immunization of female mice with OMVs induces a long-lasting immune response and results in protection of their neonatal offspring from Vibrio cholerae intestinal colonization. This study investigates the induced protective immunity observed after immunization with OMVs in more detail. Analysis of the stomach contents and sera of the neonates revealed significant amounts of anti-OMV immunoglobulins (Igs). Swapping of litters born to immunized and nonvaccinated control mice allowed us to distinguish between prenatal and neonatal uptakes of Igs. Transfer of Igs to neonates via milk was sufficient for complete protection of the neonates from colonization with V. cholerae, while prenatal transfer alone reduced colonization only. Detection of IgA and IgG1 in the fecal pellets of intranasally immunized adult mice indicates an induced immune response at the mucosal surface in the gastrointestinal tract, which is the site of colonization by V. cholerae. When a protocol with three intranasal immunizations 14 days apart was used, the OMVs proved to be efficacious at doses as low as 0.025 microg per immunization. This is almost equivalent to OMV concentrations found naturally in the supernatants of LB-grown cultures of V. cholerae. Heterologous expression of the periplasmic alkaline phosphatase (PhoA) of Escherichia coli resulted in the incorporation of PhoA into OMVs derived from V. cholerae. Intranasal immunization with OMVs loaded with PhoA induced a specific immune response against this heterologous antigen in mice. The detection of an immune response against this heterologously expressed protein is a promising step toward the potential use of OMVs as antigen delivery vehicles in vaccine design.
View details for DOI 10.1128/IAI.01139-08
View details for Web of Science ID 000262776100049
View details for PubMedID 19001078
Immunization with Vibrio cholerae outer membrane vesicles induces protective immunity in mice
INFECTION AND IMMUNITY
2008; 76 (10): 4554-4563
The gram-negative bacterium Vibrio cholerae releases outer membrane vesicles (OMVs) during growth. In this study, we immunized female mice by the intranasal, intragastric, or intraperitoneal route with purified OMVs derived from V. cholerae. Independent of the route of immunization, mice induced specific, high-titer immune responses of similar levels against a variety of antigens present in the OMVs. After the last immunization, the half-maximum total immunoglobulin titer was stable over a 3-month period, indicating that the immune response was long lasting. The induction of specific isotypes, however, was dependent on the immunization route. Immunoglobulin A, for example, was induced to a significant level only by mucosal immunization, with the intranasal route generating the highest titers. We challenged the offspring of immunized female mice with V. cholerae via the oral route in two consecutive periods, approximately 30 and 95 days after the last immunization. Regardless of the route of immunization, the offspring was protected against colonization with V. cholerae in both challenge periods. Our results show that mucosal immunizations via both routes with OMVs derived from V. cholerae induce long-term protective immune responses against this gastrointestinal pathogen. These findings may contribute to the development of "nonliving," OMV-based vaccines against V. cholerae and other enteric pathogens, using the oral or intranasal route of immunization.
View details for DOI 10.1128/IAI.00532-08
View details for Web of Science ID 000259293100022
View details for PubMedID 18678672
Transmission of Vibrio cholerae Is Antagonized by Lytic Phage and Entry into the Aquatic Environment
2008; 4 (10)
Cholera outbreaks are proposed to propagate in explosive cycles powered by hyperinfectious Vibrio cholerae and quenched by lytic vibriophage. However, studies to elucidate how these factors affect transmission are lacking because the field experiments are almost intractable. One reason for this is that V. cholerae loses the ability to culture upon transfer to pond water. This phenotype is called the active but non-culturable state (ABNC; an alternative term is viable but non-culturable) because these cells maintain the capacity for metabolic activity. ABNC bacteria may serve as the environmental reservoir for outbreaks but rigorous animal studies to test this hypothesis have not been conducted. In this project, we wanted to determine the relevance of ABNC cells to transmission as well as the impact lytic phage have on V. cholerae as the bacteria enter the ABNC state. Rice-water stool that naturally harbored lytic phage or in vitro derived V. cholerae were incubated in a pond microcosm, and the culturability, infectious dose, and transcriptome were assayed over 24 h. The data show that the major contributors to infection are culturable V. cholerae and not ABNC cells. Phage did not affect colonization immediately after shedding from the patients because the phage titer was too low. However, V. cholerae failed to colonize the small intestine after 24 h of incubation in pond water-the point when the phage and ABNC cell titers were highest. The transcriptional analysis traced the transformation into the non-infectious ABNC state and supports models for the adaptation to nutrient poor aquatic environments. Phage had an undetectable impact on this adaptation. Taken together, the rise of ABNC cells and lytic phage blocked transmission. Thus, there is a fitness advantage if V. cholerae can make a rapid transfer to the next host before these negative selective pressures compound in the aquatic environment.
View details for DOI 10.1371/journal.ppat.1000187
View details for Web of Science ID 000261481100020
View details for PubMedID 18949027
- Cholera Outbreak Training and Shigellosis Program (COTS Program). International Centre for Diarrhoeal Disease Research, Bangladesh. 2008; www.cotsprogram.org
Complexity of rice-water stool from patients with Vibrio cholerae plays a role in the transmission of infectious diarrhea
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2007; 104 (48): 19091-19096
At the International Centre for Diarrhoeal Disease Research, Bangladesh, one-half of the rice-water stool samples that were culture-positive for Vibrio cholerae did not contain motile V. cholerae by standard darkfield microscopy and were defined as darkfield-negative (DF(-)). We evaluated the host and microbial factors associated with DF status, as well as the impact of DF status on transmission. Viable counts of V. cholerae in DF(-) stools were three logs lower than in DF(+) stools, although DF(-) and DF(+) stools had similar direct counts of V. cholerae by microscopy. In DF(-) samples, non-V. cholerae bacteria outnumbered V. cholerae 10:1. Lytic V. cholerae bacteriophage were present in 90% of DF(-) samples compared with 35% of DF(+) samples, suggesting that bacteriophage may limit culture-positive patients from producing DF(+) stools. V. cholerae in DF(-) and DF(+) samples were found both planktonically and in distinct nonplanktonic populations; the distribution of organisms between these compartments did not differ appreciably between DF(-) and DF(+) stools. This biology may impact transmission because epidemiological data suggested that household contacts of a DF(+) index case were at greater risk of infection with V. cholerae. We propose a model in which V. cholerae multiply in the small intestine to produce a fluid niche that is dominated by V. cholerae. If lytic phage are present, viable counts of V. cholerae drop, stools become DF(-), other microorganisms bloom, and cholera transmission is reduced.
View details for DOI 10.1073/pnas.0706352104
View details for Web of Science ID 000251498700042
View details for PubMedID 18024592
Genes induced late in infection increase fitness of Vibrio cholerae after release into the environment
CELL HOST & MICROBE
2007; 2 (4): 264-277
The facultative pathogen Vibrio cholerae can exist in both the human small bowel and in aquatic environments. While investigation of the infection process has revealed many factors important for pathogenesis, little is known regarding transmission of this or other water-borne pathogens. Using a temporally controlled reporter of transcription, we focus on bacterial gene expression during the late stage of infection and identify a unique class of V. cholerae genes specific to this stage. Mutational analysis revealed limited roles for these genes in infection. However, using a host-to-environment transition assay, we detected roles for six of ten genes examined for the ability of V. cholerae to persist within cholera stool and/or aquatic environments. Furthermore, passage through the intestinal tract was necessary to observe this phenotype. Thus, V. cholerae genes expressed prior to exiting the host intestinal tract are advantageous for subsequent life in aquatic environments.
View details for DOI 10.1016/j.chom.2007.09.004
View details for Web of Science ID 000250362500009
View details for PubMedID 18005744
A novel lux operon in the cryptically bioluminescent fish pathogen Vibrio salmonicida is associated with virulence
APPLIED AND ENVIRONMENTAL MICROBIOLOGY
2007; 73 (6): 1825-1833
The cold-water-fish pathogen Vibrio salmonicida expresses a functional bacterial luciferase but produces insufficient levels of its aliphatic-aldehyde substrate to be detectably luminous in culture. Our goals were to (i) better explain this cryptic bioluminescence phenotype through molecular characterization of the lux operon and (ii) test whether the bioluminescence gene cluster is associated with virulence. Cloning and sequencing of the V. salmonicida lux operon revealed that homologs of all of the genes required for luminescence are present: luxAB (luciferase) and luxCDE (aliphatic-aldehyde synthesis). The arrangement and sequence of these structural lux genes are conserved compared to those in related species of luminous bacteria. However, V. salmonicida strains have a novel arrangement and number of homologs of the luxR and luxI quorum-sensing regulatory genes. Reverse transcriptase PCR analysis suggests that this novel arrangement of quorum-sensing genes generates antisense transcripts that may be responsible for the reduced production of bioluminescence. In addition, infection with a strain in which the luxA gene was mutated resulted in a marked delay in mortality among Atlantic salmon relative to infection with the wild-type parent in single-strain challenge experiments. In mixed-strain competition between the luxA mutant and the wild type, the mutant was attenuated up to 50-fold. It remains unclear whether the attenuation results from a direct loss of luciferase or a polar disturbance elsewhere in the lux operon. Nevertheless, these findings document for the first time an association between a mutation in a structural lux gene and virulence, as well as provide a new molecular system to study Vibrio pathogenesis in a natural host.
View details for DOI 10.1128/AEM.02255-06
View details for Web of Science ID 000245156800017
View details for PubMedID 17277225
Cholera stool bacteria repress chemotaxis to increase infectivity
2006; 60 (2): 417-426
Factors that enhance the transmission of pathogens are poorly understood. We show that Vibrio cholerae shed in human 'rice-water' stools have a 10-fold lower oral infectious dose in an animal model than in vitro grown V. cholerae, which may aid in transmission during outbreaks. Furthermore, we identify a bacterial factor contributing to this enhanced infectivity: The achievement of a transient motile but chemotaxis-defective state upon shedding from humans. Rice-water stool V. cholerae have reduced levels of CheW-1, which is essential for chemotaxis, and were consequently shown to have a chemotaxis defect when tested in capillary assays. Through mutational analyses, such a state is known to enhance the infectivity of V. cholerae. This is the first report of a pathogen altering its chemotactic state in response to human infection in order to enhance its transmission.
View details for DOI 10.1111/j.1365-2958.2006.05096.x
View details for Web of Science ID 000236328700014
View details for PubMedID 16573690
- Isolation and identification of Pseudoalteromonas piscicida strain Cura-d associated with diseased damselfish (Pomacentridae) eggs Journal of Fish Diseases 1999; 22: 253-260