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Toxoplasma gondii is an obligate intracellular parasite. It has extraordinary abilities to infect virtually any warm blooded animal, cross a number of biological barriers (intestine, placenta and blood-brain), and invade any nucleated cell tested thus far in vitro. Chapter 1 provides a general introduction to Toxoplasma gondii and its ability to invade the host cell. Emphasis is placed on biological barriers and molecular mechanism of invasion for the different forms of the parasite. Chapter 2 describes a study of how Toxoplasma crosses the human placenta. Chapter 3 provides an insight as to how Enteric Glial Cells contribute to oral Toxoplasma infection in vivo. Chapter 4 deals with the study of how Toxoplasma sporozoites invade host cells. All research topics shed light on details of how Toxoplasma crosses the anatomical barriers: from the tissue barriers of placenta to infection to the possible immune and barrier functions of Enteric Glial Cells to how Toxoplasma sporozoites invade host cells. Chapter 2 describes experiments demonstrating that Toxoplasma tachyzoites invade the human placenta. These experiments used the human placental explant system to determine where the tachyzoites first invade, how they travel down the anchoring villi, and whether there exist type differences in ability to invade this tissue. This study provided several insights: (i) Toxoplasma tachyzoites are unable to penetrate the syncytium, and invade only the exposed tissue (anchoring villi); (ii) There does not appear to be a significant difference in the ability of different strains to infect the tissue. Chapter 3 addresses the possible role of TGF-[beta] by Enteric Glial Cells (EGCs) in oral Toxoplasma infection. Mice expressing a Dominant Negative TGF-[beta] receptor in EGCs were orally infected with Toxoplasma tissue cysts. At the end of the time trials, mice were sacrificed and their intestines subjected to histological analysis. Experiments did not reveal a clear-cut contribution to the control of inflammation by this cell type to the progress of disease. Chapter 4 examines the function of two paralogues of AMA1 and RON2 that are present exclusively in Toxoplasma sporozoites. Dubbed sporoAMA1 and sporoRON2, respectively, that sporoRON2 and sporoAMA1 form an exclusive pairing, stabilized with unique structural features. Furthermore, pre-treating sporozoites with a C-terminal portion of sporoRON2 (but not generic RON2) inhibits sporozoite invasion. We see sporoRON2 in a different compartment from RON4, suggesting that sporoRON2 functions independently of known moving junction components. These data indicate that sporozoites' host cell invasion is dependent on a novel, stage-specific version of the AMA1-RON2 pairing.

Overview: 555 million preschool children live in developing countries where malnutrition (undernutrition) is a significant problem. The factors that contribute to this issue are multiple, with two important factors being gut microbiota and gut function. To begin to understand gut microbiota and gut function in a developing country, we performed two parallel studies: in the first study, we characterized the baseline gut microbiota of healthy Bangladeshi children, and in the second study, we investigated the association between household environmental conditions and gut function in Bangladeshi children. The gut microbiota affect nutrient metabolism, immune function, and pathogen resistance. Loss of gut function as occurs in environmental enteropathy (a multifaceted, subclinical intestinal disorder likely derived from repeated episodes of infectious gastroenteritis and chronic inflammation) contributes to malnutrition and growth faltering. It is possible that these processes are interrelated with the gut microbiota serving a critical role in environmental enteropathy. Gut microbiota: Previous studies have focused on the gut microbiota of infants and adults from developed countries, but few have characterized the gut microbiota of people living in developing countries. Furthermore, little is known about the gut microbiota and its variation over time in primary-school-age children and adolescents in either developed or developing countries. In this study, we compared the monthly variation in fecal microbiota composition of healthy children (ages 8--14 years) living in an urban slum in Bangladesh with that of age-matched children from affluent suburban communities in the United States. We analyzed 8,000 near full-length sequences of the small-subunit ribosomal RNA gene (16S rDNA) and over 845,000 pyrosequencing reads spanning the V1-V3 variable region of the 16S rDNA. The distal gut of Bangladeshi children harbored significantly higher microbial diversity than U.S. children, a result validated by the discovery of several novel lineages from various bacterial phyla in the full-length Bangladeshi dataset. Bangladeshi and U.S. children displayed distinct gut microbiota architectures. Moreover, significantly lower month-to-month temporal stability was observed in Bangladeshi children compared to U.S. children. Together, these results suggest that different environmental or genetic variables may affect the microbiota of healthy children in the two countries. Further studies are necessary to investigate the underlying mechanisms of these differences and to incorporate these insights into prevention or treatment of diseases. Gut function: Although poor water quality, sanitation, and hygiene are putative risk factors for environmental enteropathy, there are no studies investigating the relationship of household environmental conditions to environmental enteropathy. We compared levels of environmental enteropathy markers to growth patterns and measured the prevalence of parasitic infections in children living in different levels of environmental cleanliness based on indictors of water, sanitation, and hygiene in their households. We conducted a follow-up study of 119 children (< 4 years old) from an existing cohort, living in rural Bangladesh and divided into two types of households: a 'mitigated household' had good water quality, improved sanitation, and a handwashing station stocked with water and soap, and a 'contaminated household' had poor water quality, and inadequate sanitation and handwashing facilities. We measured anthropometry, intestinal parasitic infection, and the following markers to assess gut function: Lactulose:Mannitol (L:M) ratio, immunoglobulin G endotoxin core antibody (IgG EndoCAb), and total IgG. After adjustment for potential confounding, children from mitigated households had higher height-for-age Z-scores (HAZ), lower L:M ratios, and lower IgG EndoCAb titers than children from contaminated households. The L:M ratio was also strongly associated with HAZ in the cohort. Children living in environmentally mitigated households had lower levels of parasitic infection, improved measures of gut function, and improved growth compared to children living in contaminated environments. These results support the hypothesis that environmental contamination mediated through environmental enteropathy could be a cause of growth faltering in low-income countries. Interpretation: By characterizing the baseline gut microbiota of healthy children living in a developing country and correlating their gut function with household environmental conditions, our work provides a foundation to address the role of the gut microbiota and environmental enteropathy in child health.

The importance of the intestinal microbiota to the health of an organism is well recognized. Alterations in the composition of the microbiota are associated with a number of diseases. However, the extent to which the microbiota varies across healthy individuals and time and the causes of this variation are still not fully understood. In order to distinguish between microbial community states of health and disease, the range of healthy microbial communities must be determined. In addition, the effect of perturbations on the microbiota is an area of active research. Recent studies indicate that intestinal infections can have a significant impact on the composition of intestinal microbiota. However, the extent and duration of these changes during different infections have not been fully characterized. My aims were to characterize the range of healthy microbiota in an inbred mouse population over time and to determine the dynamics of alterations in the microbiota during disease. I also wanted to determine if certain characteristics of the microbiota make animals more or less susceptible to infection. In this work, 46 individual healthy mice were sampled over time to assess the range of microbial communities in health. A mouse model of Salmonella enterica serovar Typhimurium infection was used to assess the impact of infection on the microbiota. Mice were sampled prior to infection and over the course of infection. This work finds that the microbiota of healthy mice varies significantly over time and among individuals. However, there are stable differences among individual mice, which may be established at the point of weaning when adult microbial communities develop. In addition, this work finds significant changes in the microbiota during infection that differ with the level of infection. This data also shows that there are some taxa whose abundance prior to infection correlates with outcome of infection. This work provides insights into the dynamics of the microbiota in health and disease and its impact on the health of the host.