"Streptococcus agalactiae, or Group B Streptococcus (GBS), is a highly diverse species that can be found asymptomatically colonizing the gastrointestinal and genital tracts of healthy adults, but is also capable of causing severe invasive disease. GBS is a leading cause of sepsis and meningitis in neonates and the only preventative measure is antibiotic therapy given to pregnant mothers during labor to prevent transmission. Although this method was effective at reducing case rates upon implementation, case rates have remained unchanged since the initial decline and some mothers remain persistently colonized by GBS. Due to the high level of diversity among strains it is important to understand how strains differ at the various stages of disease progression in order to have a more complete understanding of GBS pathogenesis. Here, I examined how genotypically diverse strains differ in their interactions with human cells. The examination of strains of the same serotype in ability to associate with decidual cells and lung epithelial cells revealed that strains within the same serotype, and even the same sequence type (ST) differed in attachment and invasion, but this variation was dependent on host cell type. More specifically, strains of the hypervirulent lineage, ST-17 associated with decidual cells significantly more than the other STs, but the opposite was true for lung epithelial cell attachment. Mechanisms of persistent colonization was explored by comparing antibiotic tolerance and macrophage survival between ST-17 and ST-12 strains, which persisted and was eradicated after antibiotic prophylaxis, respectively. This study revealed that although the ST-17 strain was not tolerant to antibiotics, subinhibitory antibiotics enhanced phagocytic uptake of this strain where it was able to survive for an extended period of time. Additionally, intracellular survival of the ST-17 strain was dependent on acidification of the phagosome, whereas altered pH had no effect on survival of the ST12 strain, suggesting GBS can use different mechanisms of survival. Moreover, serotype III GBS strains were better able to survive phagosomal stress compared to other serotypes. Lastly, transcriptome analysis of the ST-17 strain during intracellular survival revealed temporal gene expression responses to long term survival and identified a large number of factors important for intracellular survival. Through mutagenesis studies, the roles of NADH peroxidase (Npx) and cadmium resistance protein (CadD) in GBS intracellular survival was examined. These studies demonstrated that Npx promotes resistance to reactive oxygen stress through detoxification of hydrogen peroxide and CadD serves as a heavy metal efflux pump to confer resistance to intoxication by certain divalent metal cations. The work described here reveals new insights in GBS pathogenesis and helps identify key virulence factors that can serve as targets for alternative therapeutics and vaccine development."--Pages ii-iii.
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