This study is designed to examine the interaction of group A Streptococcus (GAS) with saliva. The key goal of the proposed research is to initiate the molecular investigation of a crucial area of GAS pathogenesis. Importantly, GAS-saliva interaction has been the subject of little inquiry despite knowing for 60 years that saliva is a major vector for the transmission of GAS from patients who have pharyngitis. Also, given that saliva coats the posterior pharynx, there is constant interplay between saliva and GAS during pharyngeal infection. Knowledge of how GAS responds to saliva will augment our understanding of how GAS infects the human oropharynx and spreads from patients with pharyngitis. Aim 1: To asses GAS global gene expression when cultivated in human saliva. For this aim, we will use Affymetrix custom-designed expression microarrays to analyze the GAS transcriptome during growth in human saliva. We will optimize use of the expression microarray analysis by futher investigation of GAS genes that are highly expressed during growth in saliva or that are upregulated during growth in saliva compared to growth in standard laboratory media. We will focus our inquiry on genes encoding extracellular proteins, as these are most likely to participate in host-pathogen interaction. Aim 2: To determine whether targeted GAS extracellular protein-encoding genes highly expressed or upregulated during in vitro GAS-saliva interaction are expressed similarly in vivo in humans with pharyngitis. Aim 3: To establish whether specific GAS extracellular proteins are produced during growth in human saliva. Aim 4: To determine whether inactivation of particular GAS extracellular protein-encoding genes identified by successful completion of Specific Aims 1-3 results in altered growth in human saliva. As GAS-saliva interaction has been minimally investigated, many of the genes examined in the proposed research are likely to be part of the 40% of the GAS genome encoding proteins of unknown function. This project seeks to contribute to fundamental understanding of GAS pathogenesis in the human oropharynx which may stimulate new avenues of investigation for therapeutic or preventive measures for this common pathogen. GAS pharyngitis costs an estimated $15 billion/year in the US alone, and as the instigator of rheumatic fever, GAS remains, worldwide, the leading cause of preventable pediatric heart disease.