Thesis advisor: Tim Van Opijnen / Exposure to antibiotics has previously been shown to induce morphological changes to bacterial cells in Escherichia coli and Staphylococcus aureus . Response profiles to antibiotics representing various mechanisms of action provides as quick, reliable and cheap means of identifying the mechanism of action of novel antimicrobials. We evaluated whether similar cytological profiling was possible in the pathogenic bacteria Streptococcus pneumoniae and whether there were any strain specific differences in morphological changes resulting from antibiotic exposure. We evaluated antibiotics from various classes and with different mechanisms of action to develop strain specific models of phenotypic responses in order to identify clustering associated with particular mechanisms of action. Various antibiotics belonging to, cell wall synthesis inhibitors, protein synthesis inhibitors, and DNA synthesis inhibitors were evaluated using S. pneumoniae strains TIGR4 and 19F. Following exposure to high doses of antibiotics, cells were imaged for DNA and cell wall components and analyzed. Our data shows that antibiotics of the same mechanism of action induce similar morphological changes. While TIGR4 and 19F show similar changes there are strain specific differences between them. Our data shows that cytological profiling effectively indicates the mechanism of action through imaging in S. pneumoniae allowing this technique to be used to study novel antimicrobials as well as better understand bacterial responses to antibiotic stress. / Thesis (BS) — Boston College, 2019. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Biology.