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Advisory Committee Chair

Susan K Hollingshead

Advisory Committee Members

Bernard Beall

William H Benjamin, Jr

Elliot J Lefkowitz

David G Pritchard

Document Type

Dissertation

Date of Award

2010

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

The relative contributions of point mutations and intergenomic recombination via lateral gene transfer (LGT) determine the population structure and evolutionary style for a given bacterial species. Streptococcus pneumoniae has a weakly clonal population structure; adaptive clonal complexes can be detected; however, these can also be rapidly lost due to the high rate of recombination. To characterize the diversity of pneumococcal isolates in clonal complex, and to elucidate possible mechanisms for the long-term stability of such complexes, 35 serotype 6B strains belonging to a previously identified clonal complex, CC14, were assayed by Comparative Genome Hybridization (CGH) and Multi Locus Sequence Typing (MLST). Additionally, six isolates of serotype 4, which is known to be less diverse than serotype 6B, were also analyzed. Genome plasticity in pneumococci is observed in the acquisition of antibiotic resistance and epidemiological shifts in serotype. LGT is a prime mediator. The scope of transformation, or the number of unlinked and unselected recombination events that occur simultaneously, is an aspect not fully understood. Whole genome variation, particularly unselected exchanges, was followed after in vitro transformation by analyzing transformants along with their respective donor and recipient parent by CGH. Transformants were capsule-switch mutants, selected by one of two means: 1) screening for colony size in a TIGR4 cps::Janus recipient x GA71_19F donor transformation, and 2) selected for the acquisition of single-step resistance to penicillin or cefotaxime in a TIGR4 x GA71_19F cross; this co-selected for replacement of the capsule region because pbp2X and pbp1A are adjacent. Nearly 40 genes were found absent or divergent in at least one of the capsule-switch mutants; sequencing was used to verify CGH results. Some were unlinked to the capsule locus, indicating multiple recombination events per transformation. Unlinked variation was observed in at least 5 of 8 capsule-switch transformations. Furthermore, a gene annotated as hypothetical, SP1581, was implicated in resistance to cefotaxime. CGH confirms that transfer of gene regions up to 45.9 kb is possible during natural transformation and unselected variation is common. This supports the hypothesis that transformation is capable of driving the high level of genome plasticity in S. pneumoniae.

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