Theses and Dissertations

Issuing Body

Mississippi State University


Thornton, Justin A.

Committee Member

Gordon, Donna M.

Committee Member

Jordan, Heather R.

Committee Member

Rosch, Jason W.

Committee Member

Emerson, Joseph P.

Date of Degree


Original embargo terms


Document Type

Dissertation - Open Access


Biological Sciences (Microbiology)

Degree Name

Doctor of Philosophy


College of Arts and Sciences


Department of Biological Sciences


Homeostasis of trace metal ions is essential for a variety of cellular processes and virulence mechanisms, including resistance to oxidative stress, DNA replication, and regulation of cell adhesion/invasion. Understanding how pathogenic microorganisms overcome metal starvation and intoxication provides insight into how these mechanisms could be targeted by novel antimicrobial therapies. Streptococcus pneumoniae, or pneumococcus, is a Gram-positive, commensal of the human nasopharynx and upper respiratory tract. Though this organism is primarily an asymptomatic colonizer, it is also the causative agent of infections ranging in severity from reoccurring acute otitis media to life-threatening community acquired pneumonia or bacterial sepsis. This study aims to characterize aspects of pneumococcal physiology and infection that are responsive to changes in micronutrient zinc availability. Two zinc-binding lipoproteins of S. pneumoniae, AdcA and AdcAII, were characterized as playing redundant roles in zinc acquisition; however, this study shows that these proteins are not equally sensitive to zinc starvation and have additional functionality in adhesion and invasion. Mutant strains lacking AdcAII but not AdcA suffer decreased fitness when exposed to a zinc-chelator; and following chelation adcAII was upregulated 42old whereas adcA was only upregulated 4old. Zinc-deficient mutants lacking AdcA and AdcAII show increased invasion at levels reaching 200-300% compared to parental strains. Additionally, AdcAII-deficient strains show decreased ability to adhere to epithelial cells and colonize nasal tissues during murine challenge, suggesting a role for AdcAII or zinc homeostasis in biofilm formation. Analysis of biofilms grown in varying concentrations of metals revealed that increased zinc, specifically, resulted in the formation of larger, more architecturally complex biofilms. The increase in biofilm size was determined to be due to the formation of cell-to-cell aggregates. In addition to encountering high concentrations of metals, pathogens are competing for micronutrients from the host, and are thus adept to surviving metal starvation. A previously uncharacterized operon, SP1434-1438, was found to be sensitive to zinc-starvation and proteomics strongly suggest an importance for these genes in cellular metabolism. These results have identified roles for zinc homeostasis in cell adhesion, colonization, cell and bloodstream invasion, biofilm formation, and the maintenance of cellular metabolism.