Document Type


Date of Degree

Fall 2016

Degree Name

PhD (Doctor of Philosophy)

Degree In


First Advisor

Horswill, Alexander R

First Committee Member

Allen, Lee-Ann H

Second Committee Member

Diekema, Daniel J

Third Committee Member

Ellermeier, Craig D

Fourth Committee Member

Schlievert, Patrick M


The Staphylococcus genus comprises a diverse group of Gram-positive bacteria that are opportunistic pathogens of humans and other mammals. S. epidermidis and S. aureus are the most common human pathogens of the staphylococci, causing a variety of infections including biofilm-based medical device infections, skin infections, and pneumonia. Both of these organisms produce proteases whose functions in virulence are not fully characterized. In S. epidermidis, protein-mediated biofilm formation requires a cell wall-anchored adhesin called Aap that must be proteolytically processed in order to allow intercellular adhesion. The S. epidermidis protease(s) responsible for cleaving Aap were unknown. Chapter II describes our findings that the secreted metalloprotease SepA is required for Aap-mediated biofilm formation and cleaves Aap at two different sites. Further, this protease is negatively regulated by the global regulator SarA.

Chapter III discusses studies of the S. aureus Spl (serine protease-like) proteases. Although they are produced in vivo, their substrates and role in virulence are unknown. We found that in a rabbit model of pneumonia, a mutant lacking the spl protease operon caused more localized disease compared to wild type S. aureus. Proteomics studies of the secreted and surface proteins in wild type compared to spl mutant S. aureus revealed several changes. We also found that the SplA protease cleaves human Mucin-16, the first identification of a biological substrate of the Spls.

Finally, we found that the animal-associated species S. caprae produces an autoinducing peptide (AIP) that is a potent inhibitor of S. aureus quorum sensing. We identified the S. caprae AIP structure as an 8-residue thiolactone ring. A synthetic version of the peptide inhibits S. aureus virulence and quorum sensing induction in a murine skin infection model. This is a novel example of quorum sensing cross talk between staphylococcal quorum sensing systems. These studies are described in Appendix A.

On the whole, this work identified two substrates of S. aureus proteases and demonstrated their importance in biofilm formation and infection. We also characterized a novel inhibitor of S. aureus quorum sensing that attenuates virulence. These findings shed light on the importance of staphylococcal secreted proteases and quorum sensing cross talk in the modulation of virulence factor production and the ability to cause disease.

Public Abstract

Staphylococci are a group of bacteria that cause a tremendous amount of infections in humans. Staphylococci produce protein-cutting enzymes that are thought to be involved in infection, but their specific functions are unknown. We studied two of these enzymes and found out what proteins they cut. Further, we identified how their activities might contribute to infections caused by staphylococci. One of the enzymes is required for staphylococci to form a thick, multilayered community of bacteria called a biofilm. The other enzyme’s role in infection is less clear, but it seems to alter the ability of the staphylococci to damage an infected host. In another project, we found that one staphylococcal species is able to interfere with virulence in another staphylococcal species. This inhibitor could be used to prevent or treat infections.


bacterial pathogenesis, biofilm, protease, staphylococci


xii, 138 pages


Includes bibliographical references (pages 117-138).


Copyright © 2016 Alexandra E. Paharik

Included in

Microbiology Commons