The role of the PKS island in polymicrobial interactions between Escherichia coli and Staphylococcus aureus during wound infections

Abstract

Wound infections are often polymicrobial in nature and are associated with poorer disease prognosis. Escherichia coli and Staphylococcus aureus are among the top five most cultured pathogens during wound infections. However, little is known about the polymicrobial interactions between E. coli and S. aureus during wound infections. The aim of this thesis is to understand the molecular mechanism behind the polymicrobial interactions between E. coli and S. aureus. Using co-culture experiments, we showed that E. coli inhibits the growth of S. aureus both in vitro and in a mouse excisional wound model. To determine the E. coli factors responsible for S. aureus inhibition, we performed an E. coli transposon mutant library screen and identified mutants unable to inhibit the growth of S. aureus. The transposon mutant library screen revealed that both the E. coli pks island and the BarA-UvrY two component system (TCS) were essential for growth inhibition of S. aureus both in vitro and in vivo. To identify the pks island dependent factors involved in the growth inhibition of S. aureus, we investigated the ability of N-myristoyl-D-Asn (NMDA) and colibactin, a genotoxin that causes DNA damage in eukaryotic cells and is associated with human colorectal cancer, to inhibit S. aureus growth. Overexpression of the immunity protein, ClbS, protected S. aureus from E. coli mediated growth inhibition, suggesting that E. coli colibactin causes DNA damage in S. aureus resulting in S. aureus death. Finally, we showed that BarA-UvrY TCS is a novel regulator of the pks island, which acts through the carbon storage regulator (Csr) system, a global regulatory system. Together, our data demonstrated the role of colibactin in inter-species competition and showed that it is regulated by BarA-UvrY TCS, a previously undescribed regulator of the pks island.