Visualizing the attack of bacteria by the antimicrobial peptide human defensin 5

Abstract

Bacterial infections are a major cause of concern in healthcare because of a rise in antibiotic-resistant bacteria and an increase in hospital-acquired infections. In order to combat bacterial infections, we need fundamental understanding of the host-pathogen interaction. As a part of the innate immune response, various organisms, including humans, produce antimicrobial peptides. Human defensin 5 (HD5) is a 32-aa cysteine-rich peptide produced primarily in the small intestine that exhibits broad-spectrum activity against various bacteria, fungi and viruses. In this thesis, in order to study and understand the mechanism of action of HD5, we probed the antibacterial action of HD5 and the bacterial response. We synthesized a family of HD5 analogues with functional modifications including fluorophores. With this toolkit of HD5 derivatives, we examined the effect of HD5 on various bacteria. We demonstrated that HD5ox, the oxidized form of HD5, causes certain distinct morphological changes in Gram-negative bacteria, enters the bacterial cytoplasm and localizes near the cell poles and cell division sites. From these studies we propose that HD5ox overcomes the outer membrane permeability barrier and permeabilizes the inner membrane of Gram-negative bacteria, and it may interact with the cellular targets and interfere with the processes such as cell division. We extended these morphological studies of HD5 0x-treated bacteria and identified certain phenotypic responses in Gram-positive bacteria that further suggest that HD5ox interferes with cellular processes such as cell division.