Doctoral Degrees (Microbiology)

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Browsing Doctoral Degrees (Microbiology) by Author "Booysen, Elzaan"

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    Profiling of the secondary metabolites and the characterization two novel antilisterial peptides, xenopep and rhabdin, produced by xenorhabdus khoisanae
    (Stellenbosch : Stellenbosch University, 2022-04) Booysen, Elzaan; Dicks, Leon Milner Theodore; Rautenbach, Marina; Stellenbosch University. Faculty of Science. Dept. of Microbiology.
    ENGLISH ABSTRACT: In the early 1900’s the discovery of sulfonamides and penicillin paved the way for antibiotics and led to a boom in the discovery of other antibiotics. Unfortunately, this boom was short lived and soon the discovery and approval of novel antibiotics by the food and drug association and other similar associations dwindled. With the ever-increasing prevalence of antibiotic resistant pathogens this soon became a problem that was not foreseen. Most antibiotics currently on the market have been isolated from a select few genera. With nearly all the antibiotics from such few sources, bacteria were able to acquire resistance at an enhanced pace. This study focused on a relatively unexplored niche for novel antibiotics, from the genus Xenorhabdus. Species of this genus is mutually associated with Steinernema nematodes and have a unique life cycle. Xenorhabdus spp. are known to produce various secondary metabolites (SMs) that have antimicrobial, insecticidal, antiviral, immunosuppressant and proteolytic properties. Species from this genus use different synthesis machineries to produce these compounds, although the majority are produced via the non-ribosomal peptide synthesis. The ability of non-ribosomal peptides to incorporate non-proteogenic amino acids, D-amino acids, fatty chains, or polyketide chains result in unique resistance to proteinases and environmental stressors. Xenorhabdus khoisanae J194 is mutually associated with Steinernema jeffreyense J194, a nematode that was isolated from soil in the Eastern Cape. Culture conditions, especially oxygen, greatly affected SM production of X. khoisanae J194. PAX peptides, xenocoumacins and xenoamicins were identified in the cell-free crude extract of X. khoisanae J194 cultures. Two novel antilisterial peptides, xenopep and rhabdin, were also detected in the cell-free crude extract of. Xenopep has a narrow spectrum of activity and inhibited the growth of only, Listeria monocytogenes and Staphylococcus epidermidis, while rhabdin is active against both Gram-positive and Gram-negative bacteria. Xenopep and rhabdin share numerous characteristics and both contain a tetra-peptide in their structure including a tetra-peptide in their structure. Both peptides share the same amphipathic characteristic and behave similar suspension. Membrane potential and ATP release assays have shown that xenopep formed pores/lesions in the cell membrane of L. monocytogenes within minutes, followed by a rapid decrease in cell numbers over 3 hours. Scanning electron microscopy (SEM) images of L. monocytogenes treated with xenopep became elongated and formed filaments. This suggests that xenopep may inhibit penicillin binding protein three. This is the first study reporting on SMs produced by X. khoisanae when cultured under different conditions and is the first detailed description of antilisterial peptides produced by the species.