Master's Theses



Degree Name

Master of Science (MS)


Given the alternative functions of “antibiotics” as communication molecules or participants in metabolism, it seems probable that production might be influenced by factors such as nutrient availability, interactions with neighboring microbes, and/or colony or community structure and maturity. With this in mind, the present study aimed to broaden the scope of the search for novel antibiotics by experimenting with the following parameters: source of bacterial isolation, growth and assay media, and culturing techniques. Bacteria for this study were isolated from two categories of soil (petroleum-contaminated or uncontaminated) to compare diversity and antimicrobial activity. Compared to the uncontaminated soil, isolates of the petroleum-contaminated soil were as diverse and antimicrobial activity was as frequent. Antimicrobial assays were done on three different types of agar, including the standard Mueller-Hinton and two types of medium typically used for fungal growth, Yeast Peptone Dextrose (YPD) and Yeast Mold (YM). Compared to results on Mueller-Hinton, much more antimicrobial activity was seen when using YPD and YM. Finally, spent media assays were performed with pure and mixed cultures to determine if exposure to a target pathogen affects the production of antimicrobial substances by soil isolates. Those bacteria with activity against Pseudomonas aeruginosa in perpendicular streak tests were grown as mixed cultures with P. aeruginosa. In the case of Bacillus amyloliquefaciens and Pseudomonas marginalis, discs impregnated with concentrated spent media from these mixed cultures resulted in significant dose-dependent inhibition of P. aeruginosa. The same assay using pure cultures showed no inhibition.


Dr. Eric T. Gillock

Date of Award

Spring 2013

Document Type



© 2013 Joanna L. Fay


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