Exploring the effects of cranberry extract on the growth of sinorhizobium meliloti (ensifer meliloti) when paired with known antimicrobial compounds

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Denner, Abraham Verschelden
McDonald, David

Huanglongbing disease (HLB), also known as citrus greening disease, is devastating the citrus industry. It is caused by three species of Candidatus Liberibacter. The presentation of this disease in North America is primarily caused by the asiaticus species. Ca. L. asiaticus is spread by the Asian Citrus Psyllid Diaphorina citri during feeding. Current treatments include broad-spectrum antibiotics, and targeting of the insect vector, both having thus far demonstrated little effectiveness. This has led to our effort to find more suitable antimicrobial substances for this purpose. Finding novel antimicrobials that are effective against Ca. L. asiaticus is essential to combat the spread and severity of HLB. Sometimes, when using antimicrobials together, they can produce a powerful effect that is greater than the sum of the parts; this is known as synergy. By focusing on using candidate antimicrobials in combination in hopes of finding synergistic reactions, one can minimize the potential of antimicrobial resistance forming. As Ca. L. asiaticus cannot be readily cultured in labs, an alternative model must be used. For this research, we used Sinorhizobium meliloti (S. meliloti) as the target bacterium due to its being closely related as well as being more robust due to its free-living capabilities. Cranberries contain phenolic compounds such as proanthocyanidins that can interfere with and repress biofilm formation and alter selective membrane permeability and multidrug efflux pumps. Plant-derived antimicrobials such as methylglyoxal, oregano oil, and clove oil have a variety of antimicrobial mechanisms. This project sought to test these plant-derived antimicrobials in combination with cranberry extract to see if they demonstrated inhibition of growth against the target bacteria. Serial dilutions of cranberry extracts paired with plant-derived antimicrobials were assessed in a checkerboard assay in the presence of S. meliloti. By finding the minimum inhibitory concentrations (MICs) and the fractional inhibitory concentrations (FICs), the level of synergy was determined. While synergy was not demonstrated in this study, two of the plant-derived antimicrobials, methylglyoxal and oregano oil, combined with cranberry extract did prove to be additive. These additive combinations are still beneficial and could be further studied as potential treatment sources or as a basis for further research regarding effective antimicrobial candidates to combat Ca. L. asiaticus based on their mechanisms and efficacy together.

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Thesis (M.S.)-- Wichita State University, College of Liberal Arts and Sciences, Dept. of Biological Sciences