The dominant plant type of Pepperdine’s Malibu campus is chaparral, of which an important indicator species is Adenostoma fasciculatum. It is known that A. fasciculatum produces compounds that are deadly to many bacteria. However, California is currently experiencing unprecedented drought, which is pushing plants well beyond their normal stress levels. In this study, we investigated the effect of drought stress on the ability of A. fasciculatum to inhibit growth of bacteria. We hypothesized that extract from drought-stressed plants would be better able to kill bacteria than hydrated ones, because the plant would be induced to produce more antimicrobial compounds under stress. To test our hypothesis, we collected samples from regularly-watered and drought-stressed A. fasciculatum plants. We then took soil samples from the hydrated collection site and the drought-stressed site. We created extract by mixing leaves from each sample and soaking them in ethanol. We then cultured bacteria from both soil types and picked 3 strains from each plate to represent the bacterial population. We isolated those strains and used the Kirby-Bauer method to examine the plant’s ability to inhibit bacterial growth. Additionally, we used the same methods to test Staphylococcus epidermidis to investigate the plant’s abilities to kill totally foreign bacteria. Our data showed that drought-stressed plants were unable to kill any soil bacteria, but hydrated plants were capable of inhibiting growth of bacteria cultured from dry soil. Moreover, both drought-stressed and healthy plants were able to inhibit growth of S. epidermidis. These results went against our original hypothesis, demonstrating that drought-stressed A. fasciculatum are less resistant to bacteria than healthy plants.
Shute, Andrew; Duff, Jennifer; and Pearce-Harris, Salina, "Effects of Drought on Antimicrobial Properties of a Chaparral Indicator Species — Adenostoma fasciculatum" (2016). Pepperdine University, Featured Research. Paper 194.