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  • Writer's pictureEnid Gonzalez-Orta

The Positives and Gram Negatives of Potato Dextrose Agar by Noah N.


Noah determining the soil texture of his sample.

Selecting the right type of growth media is an essential decision to the process of culturing any bacterial samples. Various media have the ability to yield different products as a result of the nutrient sources and environment provided, an idea known as selectivity. At the beginning of my journey in the Tiny Earth Project, I was faced with this decision: Which growth media should I choose and why? After some back and forth, I chose Potato Dextrose Agar, also known as PDA, which is commonly used for the growth and cultivation of yeast and molds in agricultural and food settings (Hernandez et al, 2020, p. 196). I chose this media type because other students who have participated in this project have yielded antibiotic producing bacteria, but it seemed to be under utilized when compared to other types of media. I decided that I wanted to take on the challenge of exploring PDA culturing media and all that it has to offer.

Thus far in the project, out of 40 selected bacterial colonies from our original soil samples, I have had 20 demonstrate antibiotic properties when plated against our safe alternatives for common bacterial pathogens. Also, my results so far show a common theme: PDA seemingly appeals to gram negative bacteria more than gram positive. These two cell types possess different properties in their outer cell membranes that assist with survival in their typical environments. Our safer alternative pathogens that are gram negative have grown better on PDA than the positives, all of which required dilutions in order to observe any possible antibiotic activity. In addition to this, all of my antibiotic producing isolates have been determined to be gram negative after completing a gram stain, which is a process that allows a visual characterization of the peptidoglycan content surrounding the plasma membrane of the cell. The gram positive pathogens showed signs of growth, but only one out of six undiluted test plates grew enough of a bacterial lawn to interpret results. With all of this in mind, I am very happy.


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