Could Eating Dirt be Good for You? Inquiring Students in Microbiology Want to Know.
It is estimated that each gram of soil has about 1 billion bacterial cells. In the Microbiology class, the students searched through soil to find antibiotic producing bacteria.
Bacteria of the genus Streptomyces live in soil and are famous for producing antibiotics, so the students grew and then tested these bacteria.
Streptomyces also produce geosmin, the molecule that is responsible for the earthy smell of soil. The lab odors were quite nice during this analysis.
Left: Ashley adds soil to a buffered solution. Right: Sanjana and Angie weigh out soil from in front of Elizabeth’s apartment.
Left: Corinna spreads the dilute soil sample onto selective plates that should enable Streptomyes to grow. Right: Ghady and Ester dilute the soil solution.
Right: After 4 days, there are lots of bacterial colonies that look like Streptomyces (chalky surface), and smell like Streptomyces, but microscopic analysis is needed to confirm this. Left: Olivia and Carol look at bacteria under the microscope where they observed filamentous bacteria with spores—Streptomyces!!!
Left: Pure cultures of Streptomyces were grown in the middle of a plate for 4 days, then the antibiotic activity assay follows: All test bacteria were inoculated perpendicular to the Streptomyces at each marker line, right up to where the Streptomyces had grown. After an incubation, the test bacteria grew unless they were killed by an antibiotic released by the Streptomyces. Middle: This culture of Streptomyces (yellow, vertical) is inhibiting some of the test bacteria. Staphylococcus and Enterococcus growth is inhibited, but E. coli, Psuedomonas, and Enterococcus grew. Bonus points to anyone who knows the structure that differs between the 2 susceptible bacteria and the 3 resistant bacteria. Right: This culture of Streptomyes did not produce an antibiotic.
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