Week 10: Antibiotic Testing and Wrap-Up of our Soil Microbe

Welcome back!

This week was our last week of testing our soil microbe, and we are fairly confident at this point that we have determined what our soil microbe is based on our previous tests. We believe our soil microbe to be Pseudomonas aeruginosa, an opportunistic pathogen that can infect humans and animals, but typically only infects immunocompromised individuals.

This week, we did an antibiotic test to try to determine our soil microbe. We used an antibiotic test medium and three controls, Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli plus our unknown (or somewhat unknown) soil microbe. We streaked the antibiotic plates with each microbe and used four antibiotics for each plate: carbenicillin at 100 micrograms, tetracycline at 30 micrograms, azithromycin at 15 micrograms, and erythromycin at 15 micrograms. The plates were left to incubate at 37 degrees Celsius for 24-48 hours. Unfortunately, our results for our plates were inconclusive and we did not get any data, and could not repeat the experiment due to a lack of supplies. However, had we had data, we would have expected to see sensitivity for our soil microbe, if it is P. aeruginosa, to the carbenicillin and no other antibiotics. P. aeruginosa has developed strong resistance to many antibiotics, making it difficult to treat and more likely to continue to develop resistance to other antibiotics as well. For E. coli, we would have expected to see sensitivity to carbenicillin, azithromycin, and erithromycin and resistance to tetracycline. The S. aureus control would have shown up as resistant to carbenicillin, which makes sense due to its Gram-negative nature; it would have also shown resistance to tetracycline, and sensitivity to azithromycin and erythromycin.

The last experiment that could be done to attempt to solidify our microbe would be to determine its ability or inability to liquefy gelatin; Pseudomonas strains of bacteria typically liquefy gelatin, so that would be one way to easily verify what we almost certainly already know, but to further determine the specific species of our microbe, we could determine its efficiency at growing at 37 degrees C; according to the dichotomous key, P. aeruginosa grows readily at 37-42 degrees C, and another similar strain, P. fluorescens, does not grow or grows poorly at 37 degrees C. By testing this, we could rule out one or the other strain depending on its ability to grow at this temperature.

I hope you've enjoyed following along as we have experimented these past few months and deduced what we think our soil microbe is!

Patricia