The bacteria we decided to use for our project is Rhizobium tropici, which is naturally found in root nodules with many plant species, especially legumes. These nodules are in a symbiotic relationship with the plant since they help fix nitrogen into ammonium as well as solubilize inorganic phosphate. We wanted to improve upon this latter function, and since R. tropici already has the ability to do so, we thought it would be a good choice for us to work with.
However, not a lot of work has been done with this rhizobacterium, even among previous iGEM teams. Some the few that have are the 2019 Tacoma Rainmakers, the 2016 Yale team, the 2014 Hannover team, and the 2020 CU-Boulder team. So, our contribution to future iGEM teams is a Rhizobium tropici fact sheet that other teams can use. The information is mostly from our own experiments trying to figure out the properties and preferences of R. tropici, but some of this is borrowed from other literature. We hope that by bringing this information together all in one place, this research will be useful for future teams that want to work with this organism. We have created a PDF document that teams can download (embedded below), but we have also included some information and links to other parts of our wiki here.
This sheet is also available for download: Download
A summary of the contents in the fact sheet is provided below.
Rhizobium tropici are naturally found in the roots of plants, especially legumes, as part of a symbiotic relationship. The rhizobacteria benefit from some of the nutrients absorbed by the roots, and in exchange, they increase the uptake of nitrogen and phosphorous by converting them into biologically accessible forms.
The most common plant these rhizobacteria are found with is Phaseolus vulgaris, or the common bean. More information about it can be found on the Experiments page.
When growing cultures of R. tropici, the medium we originally found to be most effective at encouraging cell growth was Yeast extract/Mannitol with Low Calcium. It provided nearly identical readings as its high calcium counterpart, but it is easier to work with because of the decreased turbidity. The exact method of preparing this medium can be found on the Notebook page. However, after looking at data from the 2016 Yale team and conducting our own experiments, we found that tryptic soy agar is a better medium for growth. More details about this can be found on the Modeling page.
Our samples grew best when placed in an incubator shaker at a temperature of 29 °C and at a speed of 250 revolutions per minute. However, the difference in doubling time when grown at room temperature was negligible, indicating a degree of independence from temperature in terms of growth rate. This information is also detailed on the Modeling page.
We found that our R. tropici samples varied greatly in doubling times depending on the medium in which they were grown. Samples grown in yeast mannitol extract low calcium doubled in around 4.88 hours, but samples grown in tryptic soy agar doubled in as little as 1.75 hours. This is comparable to the times achieved by the 2016 Yale team, suggesting that these measurements are accurate. The only parameter we did not get to test was the effect of antibiotics on growth rates. If we were to continue this project in the future, this is one of the things we would test to expand our knowledge of R. tropici.