Many microbes secrete antimicrobial compounds to help them compete with other microorganisms for habitat. Some of the bacteria that are common antibiotic producers are the Actinomycetes (including Streptomycetes species), many of the Bacillus species, and the fruiting myxobacteria, to name just a few among many, many antibiotic producing bacteria. Today, you have already tested for the opposite: the sensitivity of your organisms (or known stock bacteria) to manufactured or secreted antibiotics. Identify how many potential antibiotic producers you might have. It's possible that you might discover the next great antimicrobial drug and get very rich by selling the patent for your discovery to a drug company. Remember that the discovery of penicillin was completely accidental.
Day 2 (Wednesday)
We will provide you with fresh cultures of Escherichia coli (Gram negative), Bacillus subtilis (Gram positive).
Use the plate(s) from Monday
1. Use a sterile swab and aseptically apply a line of inoculation of each of the provided broth cultures of : E. coli
and B. subtilis
as shown below.
2. Draw a line perpendicular to the antibiotic producer's (Your isolate) inoculation. Be careful not to touch the antibiotic producer's growth.
3. Using the same swabs, inoculate a new plate (one plate for all five cultures) by making a line across the plate for E. coli and B. subtilis
. Incubate this plate along with your test plate. It will serve as a control to make sure that lack of growth is due to antibiotic sensitivity and not to no living cells in the inoculum.
Day 3 (Friday)
Examine the plate and look for evidence of inhibition of growth of "test" organisms near the antibiotic producer's midline streak.
Draw the results and evaluate whether or not there was evidence that an antibiotic was produced by the organism and, if so, which of the bacteria tested were sensitive to it and to what degree. If you found no inhibition of growth, does that mean that your potential antibiotic producer does not secrete an any antimicrobial compounds? Why or why not?