Question

Jack, a new technician in the microbiology lab, made up a batch of nutrient agar and dispensed it into culture tubes for the BIO 135 class. He was running late and didn’t have time to autoclave it. What would you expect the tubes to look like when he came back the next day?

After having learned that lesson, he made a new batch of medium, but once again, he didn’t leave enough time to autoclave the medium. But he remembered his lessons from taking microbiology and realized that he could sterilize the medium if he added bleach to it. When he came in the next day, there was no growth in the tubes. Being very satisfied with himself, he put the tubes out for the class to use. The students inoculated the tubes with E. coli and Bacillus subtilis. When they came back in the next period, what did they see? Explain your answer.

Answer 1

Answer for the first part : The next day when he came back he could find the tubes to be full of growth (bacterial colonies) that appeared due to contamination as he didn't autoclaved (sterilised) the media (nutrient agar) after preparation.

Answer for the second part : When the students came back in the next period they found the tubes with E. coli culture has shown no growth (bacterial colonies). Whereas, they found the tubes with Bacillus subtilis has shown appearance of bacterial colony.

As the technician added bleach to the medium instead of autoclaving it, so the added bleach remained in the medium. The students inoculated those medium with E. coli and Bacillus subtilis. The bleach which was present in the medium effectively killed the E. coli bacteria. As a result no colonies were observed in the tube inoculated with E. coli.

The bleach remained in the medium could not kill the B. subtilis spores due to the presence of resistant spore coats, these spores eventually developed to produce viable Bacillus subtilis cells and colonies. That is why the students observed growth in the tubes inoculated with Bacillus subtilis.