Question

I was having some trouble with this:

A student swabs a lab bench, uses the swab to streak two TSA plates, and incubates them at 20℃ (68℉) and 37℃ (98℉). The plate incubated at 37℃ has 150 colonies while the plate incubated at 20℃ has 0 colonies. What is one possible explanation for this discrepancy? Be sure to use words that classify microbes based on their preferred environmental conditions (i.e. the philes)

Describe a dilution regime you could use to obtain a 10^-4 dilution plate from a broth culture of E. coli.

Answer 1

1. The difference in the microbial growth is temperature. Most of the microbes grow best at room temperature that is 37 degrees. Such organisms which grow at moderate temperatures is called Mesophiles. Because 20 degrees is too low. microbes did not grow at 20 degrees.

2. By the method of serial dilution, it can be made 10-4 dilution. 1 ml from the stock of E.coli is transfered to first tube in the row which has 9 ml of diluent. Mix the contents well . From this first tube 1 ml is added to the second tube in the row. From the second 1ml is transferred to third. Then mix and transfer 1 ml from third to fourth as shown in the picture below. Every time you transfer mix the contents. From the fourth tube 1 ml is plated to get 10-4 dilution plate.