Question

A medical researcher has obtained a new strain of streptococcus bacteria. it has a generation time of 15 minutes. bacteria reproduce by binary fission, in which one bacterium becomes two bacteria. after one hour of culture, there are 2000 bacteria in her petri dish. what was the initial amount of bacteria in the petri dish

Answer 1

The division rate of the streptococcus bacteria is 15 minutes/division. After a period of 60 minutes there are 2000 bacteria. The initial number of bacteria can be found by looking at the number of times in an hour the division has taken place.

Number of divisions in a hour = Total time/Division rate = $\frac{60 \times 1}{15} = 4$

So, using the logic that bacteria doubles in every division, we can backtrack the initial number.

4th division: 60 minutes has 2000 bacteria,

3rd division: 45 minutes has 1000 bacteria,

2nd division:30 minutes has 500 bacteria,

1st division: 15 minutes has 250 bacteria

0th division (initial phase): 0 minutes has 125 bacteria