Question

1. The mixed culture in an experiment was prepared by mixing overnight E. coli and S. aureus cultures in the ratio 2:8. However, there seemed to be more E. coli growth on the resulting streak plate than S. aureus growth. Explain this.  

2. It was observed that some colonies of the same bacterial species appeared in different sizes on the streak plates. Give the possible reason for this.

3. Is it better to prepare pure culture in broth or on agar plate? Explain.                  

Answer 1

1. The mixed culture in an experiment was prepared by mixing overnight E. coli and S. aureus cultures in the ratio 2:8. However, there seemed to be more E. coli growth on the resulting streak plate than S. aureus growth. The reason for the above observation is becuase of the difference of doubling time of E.Coli and S.aureus. Doubling time refers to the amount of time required by a cell to double in its number under a constant growth rate. The doubling time of E.Coli is 20 minutes wheras that of S.aureus is in the range of 2 to 4 hours. Now this explains very well that although the amount of inoculum used for S.aureus was more still the growth rate of the same was less because of extended doubling time.

2. It was observed that some colonies of the same bacterial species appeared in different sizes on the streak plates. This phenomenon is possible because of different growth rate of bacteria. Some cells of the same bacterial population would be lag or log or stationary phase. Cells present in different stages will give different colony sizes on the streak plates.

3. It is always better to prepare pure culture in agar plate as compared to broth culture. A pure culture contains only one single type of that organism and therefore it is referred to pure culture. On an agar plate, we can streak the bacterial culture available and then we can observe all colonial morphology. A pure culture would always give rise to same colony which is going to have all features same. On the otherhand Broth cultures are liquid cultures which are used to grow bacteria in a sterile growth medium. So when a culture is inoculated, we wont be able to see the indiviual colonies as all the bacterial cells are going to grow in liquid culture showing turbidity. Hence a pure culture should be grown in agar plate and not in broth culture so that morphologically we will be able to visualize the pure colonies on the agar plate which is difficult in broth culture.