8. Examine the bacterial growth curve below. Based on this growth curve there ap

Question

8. Examine the bacterial growth curve below. Based on this growth curve there appears to be little change in the optical density between 270 to 310 minutes; however, your TA assures you that the bacterial species that produced this growth curve was well into the death phase during this time frame. Luckily your TA has saved culture samples from the 200, 270 and 310-minute time points. How can you verify that this bacterial species is well into the death phase and provide a reason as to why bacterial death and the optical density readings do not correspond? [4 pts) 1.20 270 310 200 Time (minutes)

Explanation / Answer

Ans. The term “death phase” is limited to the decline in viable cell counted in a system with no flow of nutrients and removal of waste metabolites. Moreover, the death phase is irreversible in the given culture media.

#1. Verifying death phase of bacterial culture: Perform a viable cell count: Use spread plate/ pour plate or streak plate methods to enumerate the viable cell count (only viable bacterial cells form colonies on the culture plate) of the sample from 200, 270 and 300 minutes culture sample. The viable cell count shall be maximum for 200 min (stationary phase, maximum number of viable cells), and minimum for 300 min (death phase, the viable cell count gradually decreases due to death of bacteria cells) samples.

Therefore, if you find that the viable cell count at 300 min is lower than that of 200 min (and 270 min, too), the culture is in its death phase at time 300 min.

#2. Reason why OD and bacterial death do not corresponds: The OD gives total (dead + viable cells) cell count. The total cell count in decline phase is greater than stationary phase because it includes the living cells as well as the dead cells that were ever in stationary phase. So, the OD of a death phase culture shall always be higher than that of stationary phase in spite of relatively lower viable cell count.

Therefore, the OD and bacterial death do not correspond because the OD keeps on increase instead of gradual decline in viable cell count.

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