Question

Below are the results of growth on the plates of bacteria exposed to UV light.  Answers the questions below using the results in the table below.

+++ = maximum growth

++ = moderate growth

+ = Little growth

- = no growth

S. aureus	10 sec	20 sec	40 sec	80 sec	2.5 min	5 min	10 min	10* min
Survival	+++	+++	++	++	+	-	-	+++
B. megaterium	1 min	2 min	4 min	8 min	15 min	30 min	60 min	60* min
Survival	+++	+++	+++	+++	+++	++	++	+++

* Plates covered during exposure

1.  How many times more resistant were B. megaterium spores than S. aureus vegetative cells?  (S. aureus survived through 2.5 minutes and B. megaterium survived through 60 minutes)

2.  Why was half of each plate covered with an index card?

3.  What was the purpose of leaving the cover on one set of plates?  How did the growth on these plates differ from the growth on the uncovered plates that were exposed to UV light for the same amount of time?

Answer 1

1) 24 times

As the B.megaterium is survived until 60 min compared to S.aureus survived only until 2.5 min, that means it is more resistant as factor of 60÷2.5 = 24 time's more than the S.aureus.

2)half of each plate is covered with index card to understand the weather this low survivality is due to the any factors other than uv radiation. If there is loss of growth in covered area compared to other plates than there must be another region for the low number of survival's on that plate.

3)the purpose of leaving cover on set of plates that they can act as control plates which are not effected by uv radiation hence show normal growth.