Critical Thinking
1.
Explain why fungi and Bacillus sometimes grow
better after heat treatment.
Fungi and Bacillus are producing endospores.
Heat activates these endospores meaning the bacillus and the fungi are able to
divide and replicate because the heat has activated the endospores.
2.
The decimal reduction time (DRT) value is the
time it takes to kill 90% of cells present. Assume that a DRT value for
autoclaving a culture is 1.5 minutes. How long would it take to kill all the
cells if 10^6 cells were present? What would happen if you topped the heating
process after 9 minutes?
It will take1.5
minutes to kill 90% of the cells. Therefore, after
1.5 minutes, 10% remain. After three minutes, 1% remain. So, to figure out how
long it would take to kill a million cells, you repeatedly multiply 10^6 by 0.1
until your answer is less than 1. So:
10^6 x (0.1)^7 = 0.1
Therefore, you need 10.5 minutes of killing to kill one million cells. 7 x 1.5 minutes = 10.5 minutes.
After 9 minutes you would have:
10^6 x (0.1)^4 = 100 cells left
Therefore, you need 10.5 minutes of killing to kill one million cells. 7 x 1.5 minutes = 10.5 minutes.
After 9 minutes you would have:
10^6 x (0.1)^4 = 100 cells left
3.
Indicators are used in autoclaving to ensure
that sterilization is complete. One type of chemical indicator turns color when
it has reached a specific temperature; the other type turns when it has reached
a specific temperature and been exposed to steam. Which type of indicator
should be used?
Tape indicators will change color when the temperature hits over 120. This
indicates that the required temperature
was attained and you are unlikely to have heat-resistant spores around. This is
the best indicators to be used.
4.
A biological indicator used in autoclaving is a
vial containing 10^9 Bacillus stearothermophilus cells that is placed in the
autoclave with the material to be sterilized. After autoclaving, the vial is
incubated and examined for growth. Why is this species used as opposed to E.
coli or B. subtilis?
The
substance is used because it can withstand high temperatures which are needed
to be sterilized without the bacteria dying. It is a thermophilic which has the
presence of heat-resistant endospores.
For question 2, the answer should be 1 cell left. As after 9 minutes, should be in the 6th round but not 4. So, is 1 million of cells x 0.0001% = 1 cell. tq :)
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