[Cecily]

1. Why were plate counts used for analyzing bacterial growth instead of direct
microscopic counts or turbidity measurements?

Because it's an experiment over an extended period of time, plate counts would be ideal. The control concentration is too low for turbidity (about 10 - 100 million / mL) and microscopic counts (>10 million / mL), Also, turbidity and microscopic counts have trouble distinguishing live cell counts from dead cell counts. Yet another reason would be the live organism is most likely motile and therefore hard to count with a microscope.



2. How did the control cottage cheese and the experiment cottage cheese differ? What was the independent variable and dependent variable? (Hint: Scientific method)

Independent variable = preservative
Dependent variable = generation time


* I think that's right? It's been a long time since elementary school science.
"The independent variable is the one the experimenter controls. The dependent variable is the variable that changes in response to the independent variable."



3. Determine the effectiveness of the new food preservative by calculating the
generation time of bacterial population in the control versus the experimental
cottage cheese.



Experimental
End: 6.10 x 10^7 = log 7.78
Begin: 700 = log 2.85


G = t/3.3 x log (log 7.78 - log 2.85)
G = t/3.3 x 4.93
G = t/16.269
t = 38 hours x 60 = 2280 mins / 16.27 =
Experimental G = 140 mins

Control
End: 1,000,000 = 1.00 x 10^6 = log 6
Begin: 200 = log 2.30

G = t/3.3 x log (log 6 - log 2.3)
G = t/3.3 x log 3.7
G = t/12.21
t = 29 hours x 60 = 1740/12.21
Control G = 143 mins


Conclusion: The preservative was marginally successful in inhibiting microbial growth.



4. Does this type of test determine bacteriostatic or bactericidal activity?

The goal of a food preservative is to inhibit bacterial growth, not necessarily kill bacteria, so this test would determine bacteriostatic activity.


Pretty sure that's right. Haven't encountered those definitions in any chapter reading yet.




Finally done.