Take Home Quiz: Enumeration of Bacteria in a population (20 Points)
Answers due on Wednesday October 24, 2007 at 10:50 a.m. For FULL credit make certain that you
answer the questions as thoroughly AND concisely as possible. Show all of your math calculations
for full credit. All final answers must be answered using correct scientific notation Y.Y x 10 Y
1. Please answer questions (A-C) based on the data on the following page.
A. Of the 9 trials above, which data is most reliable? Why? (2 points)
B. Fill in the data table on the following page. Please show all of your calculations. (8
points)
C. In this experiment 32 grams of ground pork was suspended in 200 mL of water.
Based on the data on the following page please tell me. What was the average number of
Serratia marcescens (pink) present per mL of water, Escherichia coli O157::H7 (yellow)
present per mL of water? How many bacteria total were present in 1 g of ground pork?
Show all of your calculations. (4 points)
2. You have added 0.02mm3 of non-pasteurized milk to a Petroff-Hausser counting
chamber (hemocytometer); that is the most that the chamber will hold. The chamber has
a total of nine large squares for counting bacteria. You count 6 large squares that contain
25, 36, 22, 18, 15 and 28 bacteria, respectively. How many bacteria per milliliter (mL)
are present in the milk (6 points)? Show all of your math.
Table 1: Class data. Enumeration of bacteria (Serratia-Red) and/or E.coli (Yellow) in a population
Dilution
Factor
1:10-6
1:10-7
1:10-8
Number of Bacteria colonies (CFU’s) Yellow (Y) or Red (R)
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Trial 6
Trial 7
Trial 8
Trial 9
Y
Y
Y
Y
Y
R
Y
R
Y
R
Y
R
Y
R
R
R
R
R
Avg.
#
CFUs
Y R
Avg. #
Bacteria
Per mL
Y
R