Hello, I am about to give this problem set as a take home quiz to my students in an introductory food microbiology class. the following test items are open for critiquing. thank you so much in advance!
Microbiology 180- Introductory Food Microbiology
Problem Set 1: Media and Reagent Preparation/ Standard Plate Count
1.You are to analyze a burger sample suspected to have caused an outbreak of food poisoning in a nearby diner. As the forensic laboratory analyst on duty at that time, the burger samples were forwarded to your custody for microbiological analysis. Using the standard plate count procedure, enumerate the number of spoilage bacteria per gram of hamburger. The procedure and dilution method is as follows:
a. aseptically obtain 50g of burger sample and dilute it with 450ml of sterile peptone water in a blender jar, blend the suspension for 1 minute until homogenous, label this as your first dilution;
b. from this mixture, transfer an aliquot of 1ml to 99ml sterile peptone water; label this as your second dilution.
c. from your second dilution, obtain another 1ml aliquot and transfer it to another 99ml of sterile peptone water, label this as your third dilution.
d. pipette 1ml aliquots of your 2nd and 3rd dilutions and pour-plate in duplicates in Standard Methods Agar. Set aside an uninoculated SMA plate. This will serve as your negative control.
Answer the following:
1.a. Illustrate a dilution and plating scheme for the enumeration of spoilage bacteria in your sample. Properly label each step with the volume of aliquots to be transferred and the corresponding media.
1.b. Compute for the dilution and dilution factor for each dilution
1.c. You need to prepare at least 650 ml of peptone water as diluent. Show your computations and describe the manner by which you are to prepare the sterile diluents.
1.d. How much of each component of the standard methods agar do you need to prepare? Show your computations and outline/describe the manner by which you are going to prepare the sterile medium and glassware needed for the analysis.
1.e. After incubating the SMA plates at 37.5±2°C, the following colony counts were obtained (refer to the table below). Compute for the cfu/ml and then report your counts as cfu/g of burger sample, use a scientific notation of up to 2 decimal points only.
Plates Dilution Colony counts
a. 2nd dilution 248, 253
b. 3rd dilution 15, 26
2.You are to prepare 0.9N H2SO4 from a stock solution of 8M H2SO4 for a separate analysis. How much 8M H2SO4 do you need to dilute in order to come up with the desired final concentration? Show your computations and describe the manner by which you are to prepare this reagent.
3.You also need to prepare the following reagents for another analysis. Compute for the amounts to be weighed and outline/write down the manner of preparation.
a. 100ml of 1M KCl
b. 100ml of 3% KOH (however, only KOH●H2O is available)
c. 500ml of 70% v/v isopropyl alcohol from a 99.8% v/v isopropyl alcohol
d. 20ml 0.2% w/v bromthymol blue solution
e. 500ml sterile distilled water with 200 ppt of magnesium sulfate heptahydrate
*Accomplish all computations as neatly as possible in a separate sheet of paper and encircle all final answers. Have fun! :)
I like the calculations.
Regarding the volumes, you can reduce the dilutions to 1 in 9ml Peptone without loss in accuracy and will save a lot on Peptone water.
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