Serial Dilutions Mastering Microbiology Lab Homework

Arrange the steps to correctly arrive at an estimate of the quantity of bacteria in a sample of a solid food. Do not overlap any steps.

- Weigh 1g of sample and place into 9ml of sterile water. -Prepare serial dilutions of the sample. -Deliver inoculum to each plate. -Pour melted agar into plates. -Stack plates, invert, and incubate. -Count colonies and calculate concentration.

A student is testing a milk sample, and carries out the dilution scheme in the diagram below. What is the total dilution of Tube C? 10-1 10-4 10-3 10-5

10-4

Imagine you and a partner are preparing a serial dilution of a sample of pond water. While you were watching your agar, your partner labeled several test tubes with the numbers below. Which of these numbers correctly expresses a dilution? Imagine you and a partner are preparing a serial dilution of a sample of pond water. While you were watching your agar, your partner labeled several test tubes with the numbers below. Which of these numbers correctly expresses a dilution? 104 5.2 X 104 cfu/ml 5.2 X 10-4cfu/g 10-4

10-4

What is the total dilution of tube 5 in this serial dilution scheme? 10-6 105 10-1 103 10-3 10-4 10-5

10-5

A food microbiologist is testing ground meat to estimate the bacterial concentration. He carries out the following serial dilution scheme. What is the total dilution of Plate D's sample? 10-7 10-1 10-6 10-4

10-6

You found 67 colonies on the ground beef plate with the 10-2 dilution. What is the concentration of bacteria per milliliter of beef? 6.7 × 103 CFU/ml 6.7 × 104 CFU/ml 6.7 × 102 CFU/ml 67

6.7 × 103 CFU/ml

If you had fresh lettuce containing 9.3 × 103 organisms/ml, how many colonies would you expect to see on a plate that represents a 10-2 dilution? 93 930 9300 9

93

After running this experiment, you find that the carrots you tested had only 15 bacteria/ml. Your lab partner states that this number is so low that the carrots do not need further treatment. Why should you disagree? None of the available answers are correct. Since these organisms are Gram negative, they are definitely pathogenic, so even a small dose could cause disease. A low number of organisms does not indicate the absence of pathogens. The 15 bacteria are resistant to the erythromycin, methicillin, and bacitracin found in EMB agar.

A low number of organisms does not indicate the absence of pathogens.

Which of the following would produce a countable plate? A solution containing 1.9 × 104 at a 10-2 dilution A solution containing 1.6 × 107 at a 10-6 dilution A solution containing 2.1 × 104 at a 10-1 dilution A solution containing 2.5 × 107 at a 10-2 dilution

A solution containing 1.9 × 104 at a 10-2 dilution

A student is counting the number of bacteria in a sample. She uses two methods: Counting bacterial cells with a microscope and A plate count method using serial diluted sample. Why does the plate count have an advantage? A student is counting the number of bacteria in a sample. She uses two methods: Counting bacterial cells with a microscope and A plate count method using serial diluted sample. Why does the plate count have an advantage? It takes into account only viable cells. It uses fewer materials. The procedure is less time consuming. There is much less likelihood for error.

It takes into account only viable cells.

Which of the following best describes serial dilution? Which of the following best describes serial dilution? Inoculum from one dilution is transferred to a sequence of plates. Each tube in a set of tubes is a dilution of the previous tube. Identical volumes of a bacterial sample are transferred to several Petri dishes. Melted agar is poured into a series of sterile Petri plates.

Each tube in a set of tubes is a dilution of the previous tube.

Which of the following bacterial concentrations would produce a TFTC plate at the 10-4 dilution? 2 × 106 organisms/ml 2 × 105 organisms/ml 2 × 108 organisms/ml 2 × 107 organisms/ml

2 × 105 organisms/ml

A food microbiologist carried out a serial dilution procedure using a 1-gram sample of ground meat. After incubation, the plates had the results shown in this photograph. What was the bacterial concentration of the original sample of ground meat? 8.0 X 107 cfu/g 5.8 X 107 cfu/g 5.8 X 10-7 cfu/g 10-6

5.8 X 107 cfu/g

Why do we use the term colony-forming units, instead of cells, when expressing concentration of bacteria in a sample? Why do we use the term colony-forming units, instead of cells, when expressing concentration of bacteria in a sample? There could be different species of bacteria in the sample. Not all cells are viable. Sometimes a group of cells forms a single colony. Only the colonies on the upper surface of the agar are counted.

Sometimes a group of cells forms a single colony.

A student is learning how to perform serial dilutions, using a stock solution of E. coli. He was instructed to use the dilution scheme shown in the diagram. The stock solution was turbid, as shown below. He delivered 1 ml from each tube into a separate sterile Petri dish and poured melted agar into each dish. When he examined the plates after 48 hour incubation at 37oC, he found no growth in any of the seven plates. What could explain these results? The same pipette was used throughout the experiment. He did not use aseptic technique. The agar was not at the correct temperature when poured. He did not stack the plates while cooling.

The agar was not at the correct temperature when poured.

Why are the coliform plates incubated at 37° C? None of the available answers is correct. Coliforms, by definition, are thermophiles. They grow best at body temperature. The nutrients will not be released from EMB agar at temperatures below approximately 35° C.

They grow best at body temperature.

Why must you perform multiple dilutions for each food type? You need to get one TFTC plate, one countable plate, and one TNTC plate for each food. You do not know if the organisms in the food are Gram positive or Gram negative. You do not know if the organisms in the food are pathogenic. You do not know the initial concentration of bacteria in the food.

You do not know the initial concentration of bacteria in the food.

Which of the following is indicative of the presence of E. coli? colonies on EMB agar with a metallic sheen colonies on EMB agar with a white precipitate inhibited growth on EMB agar growth on EMB agar

colonies on EMB agar with a metallic sheen