Determine number of Bacteria

Question

Determine the number of bacteria within a 1 kilogram package of ground beef in order to provide a quantitative risk assessment and quality measure before release to public market.

Technical Data: A 1 gram sample of the ground beef was added to 99 ml of sterile saline solution and extensively mixed to resuspend all the bacteria into the solution (final volume 100ml). One millilitre of this mixture was then added to 9 ml of saline in tube A. Then 0.1 ml from tube A was spread evenly onto a growth media plate, and then the plate was incubated for 18 hours at 37 degrees to cultivate E. coli bacteria. After 18 hours, the growth media plate had 235 colonies.

From the experimental data, calculate how many bacteria are within the original 1 kg package of ground beef (considering homogeneity and representative sampling for the experiment). Write your answer as a complete number (no scientific notation, no commas, no punctuation).

Answer 1

Hi Mika,

The answer is 2,350,000,000 bacteria in the original 1 kg package of ground beef. How to get this answer is shown below.

-First, you are diluting the 1 gram of ground beef in 99 mL of sterile saline solution resulting in a 1/100 dilution. To calculate the dilution at this step you take the amount added to the first tube (1 gram) and put it on top of the fraction while you take the amount added PLUS the volume already present in the tube (1 gram + 99 mL) and put it on the bottom of the fraction to calculate your dilution of 1/100. It is important to remember that 1 gram is equivalent to 1 mL when doing dilutions in microbiology.

-Second, you perform a second dilution where you take the solution that has been 1/100 diluted and transfer 1 mL of this solution to 9 mL of saline in tube A. To calculate the dilution for this step you take the amount added to tube A (1 mL) and put it on top of the fraction while you take the amount added PLUS the volume already present in the tube (1 mL + 9 mL) and put it on the bottom of the fraction to calculate your dilution of 1/10.

-Third, you transfer 0.1 mL of bacterial solution from tube A and spread it out on a growth media plate. Since 0.1 mL is a tenth of 1 mL we can calculate a dilution of 1/10 for this step. This dilution fraction is calculated by taking the volume added to the plate (0.1 mL) and putting it on the top part of the fraction over 1 mL to get 0.1/1 which becomes 1/10 when both sides of the fraction are multiplied to generate whole numbers. (For example, if instead the problem asked us to transfer 0.5 mL from tube A to the plate then it would be a 1/2 dilution for this step - 0.5/1 which rounds up to 1/2).

-Fourth, you take the reciprocal of each of the serial dilutions you have calculated to plate your bacteria to calculate a dilution factor of 10,000. (The reciprocal is the inverse of each dilution fraction, for example, dilution 1 was 1/100 which becomes 100, dilution 2 was 1/10 which becomes 10, and dilution 3 was 1/10 which becomes 10. Then 100, 10, and 10 are multiplied together to calculate the dilution factor of 10,000.

-Fifth, take the number of bacterial colonies that grew on the plate (235) and multiply it by the dilution factor (10,000) to calculate the number of bacteria in the 1 gram of ground beef that was added to the first tube. 235 * 10,000 = 2,350,000 bacteria in 1 gram of ground beef.

-Sixth, the question asks how many bacteria are within the original 1 kg package of ground beef so you must multiply the number of bacteria in 1 gram of ground beef by the number of grams in 1 kg. There are 1000 grams in 1 kg, so to get the final answer 2,350,000 x 1000 = 2,350,000,000 bacteria are within the original 1 kg package of ground beef.