Question

5.3. CALCULATE THE NUMBER OF BACTERIA PER GRAM OF HAMBURGER MEAT. (1 gram = 1ml) 1gram 0.1ml 1 ml 0.1ml A MEAT water blanks 99ml 99.9ml 9ml duplicate 1ml samples plated colony count 201 210

Answer 1

Answer:

The number of bacteria per gram or ml also known as colony forming units per ml (CFU/ ml) can be calculated using the below formula:

CFU/ml = Number of colonies formed /(amount plated X dilution factor)

To solve the problem, first we need to find the dilution factor. Dilution factor can be calculated by using the formula = stock/final volume. Given, 1 g of meat sample (stock) was added to 99 ml of water, so the dilution factor for first dilution is 1/100 = 0.01.

Afterwards, 0.1 ml from first dilution was taken and added to 99.9 ml of water. Dilution factor in this case is 0.1/100 = 0.001.

Finally, 1 ml from second dilution was taken and added to 9 ml of water. Dilution factor in this case is 1/10 = 0.1

Final dilution factor = fisrt dilution factor X second dilution factor X final dilution factor = 0.01 X 0.001 X 0.1 = 0.000001

Number of bacteria per gram in first plate is calculated as below (Gram = ml)

Given, Number of colonies in first plate is 201 and the volume of sample plated is 1 ml.

CFU/ml = Number of colonies formed /(amount plated X dilution factor)

= 201/ (1 X 0.000001) = 201000000 Bacteria per gram of meat. or 2.01 X 108 CFU/ml or Bacteria per gram of meat.

Number of bacteria per gram in second plate is calculated as below (Gram = ml)

Given, Number of colonies in second plate is 210 and the volume of sample plated is 1 ml.

CFU/ml = Number of colonies formed /(amount plated X dilution factor)

= 210/ (1 X 0.000001) = 210000000 Bacteria per gram of meat. or 2.10 X 108 CFU/ml or Bacteria per gram of meat.

As the experiment is carried out in duplicates, the mean of CFU /ml of two plates represent the number of bacteria per gram of Hamburger meat.

= (201000000 + 210000000) /2 = 205500000 = 2.055 X 108 Bacteria per gram of hamburger meat.