In order to determine whether a newly synthesized chemical might be a useful foo
ID: 66971 • Letter: I
Question
In order to determine whether a newly synthesized chemical might be a useful food preservative, the chemical was tested for its ability to inhibit bacterial growth. Control: 500 mL of cottage cheese was inoculated with 2 mL of a 24-hr culture of Pseudomonas aeruginosa and incubated at 25 degrees C. Five hours after inoculation, a standard plate count showed there were 200 bacterial cells/ml in the cottage cheese. After 29 hours at 25 degrees C, there were 1,000,000 cells/ml in the cottage cheese. Experiment: 500 mL of cottage cheese was inoculated with 2 mL of a 24-hr culture of P. aeruginosa. After 6 hours of incubation at 25 degrees C, a standard plate count was performed. There were 700 bacterial cells/ml in the cottage cheese. After 38 hours, there were 61,000,000 bacterial cells/ml in the cottage cheese.
Questions:
1. Why were plate counts used instead of direct microscopic counts or turbidity measurements?
2. How did the control cottage cheese and the experiment cottage cheese differ? Was this a fair test?
3. Determine the effectiveness of the new food preservative?
4. Does this type of test determine bacteriostatic or bactericidal activity?
Number Log 1 0.00 2 0.30 5 0.70 6 0.78 24 1.38 32 1.51 200 2.30 700 2.85 1.00 x 106 6.00 6.10 x 106 6.79 6.10 x 107 7.79Explanation / Answer
1.
Plate counts will provide information about viable bacteria only which is described as colony forming units at a partiticular dilution. In direct microscopic or turbidity measurement both viable and non-viable bacteria will be taken into measurement. Viable is defined as the ability to multiply viabinary fission under the controlled conditions. Also, the particles in cottage cheese would interfere with direct counts and turbidity.
2.
The new chemical was added to the experiment and was lacking in the control. Yes, this is a fair test. Control cottage cheese was less prone to spoilage as compared to experimental cheese. In other words shelf life of control cheese is better than experimental cheese.
3.
The two tests had the same generation time, proving that the new food preservative was not effective.
4.
Both. The answer is “bactericidal” when the number of bacteria declines, and “bacteriostatic” if the number of bacteria stays the same.
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