A friend of yours is doing a critical experiement with E. coli and needs your he
ID: 62985 • Letter: A
Question
A friend of yours is doing a critical experiement with E. coli and needs your help to enumerate the sample. One ml of the culture is mixed with 9 ml of saline, which is then further diluted decimally (1/10) 5 times and 0.1 ml of the last three dilutions are spread onto plates of MacConkey agar and PCA triplicate. After incuation you observe the following number of colonies
TNTC
TNTC
TNTC
251
232
246
21
28
25
TNTC
TNTC
TNTC
239
223
258
23
29
30
a) Determine the number of CFU/ml of the undiluted culture from MAC plate counts.
b) Determine the CFU/ml of undiluted culture from the PCA plate counts.
C. MAC is a selective medium. Why are the nunbers of colonies that grow on the MAC and PCA plates nearly the same? If you repeated the experiment with B. cereus instead of E. coli, would you expect the number of colonies that grow on MAC and PCA plates to be the same? Explain.
Dilution in tube 10-4 10-5 10-6 PCATNTC
TNTC
TNTC
251
232
246
21
28
25
MACTNTC
TNTC
TNTC
239
223
258
23
29
30
Explanation / Answer
Dilution
10-5
10-6
No of cells in 1 ml of sample (0.1 ml is plated)
No of cells in original sample
PCA
251
21
2310
2.31 × 108
PCA
232
28
2560
2.56 × 108
PCA
246
25
2480
2.48 × 108
MAC
239
23
2340
2.34 × 108
MAC
223
29
2560
2.56 × 108
MAC
258
30
2790
2.79 × 108
Taking the mean values, the number of cfu in PCA is 2.45 × 108 /ml in original sample
And the number of cfu in MAC is 2.56 × 108/ml in original sample.
MAC is selective medium but E. coli grows in both of the media, and so the values are nearly equal
B. cereus cannot grow on MAC medium as crystal violet in the medium inhibits growth of gram positive organism. The number of cfu will be 0 in MAC if B. cereus is grown on the medium
Dilution
10-5
10-6
No of cells in 1 ml of sample (0.1 ml is plated)
No of cells in original sample
PCA
251
21
2310
2.31 × 108
PCA
232
28
2560
2.56 × 108
PCA
246
25
2480
2.48 × 108
MAC
239
23
2340
2.34 × 108
MAC
223
29
2560
2.56 × 108
MAC
258
30
2790
2.79 × 108