Question

10. Ten cm^{2} of a tabletop surface was swabbed with a
moistened sterile cotton swab, and the swab was suspended in 2 ml
of sterile water and vortexed to suspend swabbed bacteria. 10 ?l of
this suspension was added to 9.99 ml of a buffer, and a 20 ? l
aliquot was plated. Following incubation, 106 colonies appeared.
How many CFU of bacteria cm^{-2} were present on the
sampled surface? Please show work in detail with all steps.

Answer #1

**Answer:**

Based on the given information:

- Area of the tabletop surface = 10cm
^{2} - Volume of sterile water used to suspend swab= 2ml
- 10ul (0.01ml) of suspension + 9.99ml of buffer (Fold dilution = (9.99+0.01)/10 = 1000x
- Volume of aliquot plated = 20ul
- Number of colonies obtained = 106

**Calculations:**

- Number of colonies using 20ul (0.02ml) aliquot = 106 colonies
- Number of colonies per ml = 106/0.02 = 5300 CFU/ml
- Number of colonies x Fold Dilution = 5300 CFU/ml x 1000 = 5.3 x
10
^{6}CFU/ml - 2ml of suspension contains = 5.3 x 10
^{6}CFU/ml * 2ml = 1.06 x 10^{7}CFU - 10cm
^{2}of tabletop surface contains = 1.06 x 10^{7}CFU - CFU/cm
^{2}= 1.06 x 10^{7}CFU / 10cm^{2}= 1.06 x 10^{6}CFU / cm^{2} **Therefore, sampled tabletop surface contains****1.06 x 10**^{6}CFU / cm^{2}

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