Question

10. Ten cm² 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 = 10cm²
• 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 10⁶ CFU/ml
• 2ml of suspension contains =  5.3 x 10⁶ CFU/ml * 2ml = 1.06 x 10⁷ CFU
• 10cm² of tabletop surface contains = 1.06 x 10⁷ CFU
• CFU/cm² = 1.06 x 10⁷ CFU / 10cm² =  1.06 x 10⁶ CFU / cm²
• Therefore, sampled tabletop surface contains 1.06 x 10⁶ CFU / cm²