Environmental scientists are monitoring a 75 cm³ water sample collected from a geothermal hot spring to estimate the population of the thermophilic archaeon Sulfolobus acidocaldarius. They performed the following serial dilutions:

• 2 cm³ of the original sample was mixed with 8 cm³ of sterile growth medium to make Solution 2.
• 1 cm³ of Solution 2 was mixed with 9 cm³ of sterile growth medium to make Solution 3.

The scientists transferred 0.15 cm³ of Solution 3 onto a nutrient agar plate. After incubation under aerobic conditions at $75^{\circ }C$, 53 distinct colonies grew on the plate.