Spiral plate method

In the spiral plate method, a mechanical plater inoculates the liquid sample on a rotating plate agar. The volume of liquid sample decrease from the center to the edge of the plate. The microbial concentration is determined by counting the colonies on a part of the Petri dish.

The advantage of spiral plating is less plates used versus plating manually.

Figure 1. Spiral plate template for cell counting. A: first sector. B: second sector. C: Cells

Colony forming unit (CFU) is determined by counting the colony in specific grid. The formula is describe as follows:

CFU/mL = (Colonies counted in first sector + Colonies counted in opposite sector) / volume) = (31 + 30 colonies)/0.0015 mL = 4.1 x 10⁴ CFU/mL