Bacterial Spot Plating

Ten-fold serial dilution series spotted for CFU counting.

5 steps · Microbiology · Enumeration

1. 1. Lay out the dilution series

   Dispense diluent into one row per sample — enough wells for the full ten-fold series plus the neat sample.

2. 2. Serial ten-fold dilutions

   Carry 20 µL across each well in turn, mixing between steps. Change tips between dilutions or the series is meaningless.

3. 3. Spot onto the agar plate

   Spot each dilution onto a dried plate. Let the spots absorb fully before moving the plate — a running spot ruins the count.

4. 4. Incubate inverted

   Invert the plate once the spots are dry and incubate until colonies are countable.

5. 5. Count and back-calculate CFU/mL

   Count the dilution giving well-separated colonies, then multiply by the dilution factor and the spot volume.