What levels of microbes do I need in my samples in order to use a spiral plater?
UPPER AND LOWER COUNT LIMITS FOR SPIRAL PLATES In order to explain the count limits it is first necessary to understand the method of counting for spiral plates. All spiral plates are prepared in an identical manner whereby a fixed volume of sample is deposited on the surface of an agar plate at a decreasing rate from the centre to the edge, therefore fixing the volume/area relationship for any portion of the plate. In a mode where 50 µl is dispensed a dilution of 1000:1 is created across the plate which will produce a countable plate from any sample containing between 400 cfu/ml up to 4 x 105 cfu/ml without the need for further dilution. After incubation colonies appear along the track made by the deposited sample, with spacing between colonies increasing from the centre to the edge. The viewing grid for counting spiral plates is shown in Fig 1. The major divisions on this grid are the concentric circles and the 8 pie-shaped wedges or sectors which result in a number of annular segmen
