How well do you know the Spiral Plating Technique?

Method for Preparation and Enumeration of Microorganisms Using Spiral Plating Technique

• Automatic dispenser or constant pouring of sterile agar at 60-70°C into petri dishes is recommended for preparing agar plates.
• Laminar air flow hood and automated dispenser are suggested for low contamination rates.
• Spiral plating technique allows for the enumeration of microorganisms in solutions containing between 500 and 500,000 microorganisms per ml.
• The plater deposits a decreasing amount of sample in an Archimedean spiral on the surface of the agar plate.
• After incubation, colonies appear along the line of the spiral and can be counted using a special grid that associates a calibrated volume with each area.
• The counting rule of 20 is used to count colonies on the spiral plate.
• Any count irregularities in sample composition are controlled by counting the same segments in the opposite wedge and recording results.
• If 20 CFU are not within the 4 segments of the wedge, the entire plate should be counted.
• If the number of colonies exceeds 75 in the second, third, or fourth segment, the estimated number of microorganisms will generally be low because of coincidence error associated with crowding of colonies.
• Spiral plates with irregular distribution of colonies caused by dispensing errors should not be counted and should be reported as laboratory accidents (LA).
• Inhibitory substances in the sample, excessive spreader growth, or laboratory accidents may restrict the computation of SPLC.
• The counts should be rounded off and reported as SPLC or estimated SPLC per ml.