Studies on the surface plate method of counting bacteria

A study of plate preparation, inoculum volume, and technique of spreading in the surface plate method showed that predrying plates to a weight loss of 3–4 g/15-ml plate allowed the use of inoculum volumes of up to 1 ml without affecting recovery of bacteria. Satisfactory drying was achieved at 24–50 °C with the plates positioned upside down separately or in stacks. Equally good recovery of bacteria was obtained either by spreading the inoculum until it had been completely absorbed, or by spreading it for only 15 s and waiting for absorption and evaporation of excess liquid before inverting the plate. The former technique was limited to about 0.5 ml of inoculum because of the length of the spreading time required for volumes greater than this, whereas the latter method was satisfactory for volumes up to at least 1 ml as shown in recovery tests with Escherichia coli and Pseudomonas fluorescens.