Alternative Methods For Viable Cell Count Procedure

The conventional viable cell count or standard plate count method is time-consuming in terms of both operation and collection of data. Several methods have been explored to improve the efficiency of operation of the viable cell count procedure.

The spiral plating method is an automated system for obtaining viable cell counts (Spiral Biotech, Bethesda, Md.). The instrument can spread a liquid sample on the surface of agar contained in a petri dish in a spiral shape (the Archimedes spiral) with a concentration gradient starting from the center and decreasing as the spiral progresses outward on the rotating plate. The volume of liquid deposited at any segment of the agar plate is known. After the liquid containing microorganisms is spread, the agar plate is incubated overnight at an appropriate temperature for the colonies to develop. The colonies appearing along the spiral pathway can be counted either manually or electronically. Table 1 demonstrates that the spiral plating method provides the same counts as the conventional method. A skilled operator can perform 10 times more samples using the spiral system compared with conventional methods.

New versions of the spiral plater have recently been introduced as Autoplater (Spiral Biotech, Bethesda, Md.) and Whitley Automatic Spiral Plater (Bioscience International, Inc., Rockville, Md.). With these instruments an analyst need only present the liquid sample and the instrument completely and automatically processes the sample, including resterilizing the unit for the next sample.

Many instruments are now available for rapid enumeration of bacterial colonies in agar plate. Spiral Biotech (Bethesda, Md.) markets a laser system called CASBAII that has been in use in the author's laboratory for many years with satisfactory results. Recently the same company introduced CASBA III, which includes a high-resolution scanner with a high-performance personal computer and Windows-based colony image analysis software. Protos colony counter (Bioscience International, Inc., Rockville, Md.) is another automatic colony counter that can count 600 plates/hour at 0.5 s/plate. Many other companies also market automatic plate counters. Table 1 also shows that counting manually and counting by a laser counter provided the same number, but the laser counter can complete the task in a few seconds.

Another alternative method for viable cell count is the Isogrid system (QA Laboratories Ltd., San Diego, Calif.).

Table 1. Comparison of Pour Plate and Spiral Plate Counted Manually and by Laser for Bacterial Cultures

Log10cfu/ml Pour plate Spiral plate

Table 1. Comparison of Pour Plate and Spiral Plate Counted Manually and by Laser for Bacterial Cultures

Test cultures

Manual

Laser

Manual

Laser

Escherichia coli

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