Viable Counts of Bacteria’a New Method for Facultative Anaerobes

SUMMARY: Large dilutions of bacterial suspensions were made at 45° in a buffered complex medium containing agar. The tubes of medium with the serial dilutions of bacteria were solidified by cooling; subsequent incubation at 37° permitted bacterial growth in discrete colonies. The density of colonial growth was enumerated by comparison with a set of prepared standards calibrated by the pour-plate technique. A large range (log10 2·5 to 10·5) of colony counts was covered in only four test tubes without any prediction of the approximate count. The method has advantages of speed, economy of reagents and apparatus, as well as range and precision.