How many living cells in my culture? There are times when optical densities will not suffice, when you will want to have an accurate assay that determines viable cell counts. It's no wonder one of the first things a student of microbiology learns is the age-old concept of using "colony forming units" (aka CFU) as a viability assay. For a bit of background history on the topic, read this editorial by George O'Toole. The standard approach of plating serial dilutions on agar medium on Petri plates – and its derivatives, the spiral and drop plating – are often time and resource intensive. Think, for example, the thousands upon thousands of plastic pipette tips and Petri plates used daily to perform these assays.
Recently, a group of innovative investigators developed an alternative approach. One that uses a single pipette tip per sample. It's so simple that, once I read about it, I uttered, "Why didn't I think of this myself?" But, alas, I didn't! They did. This new approach takes advantage of the conical shape of a typical pipette tip, which means that the local volume increases exponentially as you go from the narrow end of the tip to its wide pipettor-fitting end. By filling the pipette tip with cells suspended in agarose medium and incubating, the number of (micro)colonies that grow along the cone increases exponentially along its length. Images of the colonies inside the tip can be analyzed with imaging software or even by simply placing the tips on top of a piece of paper with a calibrated ruler on it (Fig. 1). The dynamic range within a single tip is six orders of magnitude! And counting as few as ten colonies yields reasonable accuracy. While this approach is still in its infancy, I would not be surprised if it takes off as the method of choice in some applications, for example, in moderate- to high-throughput screens that would otherwise use thousands of Petri plates and many more pipette tips to make dilutions. The inventors claim that this new assay, which they call the geometric viability assay (GVA), reduces time and consumables by at least one order of magnitude. If the GVA sounds attractive to you, you'll find all the details in the publication and in a dedicated website. Therein you'll see many wonderful tips for CFU determination (pun intended).