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Coulter Counters in the study of bacteria

As you will probably have read by now, the Coulter Principle was discovered in the 1940s by Wallace Coulter, who used it to develop a very quick and efficient way of carrying out blood cell counts. Since its introduction, its uses have expanded dramatically. Today, Coulter Counters can be found counting and sizing anything from toners to explosives, and from food to construction materials. Even so, the Coulter Principle still has a major role in the field of human health.

As well as counting blood cells, Coulter Counters are used in many other branches of medical science and biomedical or biological research. The same principle lends itself easily to the assessment of any kind of cell, including viruses and bacteria.

The range of cell sizes in nature is well within the broad spectrum that Coulter Counters can handle. Blood cells are typically between 6 and 10 µm, while bacteria are usually less than 1 µm.

Because bacteria tend to aggregate when placed in a suspension, the sample usually needs to be shaken, to disperse them, before testing. If they are allowed to aggregate, clumps containing a number of bacteria may pass through the counter’s orifice and be counted as just one large bacterium. This would give an underestimated cell count and would distort the size distribution results.

Another key point to remember is that the Coulter Counter cannot distinguish between living and dead bacteria, which are each counted and sized in the same way. If size and count data are needed for just the living bacteria, the analysis has to be backed up by assessment of cultures in petri dishes, or some other technique, to find out what percentage of the bacterial population is living.

Bacteria-bearing samples investigated with the aid of Coulter Counters have ranged from sea water to milk. In addition to count and size data, these versatile instruments have been useful in studying bacterial concentrations and growth rates.