Serial Dilution

When the supporters were in the sports stadium they were packed together at high density. Stadiums that hold 60,000 to 80,000 people are quite common. The team supporters were at a very high concentration, i.e. one every few feet. But, when they left the stadium and spread out over the city they separated from each other more and more, until, when they finally reached their homes, they were at a very low density (one every few miles). The team supporters were now at a very low concentration.

This is the principle of dilution. A concentrated mass of team supporters, or a concentrated mass of bacteria, can be separated and 'diluted out' until they are so far apart they can be identified as individuals. Put in a more mathematical way, as the dilution takes place the number of individuals (supporters or bacteria) in a unit volume of space becomes less and less.

It takes a whole city to dilute out the supporters of a team, and it would take many liters of broth to dilute out bacteria sufficiently to cleanly separate all the individual cells. In a science laboratory this would be very wasteful, so another method is used, the one described in this story, - serial dilution. In this method, a small amount of bacteria containing broth is added to a larger amount of sterile liquid. Typically this would be 0.1 ml added to 9.9 ml of sterile liquid. This is equivalent of adding 1.0 ml into 99.0 ml, which is a 1:100 dilution.

If 0.1 ml of this suspension is added to yet another 9.9 ml of sterile liquid, another 1:100 dilution takes place. The original sample has now been diluted twice and has reached a dilution of 1:(100 x 100) or 1: 10,000. This is now the equivalent of putting one ml of the original suspension into 10,000 ml (10 liters) of sterile liquid. But by using 'serial dilution' the scientist has only used 19.8 ml of sterile liquid - quite a saving in time and money.