What is a serial dilution?
A serial dilution is a method of reducing the concentration of a solution by a constant amount. In many cases, the dilution follows a logarithmic sequence that reduces the concentration of the solution for a constant factor of 10. To make a 10-fold serial dilution, take one part of the original solution and add this volume to a new container with 9 parts solvent (usually water). Next, take one part of this diluted sample and add this volume to a new container with 9 parts solvent. Continue in the manner until the desired final solution concentration is achieved.
Why do some bacteria samples require dilution?
A way to avoid staring into a microscope to count individual bacteria in a sample is to use an alternate counting method in which the bacteria are spread out over a growth medium (such as agar). The number of colonies produced is then counted. This assumes each bacteria cell produces a single colony. A typical sample of bacteria would yield far too many colonies to count individually. To use this colony counting method, a highly dilute sample of bacteria must first be achieved through a dilution process that drastically reduces the overall number of bacteria in the sample.
What does CFU/mL represent?
CFU/mL stands for “colony forming units per milliliter” and is a unit of estimation representing the number of viable bacteria colonies in a one milliliter sample.
What is a viable plate count?
A viable sample indicates that only living and growing bacteria colonies are counted and not non-living ones. A viable plate count therefore requires a growth medium and is typically not done using a microscope. The benefit of a using a viable plate count is that the naked eye can be used to determine the number of bacteria in a sample and does not require the use of a microscope. Also, since colonies are growing and can be observed, differentiation of microbial species present is enhanced. For certain bacteria, such as E. coli, a plate count of between 30–300 is ideal for a standard-sized Petri dish. In most cases, dilutions are required to achieve the desired visible colony count.
To determine the viable plate count, use the following equation.
viable plate count = number of CFU/[(volume plated in mL) X (total dilution used)]
For example, if the number of colonies counted was 200 in 0.1 mL of plated sample of a 10-6 dilution, the viable plate count would be 200/[(0.1 mL) (10-6)] = 2 X 109.
What are E. coli?
E. coli (Escherichia coli) bacteria commonly live in the intestines of humans and other animals. The majority of E. coli are not only harmless to humans, but many aid in the digestion process. A small number of E. coli can cause human health concerns. These problems range from diarrhea to illnesses outside the intestinal tract. Major ways that harmful E. coli illnesses are transmitted are through contaminated water, contaminated food, and through contact with infected animals (including other humans). Diarrheagenic E. coli responsible for human illness occur in six pathotypes: Shiga toxin-producing (STEC), Enterotoxigenic (ETEC), Enteropathogenic (EPEC), Enteroaggregative (EAEC), Enteroinvasive (EIEC), and Diffusely adherent (DAEC).
A typical 1 mL sample of E. coli bacteria grown in a nutrient broth may contain tens of millions of bacteria. Faced with the task of counting these bacteria, is there a method to complete your count using only your naked eye? Let the countdown begin to your counting task.
