Viable counts: serial dilutions, streak/spread plating

Understanding Viable Counts

Viable counts focus on quantifying the number of living, reproductive-capable microorganisms in a sample.
Unlike total population count, this method does not include dead or non-reproductive organisms.
Viable counts are usually expressed as Colony Forming Units (CFUs) per millilitre or gram of the sample because one colony on the agar plate originates from a single microorganism.

Serial dilution is a method used to decrease the concentration of a sample. It involves sequential dilution of the original solution.
When counting microorganisms, it’s often necessary to use serial dilutions to achieve a manageable number of colonies.
A small aliquot of a sample is mixed with a diluent, creating a diluted sample. This process is repeated, with each step using an aliquot of the previous dilution.
This method involves precision and care to ensure accuracy in dilution, making sure each step uses a correct amount of sample and diluent.
Serial dilution technique provides a gradient of microbial concentration. This ensures at least one dilution will have an appropriate number of colonies for counting.

Streak plating and spread plating are methods used to isolate a pure strain from a single species of microorganism for viable counting.
Streak plating technique involves spreading a loopful of culture over the surface of an agar plate. As the loop is dragged over the surface, it thins out the sample, ideally resulting in isolated colonies at the end of the process.
Spread plating involves spreading a small amount of sample evenly over the surface of an agar medium.
After incubation, the colonies formed are counted. This number is used to calculate the original concentration of organisms in the sample given the dilution factor, usually given as CFUs.
Both streak and spread plating methods are used when the exact count of viable microorganisms is essential, as in clinical diagnostics, food, and water industries.

Factors such as the quality of dilution, the incubation conditions (temperature, time, oxygen, humidity) and the type of growth medium can directly impact the count of viable microorganisms.
Errors in sampling and plating technique can lead to an overestimate or underestimate of the microbial population.

Viable counts are crucial in various spheres of human biology including medical diagnosis, quality control in food industry, and in pharmaceutical production.
It aids in understanding the pathogenic behaviour of harmful microbes and designing specific treatments in microbial diseases.
Viable count also has significance in environmental studies such as in monitoring microbial contamination in water bodies.
In scientific research, it helps in developing genetically engineered organisms and evaluating microbial activity in soil.