Question: What are the bacteria with the least and most cells?

Simon Webster answered on 1 Apr 2025:

This is a good question. Bacteria themselves are single celled organisms often only 10-20 micrometers in size. When we talk about them growing until they are a problem (like when bacteria grow in food which then gives us food poisoning) we mean how many bacteria are produced over a period of time.
Bacteria increase in number (reproduce) by each bacterial cell sub-dividing into two bacterial cells and this process repeating by all the bacterial cells over and over again. In the right conditions (where there are lots of nutrients, water and warmth) bacteria can divide every 15-20 minutes, therefore a very small number of bacteria can very quickly become very many in a short time.

In fact 1 bacterial cell can become billions in 24 hours in the right conditions (like in the Lab). In the environment bacteria don’t divide this quickly in reality but for us in the Lab to be able to know how many there are in a sample we need to grow them quickly so we can count them.

We do this by putting a small amount of the sample (one tenth of gram or less) in a liquid that supports the bacteria and spreading it over a growth medium in a petri dish. The growth medium is a firm jelly of nutrients, water and a gelling agent called agar. We incubate the petri dishes containing the agar with sample spread across it at the optimum temperature for growing the bacteria (usually 30-35 degrees C) for at least 18 to 24 hours but sometimes longer up to 48 or 72 hours depending on the type of bacteria we are trying to count.

If there are bacterial cells in the sample they will divide on the agar. As the agar jelly is firm each bacterial cell in the sample will divide over and over again making a colony of billions of them during the incubation period at which point we can see each colony clearly without a microscope. At the end of the incubation we look at the petri dishes and count the number colonies we can see. As each colony represents one bacterial cell the number we count relates to the number of bacteria in the sample. From the colony count we can calculate how many bacteria there are per quantity of sample and understand if this is too many or not.