Science Buddies: "Ask an Expert"

How do you know how many bacteria you started with prior to the experiment? For example, if I started with an empty sterile agar filled petri dish and just swabbed it with bacteria using a cotton swab, would it just be zero?

If the petri dish has sterile agar, then it will not contain any bacteria. The sterilization process will normally kill off any microorganisms. You might want to keep a negative control, i.e. the petri dish with the sterile agar and unswabbed. Over time, this should not show any bacterial growth. If it does, then that means that there is contamination or the sterilization process was not effective.

There are many ways to find out how many bacteria you start with. The simplest way is to use the spectrophotometer at wavelength 600 nm for the suspended bacterial inoculum. Another way is to do protein assay such as the BioRad®protein assay.

Hi there,

I'm slightly confused by your question. What is your question and hypothesis, and what is the experiment you're doing to address it? Like the previous expert mentioned, if you start with an empty sterile agar plate, there should be no bacteria on that plate unless it was contaminated. What do you mean by swabbing the plate with bacteria using a cotton swab and that it would be zero? If you streak bacteria onto the plate, then the plate will no longer be sterile and there will be bacteria on it. You just have to wait a day or two before colonies show up so you can count how many are on the plate. Why do you need to know how many bacteria you have before your experiment? If you give us more details about your experiment, we can provide you with more helpful advice.

Connie