You may be wondering how to see bacteria without a microscope..
Individual cells are too small to see with the naked eye, but, when enough of them grow in one place they form a visible colony that can be observed.
What is a colony?
Wherever a cell lands on a surface that is favorable for growth (a petri dish full of yummy nutrient for example) a group of cells grows from that one cell. This forms a visible spot on the surface.
Different types of bacteria have characteristic appearances that can help identify them. Growing bacteria (also known as culturing) also allows the researcher to have enough cells to:
- do further testing to ID the bacteria
- determine the amount of bacteria in a sample
So how does a petri dish work?
A jelly-like growth media is prepared, sterilized, and poured in the dish while still liquid. It contains agar (from red algae) to make it gel. It very similar to making jello, but with a variety of nutrients to feed bacteria.
Not tasty for us, but bacteria love it, and many microbiologists love the smell- it grows on us (pun intended).
Bacteria need nutrients to grow, just like we do. They need a carbon source, nitrogen, and other things. There are several types of growth media. Some work for a broad range of bacteria while others are geared to specific types.
When the bacteria are introduced to the surface of the agar, they are in foodie heaven for microbes and begin to flourish. Some form visible colonies in as little as 8 hrs. A tool that looks like a wire loop, called inoculating loop is used to transfer a culture to a plate. It is sterilized in a bunsen burner flame to make sure only the culture of interest is transfered.
What is a streak plate?
The streak plate method, also known as the 4-quadrant streak, is a way to dilute the bacteria in order to observe individual colonies. It also helps to separate out the different types of bacteria if a mixture is being cultured.
If the growth completely covers the surface, you end up with a solid film of bacteria and won’t be able to examine the colony characteristics.
The plate is divided into 4 sections. The culture to be grown is streaked onto the first section, the inoculating loop is sterilized and then dragged from the first section into the second (this takes some of the bacteria from the first section and thins it out a little). The loop is sterilized and then bacteria from the second section is dragged to the third section and this is repeated for the 4th section (see below).
Check out the video below to see the process!
