You may recognize Petri dishes, but you may not know why microbiologists use them. Let’s talk about it!

The squishy, jelly-like material you often see inside a petri dish is called agar. Agar is made by algae. Agar is used often in cooking because it is liquid when warm and sets into a squishy solid just like gelatin when it cools down. It can thicken soups, ice creams, and jellies! It is also commonly used to grow bacteria in microbiology.  

For bacteria to grow on agar, nutrients and additives are mixed into the agar before it sterilized (heated at high temperature and pressure). This allows scientists to tailor the contents of the media1 to the bacteria they want to grow. Some media are considered selective, meaning they are designed to prevent the growth of some species, while promoting the growth of other species. Selective media are especially useful for isolating a particular microbial species from a sample. Using a selective medium to isolate a particular bacterium is like using a sieve to pan for gold! Other unwanted bacteria are removed, and you're left with exactly what you want.  

Additionally, some media are considered differential, meaning they contain indicators: compounds that change color based on chemical changes. Bacterial species have different needs and process nutrients differently, so they consume and produce different compounds. Some compounds that are produced react with indicators and change the color of the medium. Since not all bacteria can produce the same compounds, different bacteria produce different colors. Scientists can look at these color changes to quickly identify bacteria. 

Agars can be both selective and differential. The selective and differential components of agars make them extremely useful, and colorful and beautiful. Here are three examples of foodborne pathogens growing on colorful differential and selective agars.  

Here is an example of how you can color streak plates.

Please feel free to print out this coloring page and color it yourself.  

  1. In microbiology, “media" are liquids or solids designed to grow bacteria in a lab setting. “Medium" is the singular form.


Below we have provided a list of bacteria, agars, and colors for you to pick from, or you can design your own agar and bacterium! 



  • Type of medium: color of medium 
  • Organism: (description of organism) color of growth  


MacConkey Agar: clear with a pink tint 

  • Escherichia coli: (common gut flora, but can be pathogenic) magenta growth with a magenta haze around growth  
  • Enterobacter aerogenes: (common gut flora, but can be pathogenic) magenta growth 
  • Salmonella enterica: (some strains are foodborne pathogens) clear/white growth 

Mannitol Salt Agar: pink 

  • Staphylococcus aureus: (common skin and nose microbiota, but can be pathogenic) yellow growth 

PALCAM Agar: red 

  • Listeria monocytogenes: (foodborne pathogen) green/brown growth with a brown haze around growth 

XLD Agar: red 

  • Salmonella enterica: black growth 
  • Escherichia coli: yellow growth 

HE Agar: green 

  • Salmonella enterica: green/blue growth with a black center
  • Escherichia coli: orange growth with an orange haze around growth  

EMB Agar: purple 

  • Escherichia coli: metallic green growth 
  • Enterobacter aerogenes: dark purple growth 
  • Salmonella enterica: pale white growth