## What agar is

Agar is an extract from seaweed that when added to liquid sterilized and then cooled form into a gel. Agar is mixed with nutrient media at a 4% concentration, dissolved by heating and then sterilized by autoclaving. Once sterile it is then cooled to handling temperature and poured into petri dishes for use in culturing microorganisms.

## What plates are

Agar plates or more specifically, petri dishes are used to culture microorganisms on a semisolid gel-like surface. This allows the technician to isolate and characterize the microorganisms that are growing. It can also be a useful way to store growing microorganisms for short periods of time.

### Plate Pouring Protocol

# Why we might use antibiotic

## What antibiotic is

Antibiotics are chemicals that have antimicrobial activity. There are many different classes and chemistries of antibiotics but their function is to kill microorganisms. In the biotechnology lab we often isolate and manipulate small DNA molecules called plasmids and these plasmids always carry a gene that produces a protein that breaks down a certain class of antibiotic. We use this feature of plasmids to ensure that our plasmid is maintained in the growing bacteria. It also makes sure that only bacteria carrying our plasmid are grown in the presence of the antibiotic. Our bacteria carrying the plasmid will be resistant to the antibiotic while all other bacteria will be killed off. For this reason we often add different antibiotics to both liquid cultures and agar plates to select for only the correct bacteria we want to grow.

## What sorts of things grow on plates

Nutrient rich agar plates without any antibiotic will grow thousands and thousands of different species of microorganisms. These microorganisms are ubiquitous, they grow on practically every surface of everything on earth. It is estimated that there are 40 trillion microorganisms growing in and on the average human, more microorganism cells than the actual human cells. By using antibiotics we are able to isolate only the specific strain that we want to work with.

Remember: The next step is to come into the lab to practice with the materials, view a demonstration, or to even just ask questions. Also, if you are ready to take the lab final, please inform your instructor. The lab final will be performed by yourself and will be graded with a rubric.

### Learning Objectives:

- 1.2 - Pour agar plates for microbial growth with or without antibiotic.