Flame sterilize the loop using a flame or Bunsen burner until it is red hot. Let it cool for at least 15 seconds so that it does not kill your bacteria when it comes into contact with them. When flaming, start at the base and work towards the tip to prevent the formation of aerosols which you might inhale.
Dip the loop into your bacterial specimen. Bacteria are very small, so you only need a very small amount of specimen on your inoculation loop. Bacteria can be found everywhere. If you want to know what you are growing so you can perform experiments, you can order the bacteria from a science supply website. If you just want to observe growth, you can collect bacteria from the bottom of your shoe or even the faucet of the kitchen sink.
Inoculate the upper third of the agar plate by rapidly streaking the loop inoculated with the specimen back and forth across the agar. This is the primary streak. Make sure that you tilt the dish away from your face to shield yourself from breathing in airborne microbes.
Flame sterilize the loop again, remembering to cool it for 15 seconds before proceeding. Inoculate the loop by streaking it back and forth through the primary streak two or three times, and then streak the second sector of the plate. This section of the plate is called the secondary streak.
Flame sterilize the loop once more, making sure to cool it before touching the plate again. Inoculate the loop by streaking it through the secondary sector two or three times, and then streak the third sector, creating the tertiary streak.
Replace the lid back on the petri dish and turn the entire dish upside down. This practice prevents condensation that collects on the lid from contaminating the bacteria on the agar. Label the bottom of the plate with your permanent marker. Include important information such as date and type of bacteria. Never label the lid. If the lid is lost or separated from the bacteria, you will lose this valuable information.
Place the petri dish in the incubator, keeping it in the inverted position so that the plate rests on its lid. Most incubators should be kept at a constant 36 degrees C.
Incubate for 48 hours and then check your plate for growth. You will see many bacteria growing in the primary streak, less in the secondary streak and individual colonies of bacteria in the tertiary streak. The individual colonies are what will enable you to identify your microorganism under the microscope.