How to Grow & Count Organisms in a Microbiology Lab

The microbiology laboratory takes samples from human and environmental sources and identifies any bacteria in the sample. Lab technicians do this by giving the bacteria a proper environment in which to grow. Once grown, the bacteria are then counted and identified. This helps scientists and physicians know what bacteria are in the sample and how many, both key to knowing how to deal with the organisms.

Things You'll Need

• Bacterial sample
• Petri dishes with growth media
• Bottle with growth media
• Inoculation loops
• Calculator
• Personal protective equipment
• Biohazard hood

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Instructions

Inoculation

• 1

  Streak the petri dish with the sample swab from side to side on one half of the plate. If the sample is urine, take a drop of urine and use an inoculating loop to streak the petri dish from side to side on one half of the plate. If the sample is blood, place the blood in the bottle with media.

• 2

  Use a different inoculating loop to streak the second half of the petri dish from side to side, starting at the half that was initially inoculated. This carries over the bacteria initially placed on the dish and takes a small number of them over to the other half.

• 3

  Place the inoculated petri dishes or media bottles in the 37 degree Celsius incubator for the amount of time specified in your laboratory's standard operating procedure.

Counting Colonies

• 4

  Take the petri dishes from the incubator and place them under a biohazard hood. If the sample was blood, take the bottles from the incubator and streak petri dishes in a manner similar to other samples and incubate those dishes.

• 5

  Count the number of individual colonies growing on the plate. If your inoculation was done correctly, the colonies should be far enough from each other to be counted individually. If the colonies are overlapping, you may need to re-inoculate new plates.

• 6

  Use a calculator to multiply the number of colonies you counted times the volume of sample (in the case of liquid samples). This will give you the number of colony-forming units (CFU) per milliliter of sample, known as the CFU count. If the sample was not liquid, then only the number of CFUs by itself is relevant.