Aseptic Sterile Technique Used In Microbiology Laboratories

Aseptic Sterile Technique 
Used in Microbiology Laboratories

 LAB NOTES from Science Prof Online


Any medium used to grow bacteria must initially be sterile, so that the species of bacteria or clinical samples ultimately transferred to the medium remain pure and uncontaminated by microbes in the surrounding environment.

Media is typically made from a powder that is dissolved in water and then poured into bottles and sterilized in an autoclave. Although, at this stage it is sterile (free of microbes), there is opportunity for contamination with other microbes, whenever the medium is interacted with, such as when it’s poured into Petri dishes or when a poured and set media is inoculated with bacterial or clinical samples. 

Asetpic Technique for Transferring Bacteria
Once a growing medium is poured into a Petri dish and cooled, it is referred to a “plate.” Plates are made ultimately to be inoculated, that is, have a tiny amount of a pure bacterial culture or a clinical sample streaked across the surface. After inoculation, the plate is usually incubated for at least 24 hours to encourage growth of the sample.

Sterilize Inoculation Loop: Bacterial inoculate can be transferred using an inoculation loop (a.k.a. inoculation wand). This instrument is essentially a wire with a small loop at one end and a handle at the other. Since it is made of metal, it can be repeatedly used and then resterilized in a flame or microincinerator.

Allow Loop to Cool: After sterilizing the loop, it must cool briefly, so as not to kill organisms in the sample being transferred. When waiting for the loop to cool, do not wave it around to hasten cooling, and certainly don’t blow on it. Either action could introduce bacterial contamination.

Transfer Bacteria: Once the wand is sterilized and cooled, the bacterial inoculate can be obtained from the source plate and then transferred to the new plate. Streak plate technique is typically used to introduce a pure bacterial sample to a new plate and spread that sample out sparsely enough to facilitate the growth of distinct bacterial colonies.

When clinical samples are plated, a sterile swab is used to obtain the biological sample from the patient (such as throat swab) and to transfer that sample to a plate.

Sources and Resources
  • Schauer Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
  • Bauman, R. (2014) Microbiology with Diseases by Taxonomy 4th ed., Pearson Benjamin Cummings.
Article Summary: In a microbiology lab it's essential to avoid contamination of sterile materials and isolated cultures with the bacteria and fungal spores that are all around us.
Aseptic Technique Used to Reduce Contamination
Microincinerator used to sterilize inoculation loops
Microincinerator used to sterilize inoculation loops.
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Page last updated: 9/2014

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