Why Is It Important to Avoid Digging Into the Agar With the Loop?

It is important to avoid digging into agar with the loop due to the high risk of cross contamination between different specimens. Contamination renders a petri dish or streak plate unusable. The American Society for Microbiology states that improper loop sterilization also increases the risk of contamination.

According to Boston University, modern streak plate protocol calls for three or four samples per petri dish. Each segment requires a new swab or sterilized loop. The most popular tools used to inoculate agar are cotton swabs and metal loops. Cotton swabs are single-use instruments because they do not withstand flame sterilization. Reusable metal loops require sterilization after every use.

Some scientists prefer to use plastic loops. These resemble metal loops but must be thrown away after each inoculation. Metal needles are another option. Like metal loops, needles require sterilization.

The American Society for Microbiology recommends sterilizing metal loops and needles by passing them through the flame of a Bunsen burner turned to its lowest setting. To sterilize an instrument, insert it into the light blue portion of the flame and hold it there until it glows orange or red. Gradually inserting the loop or needle into the flame prevents sputtering of potentially biohazardous liquids.

Another effective way to sterilize metal loops and needles is to place them in an incineration machine designed for that purpose. This method of sterilization takes less than 10 seconds. However, the incinerator takes several minutes to reach operating temperature.