Inoculating Loop: Types and Uses

An inoculating loop is essential laboratory equipment which is also called an inoculation loopsmear loop, inoculation wand, or microstreaker. Inoculating loops are used in the laboratory to pick the bacterial colony, transfer the microorganisms, and inoculate the samples in the culture media plates or the broths in a tube. It is also used in making smears on the slide.

The wire of the inoculating loop is usually made of the nichrome wire, which is sterilized by flaming in the bunsen burner or a bacterial incinerator till it becomes red hot. The diameter of the inoculating loop used in the microbiology laboratory is usually 2mm and 4 mm, and the length of the wire is 6 cm. The calibrated loop used mostly in the laboratory is 1 µl and 10 µl, which helps in precise volume inoculation. 

Parts of Inoculating Loop

parts of inoculating loop

The inoculating loop consists of the loop and the handle. Loop is used to carry the sample, whereas the handle supports the wire. The handle of the inoculating loop is made up of aluminum. Both the insulated and the non-insulated types handle are available. Handles are usually insulated by polyvinyl chloride (PVC), which protects the hand during its use. 

Types of Inoculating Loop

There are different types of inoculating loops based on the material and the size of the loop. 

Metal inoculating loop

wire of inoculating loop
  • The inoculating loop wire is made from nichrome, platinum, or tungsten. These metal wire loops are reusable. 
  • Nichrome is an alloy made of two metals, nickel and chromium. It is cheaper than platinum and is not easily corroded. Even after heating and cooling it multiple times, it does not degrade. It heats and cools down fast and is cheaper than platinum, so nichrome wire is used in the inoculating loop. It has a high melting point of around 1400℃. 
  • Though platinum wire is a bit expensive, it is durable and can resist constant exposure to acid and flame better than nichrome wire. 

Disposable inoculating loop

disposable inoculating loop
Disposable inoculating loop, Image source: Prolab Diagnostics

Disposable inoculating loops are made from plastic resin, polystyrene, polypropylene, ABS (Acrylonitrile Butadiene Styrene), or inert biological polymer. These inoculated loops are discarded after their use. Disposable loops are available in both sterile and nonsterile conditions. Disposable inoculating loops are suitable under the hoods and in the anaerobic chamber. It minimizes the risk of infection from the aerosols generated during flaming by the pathogenic substances.

Combo loop

combo loop
Combo loop, Image source: Universal Medical

Combi loops are also called combo loops. In both ends, two loops are present. One end has a loop with a diameter of 10 µl, and the next end has a loop with a diameter of  1 µl.

Inoculating loop with needle

inoculating loop with needle
Inoculating loop with a needle, Image source: Labproinc

It consists of both the loop and needle at the opposite ends.

Calibrated loop  

calibrated loop
Nichrome wire calibrated loop

Calibrated loops of 1µL and 10 µL help take the sample’s exact volume. These calibrated loops can be of metals that can be reused or of disposable plastics.

How to use an inoculating loop?

Inoculating loop made of the metals needs to be sterilized before its use, while the plastic inoculating loop is readily sterile. 

  1. Sterilization
Sterilization of inoculating loop
  • Hold the inoculating loop in the handle using the thumb and the first two fingers just like you hold the pen. Then sterilize the inoculating loop in the blue flame of the bunsen burner till it becomes red hot. 
  • An inoculating loop can be sterilized in the bacterial incinerator in which there is no open flaming and prevents the formation of aerosols from the infectious substances.
  • Once the loop is sterilized, allow it to cool down for 15-30 seconds. But be sure not to touch any surface once it is sterilized because it will be contaminated. If you mistakenly touch any surface after sterilizing it, you need to repeat the process and sterilize it until it becomes red hot. Then it can be used for inoculations.
  • A disposable inoculating loop does not need to be flamed, so after its use, its discarded safely. Be sure to follow the laboratory protocols while discarding the waste. 
  1. Inoculation on agar plates
  • To obtain the isolated colony from the streaking, follow the protocols of the streak plate method and sterilize it at every step. It helps to reduce the microbial load so that pure and isolated colonies can be isolated. While picking the colony, also be sure you pick only the selected colony.
  • When you are holding the inoculating loop in one hand, handle the Petri dish in another hand. While performing the quadrant method of streaking, sterilize before and after inoculation in every quadrant. 

Read more: Streak plate method

Video source: Hardy Diagnostics

Inoculation on broth tubes

inoculation on broth
Inoculation on broth, Image source: Flickr
  • If you are inoculating the sample in the broth media in tubes, take the loopful of the sample and immerse it in the broth.
  • Mix it slightly with the inoculating loop and place the cotton or the caps in the tube. 
  1. Making smears
smear making on slide
Smear on a slide, Image source: Susannaheinze
  • If you are making the smear from the isolated colonies for Gram stain, gently touch the colony and make the smear. If you take a heavy inoculum by taking a loopful of colonies smear will be thick. So be sure you follow the protocol properly.
  • Once you are done with inoculating the loop, sterilize it again and keep it in the rack in the aseptic condition. 

Read more: Gram staining

Uses of Inoculating Loop

  • Inoculating loop is used in picking the colony of bacteria or fungi.
  • Inoculating loop is used to inoculate in the agar plate, the slant of biochemical media, and the broth tubes.
  • Inoculating loop is used to make the smears on the slide.
  • Inoculating loop transfers the microorganisms in an aseptic condition and prevents cross-contamination.  

Inoculating Needle

inoculating needle
Inoculating needle

The inoculating needle has the same structure as the inoculating loop, except the end of the wire is straight. It is also made of metals and is sterilized either in a Bunsen burner or the bacterial incinerator. Plastics inoculating needles are also available, which are disposable. Inoculating wire is used to pick the small colonies and inoculation by stabbing the semi-solid media. 

References

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Sushmita Baniya

Hello, I am Sushmita Baniya from Nepal. I am a postgraduate student of M.Sc Medical Microbiology. I am interested in Genetics and Molecular Biology.

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