Bacteriological loop: characteristics, types, uses

He bacteriological loop It is a work instrument used by microbiologists for the sowing of microbial cultures (bacteria or fungi). It is perhaps the instrument most used by these professionals.

Its use seems simple, but in reality it requires a lot of training. Before starting to sow, the loop must be sterilized in the Bunsen burner, then taking the microbial inoculum that can come from a liquid culture or a solid culture.

Once the inoculum is taken, it can be deposited in a liquid medium or seeded in a solid medium. After each procedure the loop is sterilized again. The handle is also used to make microbial smears on a slide.

The bacteriological loop has many names, among them we can mention: loop or platinum loop, inoculation loop or sowing loop, however its real name is Kohle’s loop.

The material with which the handle is made is generally an alloy of nickel and chromium (nichrome) or also platinum. Another variety available on the market are plastic disposable bacteriological loops.

On the other hand, there are several types of bacteriological loops, each one fulfills a function within the tasks carried out by a microbiologist. The choice of the handle will depend on what is required to be done.

For example, to carry out an exhaustion fluting in order to isolate a microorganism, the platinum loop that ends in a closed ring is used, regardless of whether it is calibrated or not.

While, if it is to sow a urine sample, where it is important to quantify the CFU, it is necessary to use the calibrated loop. Likewise, to inoculate biochemical tests, which require seeding by puncture, the needle loop is essential. It is important that the handle is handled by trained personnel.

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Characteristics

Bacteriological loops are made up of two ends. One end corresponds to the handle or base, while the other is the handle itself.

Handle base or handle

The base is used to grip the handle during handling. It is usually metallic, but has a heat insulator to prevent the operator from being burned when the tip of the handle is sterilized in the lighter. It measures approximately 20 cm, although it can be longer. It has a thread to adjust the handle.

Handle

It is the functional end, that is, it is the part that comes into contact with the microorganisms and the culture media. It consists of a fine wire that is inserted into the base.

The handle can be made of nichrome or platinum, which is why it is sometimes called a platinum handle. It is approximately 6.5cm long and has a gauge of approximately 0.20mm. The handle with the handle can measure 26.5 cm. Measurements may vary from manufacturer to manufacturer.

Guys

There are several types of bacteriological loops and each one has a specific use. The types of handles are as follows: calibrated ring-shaped handle, non-calibrated ring-shaped handle, straight or needle handle, spatulate handle, «L»-shaped handle and handle with a sharp point.

On the other hand, the handles can be for permanent or disposable use. Those for permanent use are sterilized in the lighter, being reusable and very durable. Whereas, the disposable ones come sterile from the factory, they are used only once and discarded.

Applications

Ring-shaped handle, not calibrated

It is the most used handle in the microbiology laboratory. It is used to take an inoculum from a microbial suspension (liquid medium) or a portion of the colony (solid medium), to be sown in another culture medium, either a biochemical test or a more enriched or selective medium, as the case may be. It is also useful for making microbial smears on a slide.

With this type of loop, the striation of the initial inoculum can be performed. It is also designed to glide smoothly over the agar, without damaging it. Of course, this requires trained personnel to handle the seeding technique, applying the correct force and movements to make a good streak.

Various sowing techniques can be executed with this handle. Example, solid media in petri dishes are usually seeded by depletion (there are several techniques, see video 1).

The purpose of this technique is to obtain isolated colonies in order to study and identify them, especially when the sample is polymicrobial, that is, the culture is not pure (it contains more than one type of microorganism).

Instead, the wedges are planted in a simple zigzag pattern to obtain bacterial mass, and it is not necessary in this case to isolate colonies.

Sometimes they are also used to take certain samples. For example, Rojas et al. used loop bacteriology to take scraping samples from vaginal tissue in rats.

Ring-shaped handle, calibrated

This handle, as its name indicates, is calibrated to take a specific amount of microbial suspension. It is used when it is necessary for the inoculum to comply with what is standardized in the technique.

For example, it is very common for the sowing of urine cultures. In this case, a 0.01 ml and a 0.001 ml calibrated loop can be used.

It should be noted that the calibrated handles have a margin of error, which varies according to the angle at which the sample is taken and the size of the mouth of the container.

If the 0.001 ml loop is inserted vertically into a small-mouth container (≤ 7 mm), the amount taken will be 50% less than desired.

This occurs because the smaller the diameter of the mouth of the container, the greater the surface tension and therefore, the adhesion forces between (glass – liquid) increase. This generates that the load of liquid taken by the handle is less.

Whereas, if it is taken at an angle of 45° in a wide-mouth container (≥ 22 mm), 150% more than the required amount can be collected. In this case, the surface tension is lower and the glass/plastic cohesion forces decrease, while the liquid-liquid cohesion force increases.

Because of the above, other methods are sometimes preferred for the quantification of colony-forming units for urine cultures.

Straight or needle handle

It is also called a thread handle. It is used to inoculate certain biochemical tests, especially those that require seeding by the puncture technique. Example, semi-solid media that are prepared in the form of a plug, such as: SIM, MIO and O/F medium.

In addition, in the case of MIO and SIM that evaluate motility, it is essential that the puncture be performed with a needle loop, since the interpretation of positive motility occurs when the bacteria grow to the sides of the initial inoculum.

A test inoculated with a loop with a ring can falsify the results (false positive). For this reason, andIn this type of sowing, care must be taken that the puncture is single, and that when it is withdrawn it comes out in the same direction in which it entered.

Another test that should be needled even though it is prepared in a wedge shape is the Kligler. In this test it is not recommended that oxygen penetrate into the depth of the agar, since an oxygen-poor medium is required for certain chemical reactions to occur. This loop is also used to touch a specific colony in order to subculturate.

spatulate handle

Most microorganisms give soft, easily detached colonies. However, there are some microbial agents whose colonies are hard and dry, such as mycobacteria in general and some fungi. In these cases it is useful to use the spatulated handle.

«L» shaped handle

Its most frequent use is to repair fungal cultures, especially molds.

Handle with sharp point

It is useful for taking very small colonies present in polymicrobial cultures, in order to be able to subculture them.

sowing techniques

Video 1

https://youtu.be/-TnHCd4sY24

Platinum loop sterilization

The handle must be sterilized before and after use. To sterilize the bacteriological loop in the burner, proceed as follows: place the loop in the upper part of the flame, adopting a position that is as vertical as possible, so as to sterilize from the tip upwards.

When the fine filament turns bright red, it can be said to be sterile. To use it, it is cooled and the chosen sample is taken. (See video 2).

Loop bacteriological sterilization

Video 2

References

«Bacteriological loop » Wikipedia, The Free Encyclopedia. 23 Jul 2019, 13:31 UTC. Aug 18, 2019, 10:45 p.m. Wikipedia.
Herrera D, López P, Duque J, Pérez L, Golding R, Hernández C. Calibrated metal loops for microbiologists: A nationally manufactured alternative. Soc. Come. Microbiol.2010; 30(1):37-42. Available at: ve.scielo.org
Forbes B, Sahm D, Weissfeld A. (2009). Bailey & Scott Microbiological Diagnosis. 12 ed. Editorial Panamericana SA Argentina.
Koneman E, Allen S, Janda W, Schreckenberger P, Winn W. (2004). Microbiological diagnosis. 5th ed. Editorial Panamericana SA Argentina.
Rojas A, Pardo-Novoa J, Río R, Gómez-Hurtado M, Limón D, Luna F, et al. Determination of the analgesic effect of the hexanic extract of flowers of Eupatorium arsenei in a rat model of acute pain. mex. science. farm. 2015; 46(1): 64-69. Available at: scielo.org

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