The Principle, Procedure And Interpretation Of Motility Tests

Cells and organisms can move on their own. They move through expending and using energy. This is known as motility.

The Principle, Procedure And Interpretation Of Motility Tests

The way that cells and organisms move around depends on various things, including the use of muscles in animals and structures that have the ability to propel single cell organisms along.

What Is Motility?

Motility is the way that an organism is able to move. A single cell organism is able to move by itself in a way that is propeller-like.

It is a very unique way of moving that is individual to bacteria. This way in which bacteria moves around is known as flagella. 

The way this bacteria moves around is through a locomotor appendage which extends from the plasma membrane or the wall of the cell in either a movement called a multiple flagella, or a single flagellum.

Motility is a very important tool which microorganisms use that is seen as a key characteristic of them. Motility and the presence of flagella is a very important means of identification for the bacteria family as it exists mainly in bacilli.

The ability that bacteria has to move around is a key way in which organisms have been classified and differentiated.

Motility Of Bacteria

You can see the motility of bacteria by the way in which it swarms in a semi-solid agar media. The medium that it uses is called SIM medium, short for Sulphide Indole Motility.

This media has a very soft consistency which allows the motile bacteria to move through them which shows some cloudiness.

This displays where the bacteria has moved. You can also see the proof of motility through the evidence of the zone of growth.

Bacteria that is non-motile will only grow when it is in a soft agar tube and it won’t move around or grow anywhere that it was not inoculated.

How To Test The Motility Using SIM Medium

There is a great and effective way to test the motility of bacteria using a SIM medium. As explained above, the Sulphide Indole Mobility process will show you the movement of bacteria. 

Follow these steps below: 

  1. Press a needle to your colony of a culture that is growing on the agar medium. It should be a young culture, about 18 to 24 hours old.
  2. Only enter the needle a third of an inch into the tube and make sure you pull the needle out at the exact same angle as it entered.
  3. Place the tube in incubation at around 35 – 37 degrees celsius and keep looking at it daily for 7 days. 
  4. You should be able to see a diffuse zone of growth that occurs from the area of inoculation.

What You Will See?

If you have received a positive result, you will see cloudy growth dispersing from the line of inoculation.

If you have experienced a negative result, you will see a growth that is only linked onto the line where you pushed in the needle and you will see very defined margins around this. Around this line, there will be a transparent medium.

What Does This Test Show?

This test is used to show which microorganisms have motility within the lab. It allows you to classify organisms and for the differentiation of microorganisms.

Motility tests help to identify cocci and Enterobacteriaceae, and they are often used for this purpose.

Problems With Motility Tests

Problems With Motility Tests

All lab experiments and tests come with plenty of limitations and these should not be ignored.

You should note when carrying out these tests that many organisms won’t display sufficient growth during the time of testing. You may need to retest if this happens as you won’t have an accurate result. 

It is also possible to get a false negative result in some cases if the bacteria has been damaged due to heat or some other trauma that could damage it.

This may make the organism non-motile and will mean that the test may not be accurate. This could also occur if the organism is motile but does not have much motility.

When you are inserting the needle, you should make sure that you are removing the needle at the same angle as you have entered it. You can get a false-positive if you have moved the needle around at this stage.

What Are Some Examples Of Motile Organisms?

Most animals are motile organisms as they move around. They do this in different ways, such as through walking, swimming or flying. Through these means, they propel themselves through the air, sky or sea.

There are also lots of motile organisms that are single celled and microscopic. This is what this test will look for, showing which microscopic organisms are motile.

What Is It Called When An Organism Is Immobile?

Immobility is known as sessility when you are talking about an organism that does not have the ability to move or propel itself around. These are immobile organisms and can be identified using the test explored above. 

Final Thoughts 

You should now have all the information on motility tests, what they are and how to carry them out. You should also have an understanding of the result you should see from these tests and how to interpret these.

Always remember that experiments and tests like these should be performed multiple times to ensure their accuracy as sometimes they can be inaccurate for various reasons.

Jennifer Dawkins

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