Generally, Motility is the ability of the organism to move itself by using a flagella or some other propellers which are unique for each of the organism and helps in its locomotion.
Where as bacteria confines a unique form of locomotion by using its specific fibrils which looks like a gliding sort of motion.
The motile bacteria move either by using a flagellum, a thread like locomotor which extends outward from the cell membranes.
Motility is too long to be recognized as it has it own important characteristic feature in the world of taxonomy.
Usually, the flagella are present primarily in all kind of basic bacilli’s, but few of them are flagellated cocci, which have their motility varying manner depending upon the different genera.
However, identification of motility is very important for categorizing the family of Enterobacteriaceae.
In microbiology, from early days, differentiation and classification of Enterobacteriaceae is identified by means of performing the kinds of motility test.
Motility Test Objective
• The main aim of the test is to detect the motility of the bacterium.
• To differentiate between the motile and a non-motile species of bacteria.
Motility Test Principle
Usually, the motility of the bacterium is demonstrated by using a semi solid medium of agar. As in the semi solid medium, motile bacteria swarm and it gives a diffused spread of growth which can be recognized in a naked eye.
The agar medium is usually used in the Sulphide Indole motility medium which is a combination of the differential medium where the test is performed using three different parameters such as reduction of Sulphur, Indole production, and motility.
This media has a very soft consistency which allows the bacterium to migrate along with and causing cloudiness in the medium, the inoculum is stabbed to the center of the semi solid medium deeply.
Bacterial motility is identified by a diffuse zone of growth which extends from the line of the inoculation. Whereas some organisms which grow along the entire medium shows very small areas of growth or there will be formation of nodules in a particular area.
On the other hand, non-motile bacteria grow only in the soft agar tube and only in areas where they are inoculated.
Motility Test Reagents
Here SIM medium is generally used. It consists of the following ingredients.
|Ingredients||Gram per liter|
|Pancreatic digest of casein||20.0|
|Peptic digest of animal tissue||6.1|
Motility Test Procedure
Initially, a straight needle is used to touch a young colony which has been kept for about 18-to-24-hour culture in a medium of agar.
Then the medium is stabbed once in the middle of the tube up to half an inch of the tube.
It should also be made sure that the needle should be removed from the tube as the same way it entered.
Then the medium is incubated at a temperature of 35 to 37ºC and it is observed frequently up to 7 days.
Within seven days, the diffused zone of growth which is flaring out from the line of inoculation can be seen.
Motility Test Results
Positive Motility Test Result: If the medium gives a positive result, then it gives a diffuse, hazy growth that spreads across the medium which is renders as slightly opaque one.
Negative Motility Test Result: In case of negative results, the growth is confined to the stab line, which has sharp defined margins and leaves the surrounding area of the medium opaque.
Motility Test Uses
• Motility Test is significantly used for differentiating the microorganisms on the basis of their motility performed in a laboratory setting
• Motility Test is also useful for assigning a taxonomic classification of the organisms.
• Motility Tests also plays an important role in characterizing the pathogens.
• Motility Test are usually employed for identifying the protocols which belongs to the family of Enterobacteriaceae.
• Motility Test is used for the specific species which helps to differentiate the gram-positive cocci, Enterococci, Faecalis, E. casseliflacus, Enterococcus faecium.
Motility Test Limitations
Some kind of organisms will not show the sufficient growth in the medium, which makes an accurate determination and the additional follow-up testing is needed.
It is also recommended that the biochemical, molecular, mass, spectrometry testing; are all performed in the colonies that are taken from the pure culture for complete identification of the species.
If the flagella of the bacteria are damaged due to heating, shaking or any other trauma it results in false-negative results. On the other hand, if the organisms are found to have a weak and unstable mode of motility, it leads to false-negative reactions.
While inoculating in a semi-solid media, it is important to note that the inoculating medium is removed along the straight line in the same way it is inserted. A fanning motion of removing a needle may result in false-positive results.
Motility Test Citations
- Evaluation of 15 motility media and a direct microscopic method for detection of motility in enterococci. J Clin Microbiol . 2002 Jul;40(7):2476-9.
- Use of Two Selective Media and a Broth Motility Test Can Aid in Identification or Exclusion of Bacillus anthracis. J Clin Microbiol. 2005 Sep; 43(9): 4336–4341.
- Bacterial Motility .J Bacteriol. 1934 Feb; 27(2): 165–174.