Fundamentals of Microbiology

Background

The motility of bacteria can be observed directly using a microscope or indirectly by observing the mass movement of bacteria in semisolid media by culture methods. Motility is the movement of organisms from their axis. They change their position. Motility is brought about by the organelle, flagella which are hair like structures that may be present at one end or both ends or in some cases all over the bacteria either in single or in tuft.

Different types of motility are exhibited by different groups of bacteria

1.      Darting motility (or) gun shot motility. This type of motility is exhibited by Vibrio cholerae

2.      Wriggling motility - exhibited by Proteus sp.

3.      Tumbling motility - exhibited by Listeria monocytogenes

In the case of culture methods, the bacteria are grown in a semisolid medium by inoculating with a straight wire by stabbing. Motile bacteria grow along the sides in line perpendicular to the line of stabbing. They can also be grown using Craigie's tube method, where a 'U' shaped tube is kept in a semisolid medium. Bacteria are inoculated through one end and if motile, growth can be observed at the other end of the tube. It is essential to differentiate between actual motility and Brownian movement in direct observation. Brownian movement is a vibratory movement of the cells because of their bombardment of water molecules in the suspension. However, the slight movement that is observed is on its own axis only, without causing any displacement. Care should be taken to differentiate drifting of bacteria by water movement I wind and the true motility.