Techniques for Observing Microorganism
A considerable amount of information can be gained by careful microscopic examination of microorganisms. There are two general techniques used in the preparation of microbial specimens to observe them under microscope. First technique employs the unstained preparation of living cells and second one employs stained preparations of killed microorganisms.
Living microorganisms can be examined directly by wet mount or by hanging drop preparations. Both the techniques are very useful in determining size, shape and motility of the microorganisms. The spirochetes (spiral bacteria) are normally examined in wet preparation through Dark-field microscope. Some cell inclusion bodies such as vacuoles and spores can be readily observed even without staining.
· A wet mount is made by keeping a drop of liquid containing microorganisms (culture) on a microscope slide and placing a cover slip over the drop. (Figure 3.1a)
· A hanging drop mount is made by using a cover slip and a cavity slide. Vaseline is applied on each of the four corner of the cover slip or around the cavity using a match stick. A drop of culture (liquid containing microorganisms) is placed on a cover slip. The cavity slide is placed upside down on the cover slip and inverted such that the drop is hanging (Figure 3.1b).
Since microbial cells are colourless and transparent, observation of microorganisms in wet preparation by bright field microscope is difficult. But, dark-field and phase contrast microscopes give contrast and make structures within the cells to appear clear. Therefore, these microscopes are useful for examination of unstained preparation.
Staining enables better visualization of microorganisms under a microscope. Microscopic examination of stained cells helps to reveal the size, shape and arrangement of microbial cells. Microbial cell staining is important in the identification of infectious pathogens.