VMC 211: General Veterinary Microbiology (1+1)

PREPARATION OF SMEARS

• Bacteria differ a slightly in refractive index from the surrounding medium. Hence, it is difficult to see unstained bacteria in ordinary microscope. So staining is the primary requirement to see the bacteria in the light microscope.
• Smear can be prepared from fluid materials like culture, urine, sputum, pus etc by taking a loopful of the material in an inoculating loop and spread it thinly on a clean glass slide. The smear is then allowed air dry.
• The smear can also be dried by holding it high over a bunsen flame.
• The dried smear is then fixed by passing it through the flame slowly three times with the smear upwards.
• Alternatively the fixing can also be done by heating through the slide. In this method the slide is held with the smear on top in the top of the Bunsen flame for a few seconds so that the slide becomes hot.
• The slides are then marked on one end with a diamond or grease pencil on the side having the smear. In slides with a ground matt surface at one end ordinary graphite pencil can be used to mark the slide.
• For preparation of smears from material like cultures on agar first a loopful of water or saline is placed on the slide and then with a sterilized loop a minute quantity of material is taken from the culture and put on the water placed on the slide and emulsified. Then a thin smear is prepared.