Viewing Stained Bacteria
Bacteria are among the smallest cells, and are best viewed using oil immersion microscopy. Because bacteria are so small, their shapes can be distorted or hidden and staining properties can be obscured by the lack of resolution with dry objectives. In practice, bacteria smears (Gram stains, negative stains, spore stains) are viewed with dry objectives solely to find a suitable section of slide to look at. The switch to oil immersion mode is then made and the improvement in resolution is dramatic.
Make sure that the smear is up. The lOOx oil immersion lens and possibly the 40x high dry lens cannot focus through the thickness of a glass slide. Start with the scanning lens and focus on the smear. At 40x stained bacteria look like a smudge. Switch to 1OOx. The largest bacteria may begin to take shape, although most smears still look like a smudge. At 400x the shape of cells should be evident, that is, you can tell long rods from cocci. Colors will be distorted, however, and small rods may be mistaken for pairs of cocci.
Switch to oil immersion mode. Place a generous amount of oil on the slide so that you can move around to find a suitable area. After adding the oil, don't try to go back to the high dry lens.
Phase contrast, if available with oil immersion, can help you determine cell shape. Gram stain and spore stain results should be very clear in oil immersion. Never rely solely on the high dry lens for color determination. There is too much distortion.