In this course, we will be using the proteobacterium, E. coli, to propagate copies of METgenes on plasmids, which are small circles of DNA that replicate in the E. coli cytoplasm. Unlike the E. coli strains that you hear about in food-borne outbreaks, our lab strain has been engineered for use in molecular biology labs and is unable to colonize the human intestinal tract. E. coli
are particularly useful to molecular biologists because they rapidly grow to very high densities
in laboratory culture media, reaching densities of 1-10 billion cells per mL. Although E. coli are 10-100 times smaller than yeast cells, their sheer numbers and their distinct motion renders them visible with the light microscope.
Leica DM500 Light microscope
To observe E. coli with any detail, you will need to use the 100X lens, which is also known as an oil immersion lens. This is the longest, most powerful and most expensive lens on the microscope, requiring extra care when using it. As the name implies, the 100X lens is immersed in a drop of oil on the slide. Immersion oil has the same refractive index as glass, so it prevents light from bending as it enters the lens. The oil should be removed immediately from the 100X lens after use. Oil should NEVER touch the 4X, 10X or 40X lenses, which are destroyed by the oil.