There are two types of scanning probe microscopy. The first is called scanning tunneling microscopy or STM and the second is atomic force microscopy or AFM. The STM technique passes a probe above the specimen, creating an electric current between the specimen and the probe. The intensity of the current is measured. The AFM also passes a probe across the specimen and it moves up and down because of the different forces between the atoms and the probe. The amount of deflection is measured and an image is constructed.
STAINING OF MICROORGANISMS Without staining, it would be difficult to see most specimens because they do not have a lot of contrast between the different structures. For this reason, staining is used in many microscopy settings in order to see the structures better. In preparing the specimen, there are wet mounts and dry mounts available. Wet mounts involve suspending the specimen in a liquid medium. Water and sometimes stains are used after which a coverslip is applied to the glass slide. Fixation actually involves attaching the cells to the slide. It involves heat fixation or chemical treatment to kill the specimen. The organisms do not move and can easily be stained. Chemical fixating agents include acetic acid, methanol, ethanol, glutaraldehyde, and formaldehyde. Stains are also added to cause coloration of the specimen. All stains are salts that have both a negative and positive ion. One ion will be the chromophore and the other will be uncolored. If the chromophore belongs to the positive ion, it will be called a basic dye. If it belongs to the negative ion, it will be called an acidic dye. There will be a positive stain that is absorbed by what you want to see and a negative stain, which is absorbed by the background. Most positive chromophores are basic dyes that stick to bacterial cell walls. These include crystal violent, basic fuchsin, malachite green, safranin, and methylene blue. Negatively charged chromophores made up of acidic dyes are not taken up by the cell wall; these include rose Bengal, eosin, and acid fuchsin. Simple staining involves a single dye, while differential staining uses multiple dyes in order to show different structures as different colors. Gram staining, endospore 15
