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4. Bacterial Smears and Simple Stains Introduction The refractive index of most bacteria is only slightly greater than that of water; therefore bacteria are difficult to see in liquid suspension because they are almost transparent. To visualize bacteria, stains are used. Bacteria usually carry a negative charge (-). Most stains are either positively (+) or negatively (-) charged. Some stains are neutral. Positive or basic stains are attracted to the negatively charged bacteria (crystal violet, safranin, methylene blue) whereas negative stains are repelled from bacteria. A few stains (e.g. India ink) are composed of particles too large to enter the cells and are also referred to as negative stains. Staining allows observation of cell morphology (e.g. size and shape) and arrangements (clusters, chains, strands). Relationship to Class Instruction: Chapters 3 and 4. Lab report due on: _______________________________________ Simple stain of Staphylococcus with methylene blue Purpose of Laboratory  Use aseptic techniques to remove a bacterial sample from a culture  Prepare bacterial smears from solid cultures  Perform a simple stain Materials and Methods Organisms Escherichia coli: rod-shaped bacteria, usually single cells Bacillus megaterium: rod-shaped bacteria, arranged in strands Staphylococcus epidermidis: round-shaped bacteria, arranged in clusters Candida sp.: yeast cells 4-1

Bacterial Smears and Simple Stains

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