Microbiology - Microscopy - Quick Guides | Microbiology

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Study GuideMicrobiology–Microscopy1.Staining TechniquesMicroorganisms are usuallycolorless and transparent, which makes them difficult to see under alight microscope. To make them visible and easier to study, scientists usestaining techniques.Stains add color to microbial cells or their surroundings, helping us observe size, shape, andstructure.1.1When Staining Is Not NeededIn some situations, staining is unnecessary. For example:•When microorganisms arevery large•Whenmotility (movement)needs to be observedIn these cases, awet mountis prepared. A drop of the microbial sample is placed directly on a slideand observed.Another method is thehanging drop technique, where:•A drop of culture is placed on a cover slip•The cover slip is inverted over a hollow slideThis method is especially useful for studying motility in living microorganisms.1.2Preparing a Smear for StainingBefore staining, the microorganisms must be prepared properly:1.A small amount of microbial sample is placed on a slide2.The sample is allowed toair dry3.The slide isheat fixedby passing it quickly through a flame

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Study GuidePurpose of Heat Fixing•Kills the microorganisms•Makes them stick to the slide•Helps cells absorb stains more effectivelySimple Staining TechniquesSimple StainAsimple stainuses a single dye to color the cells.•Common dyes:crystal violet,methylene blue•These arebasic dyes(positively charged)•They are attracted to thenegatively charged cytoplasmThis method highlights:•Cell shape•Cell arrangementNegative StainNegative staining usesacidic dyes, such as:•Nigrosin•Congo redThese dyes arenegatively chargedand are repelled by the microbial cytoplasm. Instead of stainingthe cell, they color the background.Result:•Cells appear clear•Background appears darkThis method is useful for observing:•Cell size•Cell shape•Capsules (if present)

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Study Guide1.3Differential Staining TechniquesDifferential stains usemore than one dyeto distinguish between different types of microorganisms.Gram Stain TechniqueTheGram stainis the most important differential stain in microbiology. It divides bacteria into twomajor groups:•Gram-positive•Gram-negativeSteps in the Gram Stain1.Crystal violet (primary stain)2.Iodine (mordant)3.Alcohol wash (decolorization)4.Safranin (counterstain)

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Study GuideThe Gram stain procedure used for differentiating bacteria into two groups.Results•Gram-positive bacteriaoRetain crystal violet-iodine complexoAppearpurple or blue•Gram-negative bacteriaoLose crystal violet during alcohol washoTake up safraninoAppearred or pinkObservation is done under theoil-immersion lens.

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