CHAPTER 3-4 STUDY GUIDE (2).pdf

1. Understand what a "bacterial smear" is; why and how to prepare it. A bacterial smear is a thin layer of bacteria placed on a slide for staining and microscopic examination. It is prepared to fix the bacteria so that they adhere to the slide and to allow the stain to penetrate the cells fo...

1. Understand what a "bacterial smear" is; why and how to prepare it. A bacterial smear is a thin layer of bacteria placed on a slide for staining and microscopic examination. It is prepared to fix the bacteria so that they adhere to the slide and to allow the stain to penetrate the cells for better visualization. How to Prepare: a) Spread a small amount of the bacterial culture in a thin layer onto a clean slide. b) Allow the smear to air dry. This ensures that the water evaporates, leaving behind just the bacteria. c) Heat-fix the smear by quickly passing it through a flame. This kills the bacteria, making them adhere to the slide and readying them for staining without distorting their natural shape. 2. Differentiate an acidic dye from a basic dye; explain the purpose of simple staining. Acidic Dyes: These dyes, negatively charged, are repelled by the negatively charged bacterial cell walls, staining the background instead of the cells (e.g., nigrosin, India ink). They are used in negative staining. Basic Dyes: These dyes are positively charged and attract the negatively charged components of bacterial cells, staining the cells themselves (e.g., crystal violet, methylene blue). They are used in simple and differential staining. Purpose of Simple Staining: a) To make the bacterial cells more visible under the microscope by imparting color to the cells. b) Helps in observing the morphology (shape), size, and arrangement of bacteria. 3. Why is the Gram stain so useful? The Gram stain is crucial in microbiology because it differentiates bacteria into two groups: Gram- positive and Gram-negative, based on cell wall properties. This distinction helps in: o Diagnosis: Quickly suggests potential identities of the bacteria and their pathogenicity. o Antibiotic treatment decisions: Some antibiotics are more effective against one type of bacteria. o Epidemiological research: Helps track the spread of infections. 4. Why doesn't a negative stain color a cell? Negative stains do not color the cells because the dye is repelled by the bacterial cell walls' negative charge. Instead, the dye stains the background, creating a contrast that makes the clear, unstained cells stand out against the dark background. This is useful for viewing cell morphology and size without penetrating or altering the cells. 5. Why is heat-fixing necessary for most staining procedures? Heat-fixing serves several purposes: o Kills the bacteria, making them safe to handle. o Fixes the bacteria to the slide, preventing them from being washed off during the staining process. o Preserve the morphology of the bacteria, allowing for more accurate observations under the microscope. 6. How should the cells and endospores appear in an endospore stain? In an **endospore stain, bacterial cells are stained one color while the endospores are stained a different color due to their resistant nature. Typically: Cells: Stain pink or red with safranin. Endospores: Stain green with malachite green. This dual staining highlights the presence and location of endospores within bacterial cells. 7. Describe the evolution of the eukaryotic cell and the endosymbiotic theory. The endosymbiotic theory suggests that eukaryotic cells evolved from prokaryotic organisms through a symbiotic relationship. According to this theory: a) A larger prokaryotic cell engulfed smaller prokaryotic cells. b) These engulfed cells, instead of being digested, became beneficial to the host cell, evolving into organelles like mitochondria and chloroplasts. c) Over time, this symbiotic relationship became permanent, leading to the complex eukaryotic cells seen today. 8. Compare and contrast the cellular structure of prokaryotes and eukaryotes. Prokaryotes: o Smaller, simpler structure. o No nucleus; DNA is circular and located in a nucleoid region. o Few internal structures and organelles. o Cell wall made of peptidoglycan (in bacteria). Eukaryotes: o Larger, more complex cells. o Contain a nucleus where DNA is linear and enclosed. o Many membrane-bound organelles (mitochondria, ER, Golgi apparatus). o Plant cells have cell walls made of cellulose; animal cells lack cell walls.