Microbiology Lab Midterm Exam Questions and Answers 100% Pass

Microbiology Lab Midterm Exam Questions and Answers 100% Pass Compound microscope - 1. Ocular-eyepiece lens (usually 10x) 2. Objective-nosepiece lenses (commonly 4x, 10x, 45x, and 100x) At what magnification do you use the oil immersion technique? - 100x. Total Magnification = - Magnification (Objective) x Magnification (Ocular) Important Parts of the Microscope - Framework, Stage, and Lens System Framework - Arm and base are the structural parts of the microscope which support the basic frame. Stage - Holds the slide. The mechanical stage clamps the slide and moves the slide around the stage. Lens System - Oculars, Objective, and Condenser Oculars - Eyepiece lenses (usually 10x magnification) Objectives - Lenses attached to rotatable nosepiece, common magnification of 4x, 10x, 45x and 100x. Parfocalized microscope focusing adjustments are not to be made when... - changing the objective lenses. The oil has approximately the same refractive index as glass to prevent... - light loss due to diffraction (bending of light rays) which would occur if light traveled from one refractive index to another (eg. glass to air) As magnification of the objective lens increases... - the working distance decreases. 2 | P a g e Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE Working distance - distance between the object on slide and the objective lens, when in focus. Condenser - directs light towards the objective lens in bright field microscopy. In dark field microscopy, the condenser... - directs light at oblique angles away from the objective lens in a manner that allows only objects in the field of view to redirect or scatter light into the objective lens. This causes objects to appear white on a dark field. Iris Diaphragm - lever located in the condenser that adjusts the diameter of the cone of light so that it just fills the objective lens. As you close down the Iris Diaphragm... - 1. The light intensity decreases 2. Contrast improves 3. Depth of field increases 4. Limit Resolution (with oil immersion lens) Resolution - Resolving Power. Expressed as d. d = the smallest distance between two objects which can be seen as separate d = the diameter of the smallest resolvable object d = wavelength / 2 NA NA = numerical aperture To improve resolution... - lower d. d can be decreased by... - lowering wavelength or increasing the NA. Cocci - More or less spherical organisms. Bacilli - Cylindrical organisms. 3 | P a g e Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE Helicoidal - Spiral shaped organisms. Incompletely separated cocci may appear in a number of different patterns depending upon... - the plane in which they divide and how they remain attached. Diplococci - pairs. Divide in one plane. Streptococci - chains. Divide in one plane. Tetracocci - tetrads. Divide in two planes. Staphylococci - clusters. Divide in three planes irregularly. Sarcinae - cuboidal packets. Divide in three planes regularly. Coccobacillus - very short and almost appears spherical, but they are just slightly longer in one direction than the other. Fusiform Bacilli - tapered at the ends, appearing as football like in shape. Filamentous Bacillary Forms - grow in long threads. Most microorganisms are difficult to see using light microscopy due to... - their size and the lack of contrast between the cell and the environment. The contrast is improved with... - the help of dyes. Dyes are... - organic compounds containing a chromophore with affinity for cellular material. Types of dyes - 1. Cationic (basic dyes, positively charged chromophore) - Methylene Blue, Crystal Violet 2. Anionic (acidic dyes, negatively charged chromophore) - Acid fuschin, Congo Red, Nigrosin 3. Fat Soluble (no charge): Sudan Black stains granules of Poly-B-OH-butyric acid 4 | P a g e Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE 4. Insoluble Dyes (water insoluble): India Ink (colloid suspension of carbon particles) Negative Staining - Stains background, not the cell in brightfield microscopy (Not Dark field Microscopy) Types of dyes used in Negative Staining - 1. Nigrosin - a black anionic (negatively) charged dye. The negatively charged dye is repelled by the negatively charged surface of the bacterial cell. 2. India Ink - an insoluble dye (a colloidal suspension of carbon particles) which does not penetrate the cell surface. Simple Staining - One dye used to stain all cells the same color. Can be used to tell morphology (shape) and size [although negative staining is better for size]. Cationic dyes are positively charged and are attracted by ionic forces to the negatively charged surface of the bacterial cell. Common dyes are methylene blue and crystal violet. Differential Staining - Staining procedure which causes cells to stain differently based on properties of the cell. Gram stain - a differential stain procedure that causes cells to stain differently based on characteristics of their cell wall. Cells are stained with crystal violet, then fixed with iodine forming a crystal violet- iodine complex within the cell. Ethanol is then added as a decolorizer. The gram neg. cells are then counterstained with saffranin. Gram-positive microorganisms have a higher ____________ and lower ______ content than gram-negative microorganisms. - 1. peptidoglycan 2. lipid Why are gram negative cells easily decolorized? - because the ethanol dissolves the high lipid cell wall allowing the crystal violet-iodine complex to readily exit the cell. 5 | P a g e Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE How do gram positive cells resist colorization? - due to the difference in cell wall consistency retaining the crystal violet-iodine complex. What color will the gram positive cells be after the primary stain (crystal violet)? - purple. What color will the gram negative cells be after the primary stain (crystal violet)? - purple. What color will the gram positive cells be after the mordant (iodine) is added? - purple. What color will the gram positive cells be after the decolorizer (ethanol) is added? - purple. What color will the gram positive cells be after the counterstain (saffranin)? - purple. What color will the gram negative cells be after the mordant (iodine) is added? - pur