Mastering Microbiology Ch 3

What happens to the light rays when they hit the specimen?

They are reflected, refracted, or absorbed by the specimen.

In a typical brightfield microscope (seen in the animation), at which point does magnification begin?

The objective lens

To communicate effectively about use of the microscope in the laboratory setting, you need to know the parts of the microscope. This activity asks you to label the parts of a typical compound light microscope. Drag the label to the appropriate part of the microscope.

switch condensor and diphragm

Why is a specimen smaller than 200 nm not visible with a light microscope?

Anything smaller than 200 nm cannot interact with visible light.

What is meant by light rays being divergent?

It is spreading out

A student from one of the research labs is having trouble preparing a slide for examination and photographing. The bacterial slide that he has brought to you was prepared using a commercially purchased stain. He has asked for your help in determining what he is doing wrong so that he can change the lab protocols and continue on with his project. After examining the slide under oil immersion, you determine that no bacteria are present even though the student is able to show you the culture he used to make that slide that has visible growth in the liquid medium. Which of the following statements does NOT explain the fact that there are no bacteria present on the student's slide? Select all that apply.

Rinsing with alcohol during the washing step stripped the bacteria off the glass slide.

What is the role of the ocular lens?

To recreate the image in the viewer's eye

In addition to knowing the names of the parts of the compound light microscope, you also need to know the functions of these parts. This activity asks you to match the appropriate function with each part of the microscope. Match each microscope part with its function. Drag the labels in the left column to indicate the microscope part that performs each function listed on the right.

1. Fine focus knob:used after initial focusing to sharpen the image 2. Ocular lens:lens that you look through 3. Objective lens:lens that is closest to the slide and provides initial magnification of a specimen 4. Coarse focus knob:used for initial focusing; should never be used when the high-power objective lens or oil immersion objective lens is in place 5. Stage:platform on which the slide is placed for viewing 6. Condenser lens:used to focus the light from the illuminator onto the slide 7. Diaphragm:controls how much light from the illuminator reaches the specimen

Chlamydia trachomatis elementary bodies measure approximately 0.25 micrometers in diameter. What is this measurement expressed in nanometers (nm)?

250 nm

You volunteer to help the student with his fixation technique and in choosing a proper stain for the project he is working on. After watching and helping the student correct any problems with his fixation technique, you now need to determine which stains to use. The single criterion for the project is to be able to determine cell shape and size in a pure broth culture after some treatments. No differentiation between cell types is required (i.e., Gram-positive or Gram-negative), so you want to convince the student that a simple stain would be his best option. The charged dyes used in simple staining will penetrate the bacterial cell and will be retained after rinsing the slide with water to remove surplus dye. Which staining procedure would be best to use to stain the slides required for this student's project? Why?

The positively charged methylene blue will be attracted to the negatively charged components of the cell wall and will be retained.

When observing a specimen in a microscope you are told that the total magnification of the specimen is 630x assuming you are using a standard occular lens with a magnification of 10x what is the magnification of the objective lens?

63x

You have been asked to lead a demonstration for the undergraduate microbiology lab course about the uses of negative staining when studying bacteria. A "negative" stain does not stain the bacterial cell itself but stains the space between cells. Under magnification, the acidic (negatively charged) nature of the stain will be repelled by the negatively charged bacterial cell wall and will leave the cell colorless in a stained background. Negative stains are used primarily to reveal the presence of negatively charged bacterial capsules; therefore, they are also called capsule stains. Encapsulated cells appear to have a halo surrounding them. The negative stain procedure does not require heat fixation, which limits any chances of alteration in bacterial cell shape and size. The bacterial suspension is added to a drop of stain, such as nigrosin or eosin, and drawn across the glass slide using a coverslip. Nigrosin staining-not safranin staining-of Klebsiella pneumoniae will allow for the visualization of the cell shape and the determination of the presence of a capsule.

true

A paramecium is approximately 150 micrometers in length. What is this measurement expressed in millimeters (mm)?

0.15 mm

This activity asks you to sort items according to the most appropriate method for viewing. For each of the following items, indicate which viewing method is most appropriate-the unaided eye, light microscopy, scanning electron microscopy, or transmission electron microscopy-by dragging it into the correct bin.

ResetHelp unaided eye examining your cat for the presence of fleas light microscope examining a clinical specimen, such as a sputum smear, for the presence of bacteriaexamining a blood smear for the presence of malarial parasites scanning electron microscope viewing the three-dimensional shape of a protozoan and the arrangement and distribution of cilia on its surfacedetermining the three-dimensional shape and arrangement of bacterial cells transmission electron microscope confirming the 9 + 2 microtubule arrangement in a eukaryotic flagellumviewing ribosomes (20 nm) within a bacterial cellviewing a cross section of poliovirus (30 nm)viewing the layers of the gram-negative cell wall in cross section

A new project has come up in the core center - a researcher wants to study bacterial biofilms and will need you to document the process of biofilm formation. Bacterial biofilms are colonies of different species of bacteria that interact and allow the bacteria to grow in a new environment. Since this will be a mixed culture, you will need to use a staining protocol that allows for differentiation between bacteria based on cell wall properties. The Gram staining procedure uses a series of stains and alcohol decolorization to differentially color different species of bacteria that may be present in the growing biofilm. This differential colorization will allow for determination of bacterial morphology and give some insight into the bacterial cell wall composition. Put the following descriptions in order for the staining reactions in the cells of a bacterial smear during the Gram staining procedure.

Unstained cells (start of process) All cells are stained purple. Dye molecules are rendered insoluble in presence of mordant; dye is rinsed away. Cell wall of Gram-negative cells is broken down; Gram-positive cells remain intact. Clear cells are stained a red or pink whereas the color of other cells remains unchanged. Stained cells (end of process)