Cindy, a 17-year-old counselor at a summer sports camp, scraped her knee playing basketball 2 weeks ago. At the time, she thought it was only a minor abrasion that would heal, like many others before it. Instead, the wound began to look like an insect bite and has continued to become increasingly painful and swollen. The camp nurse examines the lesion and observes a large amount of pus oozing from the surface. Concerned that Cindy may have developed a potentially aggressive infection, she swabs the wound to collect a sample from the infection site. Then she cleans out the pus and dresses the wound, instructing Cindy to keep the area clean and to come back the next day. When Cindy leaves, the nurse sends the sample to the closest medical lab to be analyzed under a microscope. QUESTION 1: What are some things we can learn about these bacteria by looking at them under a microscope? Part 2 Wound infections like Cindy’s can be caused by many different types of bacteria, some of which can spread rapidly with serious complications. Identifying the specific cause is very important to select a medication that can kill or stop the growth of the bacteria. After calling a local doctor about Cindy’s case, the camp nurse sends the sample from the wound to the closest medical laboratory. Unfortunately, since the camp is in a remote area, the nearest lab is small and poorly equipped. A more modern lab would likely use other methods to culture, grow, and identify the bacteria, but in this case, the technician decides to make a wet mount from the specimen and view it under a brightfield microscope. In a wet mount, a small drop of water is added to the slide, and a cover slip is placed over the specimen to keep it in place before it is positioned under the objective lens. Under the brightfield microscope, the technician can barely see the bacteria cells because they are nearly transparent against the bright background. To increase contrast, the technician inserts an opaque light stop above the illuminator. The resulting darkfield image clearly shows
that the bacteria cells are spherical and grouped in clusters, like grapes.

QUESTION 2: Why is it important to identify the shape and growth patterns of cells in a
specimen?
QUESTION 3: What other types of microscopies could be used effectively to view this
specimen?

Part 3
Viewing Cindy’s specimen under the darkfield microscope has provided the technician with
some importante clues about the identity of the microbe causing her infection. However, more
information is needed to make a conclusive diagnosis. The technician decides to make a Gram
stain of the specimen. This technique is commonly used as an early step in identifying
pathogenic bacteria. After completing the Gram stain procedure, the technician views the slide
under the brightfield microscope and sees purple, grape-like clusters of spherical cells (Figure
2.35).

Figure 2.34 In this specimen, the gram-positive bacterium Staphylococcus aureus retains crystal violet
dye even after the decolorizing agent is added. Gram-negative Escherichia coli, the most common Gram
stain quality-control bacterium, is decolorized, and is only visible after the addition of the pink
counterstain safranin. (Credit: modification of work by Nina Parker)
QUESTION 4: Are these bacteria gram-positive or gram-negative?
QUESTION 5: What does this reveal about their cell walls?