Introduction to Infectious Diseases

Assignment Brief

HLTH-320 Introduction to Infectious Disease

LAB QUIZ 2

1. A scientist engineered a Gram-positive bacterium to express a polysaccharide-rich outer capsule. What effect would this have on a Gram stain of this engineered organism? Explain your answer in one sentence. (9 points)
2. A scientist engineered a strain of S. aureus so that it no longer produced protein A. What effect would this have on a StaphTEX test of this engineered organism? Explain your answer in one sentence. (9 points) 
3. Suppose you performed a Gram stain on a culture of Klebsiella pneumoniae but you accidentally skipped the decolorizer step. What color would the cell appear under the microscope? (9 points) 
4. Suppose you performed a Gram stain on a culture of S. aureus but you accidentally skipped the Gram’s iodine step. What color would the cell appear under the microscope? (9 points) 
5. What is the difference between Klebsiella pneumoniae and Klebsiella pneumonia? (4 points) 

Sample Answer

1. A scientist engineered a Gram-positive bacterium to express a polysaccharide-rich outer capsule. What effect would this have on a Gram stain of this engineered organism?

If a Gram-positive bacterium is engineered to produce a thick polysaccharide-rich outer capsule, the Gram stain would still show purple (violet) cells because of the thick peptidoglycan layer that retains the crystal violet-iodine complex. However, the presence of a capsule can alter the visual appearance under the microscope. The capsule itself does not absorb the stain, so the cells may appear surrounded by a clear or faint halo. In addition, because the capsule can sometimes mask cell wall structures, the cells may appear slightly larger and rounder than non-capsulated versions. This outer capsule may also affect how clearly defined the cell edges appear, giving a more blurred or diffused outline.

2. A scientist engineered a strain of S. aureus so that it no longer produced protein A. What effect would this have on a StaphTEX test of this engineered organism?

Protein A is a surface protein produced by Staphylococcus aureus that binds to the Fc region of antibodies and plays a major role in immune evasion. The StaphTEX test is a rapid latex agglutination test that detects S. aureus by recognising Protein A, clumping factor, and sometimes capsular antigens. If the organism no longer produces protein A, the test may result in reduced or no visible agglutination (clumping), depending on whether the remaining antigens are sufficient for a positive reaction. In some cases, the absence of protein A could lead to a false negative result, meaning the test might not identify the bacterium as S. aureus, even though it is. This demonstrates how important protein A is in rapid identification methods for this pathogen.

3. Suppose you performed a Gram stain on a culture of Klebsiella pneumoniae but you accidentally skipped the decolorizer step. What colour would the cell appear under the microscope?

Klebsiella pneumoniae is a Gram-negative bacterium, which typically appears pink or red after a correct Gram stain because the alcohol decolorizer removes the crystal violet, allowing the safranin counterstain to be absorbed. However, if the decolorizer step is skipped, the crystal violet will remain trapped in the cell, even though the outer membrane and thin peptidoglycan layer of Gram-negative bacteria cannot usually retain it. As a result, K. pneumoniae cells would incorrectly appear purple, like a Gram-positive organism. This error would lead to a misidentification and confusion during diagnosis, as staining is one of the first steps in bacterial classification.

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