Test on Gram staining
Gram Staining: A Comprehensive Guide for Students
Test: Gram Staining
20 questions
Question 1: Gram-negative bacteria in older cultures, typically over 24 hours, can appear Gram-positive due to altered cell wall integrity.
A. Yes
B. No
Explanation: The study materials state that older bacterial cultures can cause Gram-positive bacteria to lose their ability to retain the crystal violet-iodine complex, making them appear Gram-negative (Gram-variable or false Gram-negative). The materials do not mention Gram-negative bacteria appearing Gram-positive due to the age of the culture.
Question 2: Under-decolorization during Gram staining can cause Gram-negative cells to incorrectly appear Gram-positive.
A. Yes
B. No
Explanation: If the decolorizer is not applied for long enough, Gram-negative cells may not completely lose the crystal violet-iodine complex and will retain some purple color, appearing Gram-positive. This is described as a false Gram-positive result.
Question 3: The Gram stain can provide immediate information that allows clinicians to initiate empirical antibiotic treatment before definitive culture results are available.
A. Yes
B. No
Explanation: A Gram stain rapidly reveals the presence, Gram reaction, and morphology of bacteria within minutes of receiving a clinical sample. This immediate data is crucial for clinicians to start preliminary antibiotic treatment, as waiting for definitive culture results can take 24-72 hours, and early intervention significantly improves patient outcomes.
Question 4: After undergoing a complete Gram staining procedure, *Staphylococcus aureus* appears pink or red under a light microscope.
A. Yes
B. No
Explanation: The study materials state that *Staphylococcus aureus* is a Gram-positive coccus. For Gram-positive bacteria, after the application of crystal violet, Gram's iodine, decolorization, and safranin, the cells remain purple because the thick peptidoglycan layer retains the crystal violet-iodine complex, and safranin cannot displace it. Gram-negative bacteria are the ones that appear pink or red after counterstaining with safranin.
Question 5: Over-decolorization in Gram staining causes Gram-negative cells to incorrectly appear Gram-positive.
A. Yes
B. No
Explanation: Over-decolorization causes Gram-positive cells to lose their primary stain and pick up the counterstain, making them appear Gram-negative, which is described as a false Gram-negative result. It does not cause Gram-negative cells to appear Gram-positive.