A wound culture grows on MSA and turns the medium yellow. What is the most likely pathogen?
Staphylococcus aureus
Enterococcus faecalis
Salmonella enterica
Pseudomonas aeruginosa
The Correct Answer is A
A. Staphylococcus aureus: Mannitol Salt Agar (MSA) is both selective and differential. It contains high salt concentration, which inhibits most bacteria except staphylococci, and mannitol with a pH indicator to detect fermentation. Staphylococcus aureus ferments mannitol, producing acid that lowers the pH and turns the medium yellow.
B. Enterococcus faecalis: Enterococcus faecalis can tolerate salt but does not typically ferment mannitol on MSA, so the medium generally remains red or pink. It is more commonly identified using other selective media like bile-esculin agar.
C. Salmonella enterica: Salmonella species do not grow well on high-salt MSA and do not ferment mannitol, so they would not produce yellow colonies on this medium. They are typically detected using selective media like XLD or Hektoen enteric agar.
D. Pseudomonas aeruginosa: Pseudomonas aeruginosa is inhibited by the high salt concentration in MSA and does not ferment mannitol. While it may grow slowly under some conditions, it will not produce yellow colonies, making it unlikely to be the pathogen here.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is C
Explanation
A. Pink: Pink indicates Gram-negative bacteria, which do not retain the crystal violet stain during the decolorization step and are counterstained with safranin. Gram-positive bacteria are not pink after Gram staining.
B. Red: Red is another way to describe the color of Gram-negative bacteria after counterstaining with safranin. Gram-positive bacteria do not take up the red counterstain because they retain the primary crystal violet stain.
C. Purple: Gram-positive bacteria appear purple after Gram staining. Their thick peptidoglycan cell walls trap the crystal violet-iodine complex even after alcohol or acetone decolorization, allowing them to retain the purple color and be distinguished from Gram-negative bacteria.
D. Colorless: Colorless cells would indicate a failure to stain or a decayed sample. Gram-positive bacteria are never colorless after a proper Gram staining procedure.
Correct Answer is B
Explanation
A. The organism cannot tolerate salt: Mannitol Salt Agar (MSA) contains 7.5% NaCl, which inhibits the growth of most bacteria except halotolerant organisms. Growth on this medium indicates that the organism can tolerate high salt concentrations, so inability to tolerate salt is not supported by the observation.
B. The organism ferments mannitol: MSA contains mannitol and a pH indicator (phenol red). When bacteria ferment mannitol, acidic byproducts lower the pH, turning the agar yellow around the colonies. The presence of a yellow zone surrounding growth indicates that the organism is a mannitol fermenter, commonly seen with Staphylococcus aureus.
C. The organism is Gram-negative: MSA is selective for Gram-positive bacteria due to its high salt concentration. Gram-negative bacteria are typically inhibited and do not grow well on this medium. Therefore, growth on MSA suggests the organism is Gram-positive, not Gram-negative.
D. The organism produces hemolysin: Hemolysis is detected on blood agar, where lysis of red blood cells produces clear or greenish zones around colonies. The yellow zone on MSA is due to mannitol fermentation and not hemolysin production, so this observation does not indicate hemolysis.
Whether you are a student looking to ace your exams or a practicing nurse seeking to enhance your expertise , our nursing education contents will empower you with the confidence and competence to make a difference in the lives of patients and become a respected leader in the healthcare field.
Visit Naxlex, invest in your future and unlock endless possibilities with our unparalleled nursing education contents today
Report Wrong Answer on the Current Question
Do you disagree with the answer? If yes, what is your expected answer? Explain.
Kindly be descriptive with the issue you are facing.