EMB agar is considered selective because:
It enhances growth of Gram-positive bacteria
It contains dyes that inhibit Gram-positive organisms
It only supports anaerobic growth
It contains high salt concentration
The Correct Answer is B
A. It enhances growth of Gram-positive bacteria: EMB agar does not promote the growth of Gram-positive bacteria; in fact, it inhibits them. Its formulation is designed to favor Gram-negative organisms, particularly enteric bacteria, by incorporating selective agents.
B. It contains dyes that inhibit Gram-positive organisms: EMB (Eosin Methylene Blue) agar contains the dyes eosin Y and methylene blue, which selectively inhibit the growth of most Gram-positive bacteria while allowing Gram-negative bacteria to grow. This selectivity makes EMB useful for isolating enteric Gram-negative bacilli and differentiating lactose fermenters from non-fermenters.
C. It only supports anaerobic growth: EMB agar supports the growth of facultative and aerobic Gram-negative bacteria. Anaerobic growth is not a specific feature of EMB, and oxygen is not restricted in its use.
D. It contains high salt concentration: High salt concentration is a characteristic of selective media like Mannitol Salt Agar (MSA), which inhibits non-halotolerant bacteria. EMB’s selectivity is based on dyes, not salt concentration, so this is not the mechanism for inhibiting Gram-positive bacteria.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is B
Explanation
A. Enzymes that inhibit bacterial growth: Hemolysins do not inhibit bacterial growth; instead, they act on host cells. They are not antimicrobial agents but rather virulence factors that facilitate bacterial survival and spread within the host.
B. Toxins that lyse red blood cells: Hemolysins are exotoxins produced by certain bacteria, such as Streptococcus and Staphylococcus species. They disrupt the membrane of red blood cells, causing hemolysis and releasing hemoglobin. This action provides bacteria with access to nutrients, particularly iron, which is essential for their metabolism and pathogenicity.
C. Structural proteins in bacterial capsules: Capsules are composed of polysaccharides or proteins that protect bacteria from phagocytosis. Hemolysins are not structural components of capsules; they are secreted proteins that damage host cells rather than forming protective layers around the bacterium.
D. Antibiotic resistance factors: Hemolysins do not confer resistance to antibiotics. Antibiotic resistance factors include enzymes such as beta-lactamases or efflux pumps that prevent drugs from killing the bacteria. Hemolysins function solely as cytotoxins targeting host cells.
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.