Selective effect of crystal violet: On blood agar containing crystal violet at 1:500,000, which group of bacteria is predominantly inhibited, a property used to facilitate isolation of streptococci?

Introduction / Context: Selective agents in culture media suppress certain flora to highlight target organisms. Crystal violet is one such agent that differentially affects Gram-positive cocci, aiding in the observation of streptococcal hemolysis.

Given Data / Assumptions:

• Blood agar is supplemented with crystal violet at a dilution of 1:500,000.
• Goal is to reduce background growth of competing Gram-positive cocci.
• We evaluate inhibition patterns for streptococci versus staphylococci.

Concept / Approach: Crystal violet inhibits the growth of many Staphylococcus species more than that of streptococci, thereby facilitating detection of streptococcal hemolysis. Thus, the medium is used when we want to see β-hemolytic streptococci without heavy overgrowth by staphylococci.

Step-by-Step Solution: Identify the selective agent: crystal violet. Recall its effect: greater suppression of staphylococci. Choose the group predominantly inhibited: staphylococci. Confirm that streptococci remain relatively less affected, improving isolation.

Verification / Alternative check: Many laboratory protocols specify crystal violet blood agar or colistin–nalidixic acid media to favor recovery of streptococci by limiting staphylococcal growth.

Why Other Options Are Wrong:

• Streptococci: not the main group inhibited at this concentration.
• Both/None: do not reflect the selective purpose of the additive.

Common Pitfalls: Assuming crystal violet equally inhibits all Gram-positives; its selectivity is leveraged intentionally to favor streptococci.

Final Answer: Staphylococci