Which additive in selective media inhibits most saprophytic fungi?

Which additive in selective media inhibits most saprophytic fungi?

• A. Chloramphenicol
• B. Cycloheximide
• C. Gentamicin
• D. Ciprofloxacin

Answer: (B)

When isolating dermatophytes, selective media use additives to suppress competing organisms so the fungi of interest can grow. The additive that most effectively inhibits saprophytic fungi is cycloheximide. It blocks eukaryotic protein synthesis by interfering with the 60S ribosomal subunit, which stops growth of many saprophytic molds and yeasts. Dermatophytes, on the other hand, can tolerate cycloheximide well enough to grow, letting you detect them more cleanly on the plate. The other options are antibacterial agents; they target bacteria, not fungi. Chloramphenicol, gentamicin, and ciprofloxacin suppress bacterial contaminants but don’t prevent saprophytic fungi from growing, so they don’t provide the same selective pressure for isolating dermatophytes.

When isolating dermatophytes, selective media use additives to suppress competing organisms so the fungi of interest can grow. The additive that most effectively inhibits saprophytic fungi is cycloheximide. It blocks eukaryotic protein synthesis by interfering with the 60S ribosomal subunit, which stops growth of many saprophytic molds and yeasts. Dermatophytes, on the other hand, can tolerate cycloheximide well enough to grow, letting you detect them more cleanly on the plate.

The other options are antibacterial agents; they target bacteria, not fungi. Chloramphenicol, gentamicin, and ciprofloxacin suppress bacterial contaminants but don’t prevent saprophytic fungi from growing, so they don’t provide the same selective pressure for isolating dermatophytes.