Microbiology Practice Exam 2026 – Comprehensive All-in-One Guide for Exam Success!

Mannitol salt agar is specifically selective for Gram-positive bacteria due to its high salt concentration, which inhibits the growth of most Gram-negative bacteria while allowing for the proliferation of staphylococci, which are Gram-positive. The medium contains mannitol, which can be fermented by certain staphylococci, leading to a color change in the agar that aids in distinguishing those that can ferment mannitol from those that cannot.

This property makes Mannitol salt agar an excellent selective medium for isolating and identifying species such as Staphylococcus aureus, which ferments mannitol and produces acid, thus changing the medium's color. Other types of culture media mentioned do not selectively favor Gram-positive bacteria in the same way; for instance, nutrient agar supports a broad range of bacteria regardless of their Gram status, MacConkey agar is selective for Gram-negative bacteria (especially enteric bacteria), and bile esculin agar is primarily selective for enterococci and some streptococci, but not exclusively for Gram-positive bacteria.