Which of the following best describes the characteristic of Eosin Methylene Blue Agar (EMB)?

Eosin Methylene Blue Agar (EMB) is specifically designed to select for Gram-negative enteric bacteria. This medium contains eosin Y and methylene blue dyes, which inhibit the growth of most Gram-positive bacteria, thereby allowing for the selective isolation of Gram-negative organisms.

Enteric bacteria, such as Escherichia coli and Enterobacter species, can ferment lactose and produce acid, which reacts with the dyes in the medium, often leading to color changes. E. coli typically produces a distinctive greenish sheen, enabling easy differentiation from non-lactose fermenters. Therefore, the characteristic of EMB as selective for Gram-negative enteric bacteria is accurate and reflects its practical application in microbiology for isolating and identifying these types of microorganisms.

The other options do not accurately represent the properties and intended use of EMB: the medium does not promote growth for Gram-positive bacteria nor does it inhibit all Gram-negative bacteria or promote mixed acid fermentation in all bacteria. Such distinctions are crucial for understanding the selective nature of culture media in microbiological diagnostics.