How does emb inhibit gram positive

EMB agar contains sucrose and lactose, utilized as fermentable carbohydrates substrates, which encourage the growth of some gram-negative bacteria, especially fecal and non-fecal coliforms. Differentiation of enteric bacteria is possible due to the presence of the sugars lactose and sucrose in the EMB agar and the ability of certain bacteria to ferment the lactose in the medium.

Eosin Y and methylene blue are pH indicator dyes that combine to form a dark purple precipitate at low pH; they also serve to inhibit the growth of most Gram-positive organisms. Peptic digest of animal tissue serves as a source of carbon, nitrogen, and other essential growth nutrients.

Phosphate buffers the medium. Levine modification contains 10g of lactose twice as in EMB agar and contains no sucrose. Any growth on the media should be regarded as contamination and the whole lot should be discarded. Performance testing of prepared EMB agar plates can be done by inoculating known strains of bacteria into the medium and observing growth and colonial characteristics.

News Ticker. About Acharya Tankeshwar Articles. Hello, thank you for visiting my blog. The medium contains peptone, lactose, sucrose, dipotassium phosphate, eosin and methylene blue dyes. Eosin and methylene blue act as indicators to differentiate between Gram negative organisms that ferment lactose and those that do not ferment lactose. Most bacteria that ferment lactose form colonies on EMB agar that are dark blue to black with a metallic sheen due to precipitation of the dyes by the acid by-products of fermentation.

Colonies produced by lactose non-fermentors are not dark blue or black. The growth of Gram positive bacteria is generally inhibited on EMB agar because of the toxicity of methlyene blue dye. In low concentration, the protective lipid outer membrane of Gram negative bacteria prevents entry of the toxic water soluble dye while the more porous cell wall of Gram positive bacteria without the protective outer membrane makes them more sensitive to the toxicity of methyene blue.

Recipe g peptone, 10g Lactose, 2g dipotassium phosphate, 0. A differential medium can be used to differentiate Gram negative enteric organisms based on the colony color.