Differential and Selective Culture of Bacteria

The Selective Media allow one type of organism to grow preferentially because of the components included in the agar. Examples are the agars that selectively inhibit the growth of most organisms, while allowing skin bacteria to grow (Mannitol-Salt Agar for Staph. culture), those that inhibit Gram-positive organisms due to bile salts (MacConkey agar) and dyes like basic fuchsin, eosin, and methylene blue (EMB agar).

The Differential Media allow different groups of bacteria to be distinguished based on their appearance and/or biochemical reactions. Blood agar (BAP), as described for the culture of Streptococci, is one such agar. It allows bacteria to be distinguished based on hemolysis of the blood in the agar. However, it does not prohibit the growth of other types of bacteria, so it is not a selective media. Some strains of Staphylococcus produce beta-hemolysis on BAP.

Some differential media are sophisticated enough to allow preliminary identification of bacterial species to be made simply from culture results. MacConkey agar is both differential and selective. Since it contains lactose and neutral red dye, so lactose-fermenting colonies appear pink-to-red in color and are easy to distinguish from nonfermenting colonies. This is because of a reaction between the acid metabolic by-products of bacterial growth and the agar components.

Differential Culture Media and Characteristics

MEDIUM	IMPORTANT INGREDIENTS	PURPOSE AND DIFFERENTIAL INGREDIENTS
Selective Isolation Media
Manitol-Salt Agar (MSA)	7.5% NaCl (750g/l) Mannitol Neutral Red	Purpose: Selective isolation of Staphylococcus species and preliminary identification of pathogenic species by fermentation of mannitol (pink-to-yellow color change with acid production). Inhibitor: High sodium chloride concentration inhibits many types of bacteria. Carbohydrate Source: Mannitol pH Indicator: Phenol Red
Salmonella-Shigella agar (SS Agar)	Bile salt (8.5g/l) Sodium citrate (8.5g/l) Brilliant green Sodium thiosulfate Ferric citrate Lactose Neutral Red	Purpose: Inhibits the growth of coliform organisms & permits the growth of Salmonella & Shigella species from clinical specimens. Inhibitor: High Bile Salt concentration & Sodium Citrate inhibit all G+ bacteria and many Gram- bacteria. bacteria and many Gram- bacteria Carbohydrate Source: Lactose pH Indicator: Neutral Red
Primary Isolation Media (Inhibitory).
MacConkey agar (MAC)	Bile salt (1.5g/l) Crystal violet Lactose Neutral Red	Purpose: Recover the Enterobacteriaceae & related enteric Gram-negative bacilli. Inhibitor: Bile salt & Crystal Violet. Inhibit the growth of Gram-positive bacteria and some fastidious Gram-negative bacteria. Carbohydrate Source: Lactose pH Indicator: Neutral Red
Eosin-Methylene-Blue Agar (EMB)	Eosin Y Methylene-Blue Lactose	Purpose: Isolation & detection of the Enterobacteriaceae or related coliform bacilli from specimens with mixed bacteria. Inhibitor: Eosin & methylene blue. Inhibits the growth of Gram-positive bacteria and some fastidious G- organisms Carbohydrate Source: Lactose pH Indicator: Eosin & methylene blue combine to form a precipitate at acidic pH.