Examples of selective media:

A microbiological culture medium is a substance that encourages the growth, support, and survival of microorganisms. Culture media contains nutrients, growth promoting factors, energy sources, buffer salts, minerals, metals, and gelling agents (for solid media) [2].

These are classified into six types: (1) Basal media, (2) Enriched media, (3) Selective (4) Indicator media, (5) Transport media, and (6) Storage media. 1. BASAL MEDIA. Basal media are those that may be used for growth (culture) of bacteria that do not need enrichment of the media.

• Eosin methylene blue contains dyes that are toxic for Gram-positive bacteria.
• YM (yeast extract, malt extract agar) has a low pH, deterring bacterial growth.
• MacConkey agar is for Gram-negative bacteria.
• Hektoen enteric agar is selective for Gram-negative bacteria.

Examples: Nutrient broth, nutrient agar and peptone water. 2. ENRICHED MEDIA The media are enriched typically by adding blood, serum or egg. Examples: Enriched media are blood agar and Lowenstein-Jensen media.

According to Altheide and Snow, media culture means that within a culture, the media increasingly influences other institutions (e.g. politics, religion, sports), which become constructed alongside a media logic. Since the 1950s, television has been the main medium for molding public opinion.

Blood agar is differential media because 3 different types of hemolysis, or lysing of red blood cells, can be seen on this plate.

Blood agar is a general purpose enriched medium often used to grow fastidious organisms and to differentiate bacteria based on their hemolytic properties. In the U.S., blood agar is usually prepared from tryptic soy agar or Columbia agar base with 5% sheep blood.

Blood Agar is used to grow a wide range of pathogens particularly those that are more difficult to grow such as Haemophilus influenzae, Streptococcus pneumoniae and Neisseria species. It is also required to detect and differentiate haemolytic bacteria, especially Streptococcus species.

Chocolate agar (CHOC) or chocolate blood agar (CBA), is a nonselective, enriched growth medium used for isolation of pathogenic bacteria. It is a variant of the blood agar plate, containing red blood cells that have been lysed by slowly heating to 80°C.

MacConkey agar is used for the isolation of gram-negative enteric bacteria and the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria.

Nutrient agar is a general-purpose media that is mostly used for routine culture or to ensure prolonged survival of microorganisms. It is one of the most important and commonly used non-selective media for the routine cultivation of microorganisms.

Escherichia coli (E. coli) is a Gram-negative, rod-shaped, facultative anaerobic bacterium.

Selective and Differential Media MacConkey agar not only selects for Gram-negative organisms by inhibiting Gram-positive organisms and yeast but also differentiates the Gram-negative organisms by lactose fermentation. Escherichia coli and other lactose ferments will produce yellow or orange colonies.

Cultural Characteristics of Escherichia Coli: On Nutrient agar, colonies are large, thick, greyish white, moist, smooth, opaque or translucent discs. Some strains may form “mucoid ” colonies. On MacConkey agar medium, colonies are bright pink due to lactose fermentation.

Preparation of MacConkey Agar

1. Suspend 49.53 grams of dehydrated medium in 1000 ml of distilled water.
2. Heat to boiling to dissolve the medium completely.
3. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
4. Cool to 45°C -50°C.
5. Mix well before pouring into sterile Petri plates.

Preparation of Nutrient Agar

1. Suspend 28 g of nutrient agar powder in 1 litre of distilled water.
2. Heat this mixture while stirring to fully dissolve all components.
3. Autoclave the dissolved mixture at 121 degrees Celsius for 15 minutes.
4. Once the nutrient agar has been autoclaved, allow it to cool but not solidify.

Incubation conditions: aerobically at 35 ± 2°C for 18-24 hours. 18-72 h at 30-35°C for E. coli (Pharmacopoeia growth promotion).