Is nutrient agar selective

Answer 1

By definition, a selective medium is a medium that contains a substance that inhibits the growth of some types of bacteria but doesn't affect the growth of other types of bacteria (this is may be required when you want to isolate a specific type of bacteria from a specimen that contains other types of bacteria). Nutrient agar medium contains the following ingredients: peptone, meat extract, yeast extract, NaCl and agar. Nutrient agar doesn't contain any inhibitory substance for bacteria, therefore, it is not considered selective and instead it is called all-purpose medium. Answer An alternative definition of selective media is a media which allows the growth of some organisms whilst supressing the growth of others. Whilst nutrient agar contains no ingredients designed to supress growth, the composition is not suitable for the growth of certain microbes (fastidious bacteria) and so can be considered selective on this basis. For a case example try Mycobacterium lepraewhich is generally culitaved on living tissue rather than agar media. Nutriet agar is therefore selective in that certain microbes cannot grow on it. However, nutirent agar is generally considered the least selective media avalible. The confusion in answering the question arises in the catagories used to classify media in textbooks. Nutirent agar is suitable for the growth the vast majority of culturable bacteria, (however, it is estimated that only around 5% of the worlds bacteria can be cultured in the lab at present). As nutirent agar is sutiable for the growth of so many bacteria it may be catagorised as an "all purpose" media due to its suitablity for many different applications. However to researchers investigating new areas it is important for "all purpose" not to be confused with "all bacteria" media due to its selectivity. In summary, nutrient agar is selective but is classified "all purpose" as it is designed to allow the growth of many different species.

Answer 2

McConkey is selective to gram, negative enteric bacteria because it contains bile salts which inhibit most gram positive bacteria. It also contains lactose which differentiates between gram negative lactose fermenter(E.coli)and Gram negative non-fermenters (salmonella). This is typically useful for testing water samples for coliform bacteria.

Answer 3

MAC is a differential medium, meaning that this medium differentiates or distinguishes between groups of bacteria on the basis of a color change reaction. MacConkey’s contains two additives that make it differential; neutral red (a pH indicator) and lactose (a disaccharide).

Answer 4

The MacConkey agar is selective for Gram-negative and enterobacteria.