Question

4a. Describe why the EMB, Hektoen Enteric and Mannitol Salt agar are selective. Include

specific selective ingredients added to each media and what organisms these media select for and

against.

4b. Describe why the EMB, Hektoen Enteric and Mannitol Salt agar are differential. Include

specific differential ingredients added to each media and which types of organisms produce

positive differential results.

Answer 1

Ans 1: EMB (Eosin methylene blue) is selective and differential stain for coliforms (Gram negative bacteria). It is a blend of two stains, eosin and methylene blue. These dyes are toxic to Gram positive bacteria. Hektoen enteric and Mannitol Salt agar is also selective and differential growth medium in microbiology. The media contains high concentration of salt (NaCl), which makes it selective for most of the Gram negative bacteria and some Gram positive bacteria as well such as, Staphylococcus, Enterococcus and Micrococcaceae. The high salt concentration is inhibitory to most other bacteria. The media is used to selectively distinguish pathogenic microbes from non-pathogenic ones in a short period of time.

Ans2: The combination of stain present in EMB inhibits the growth of the Gram positive bacteria and acts as a colour indicator which distinguishes between organisms that produce lactose (e.g., E. coli) and those that does not (e.g., Salmonella). The organisms that produces lactose by fermentation displays 'nucleated colonies i.e., colonies with dark centres'.

Mannitol agar is a differential medium for mannitol fermenting Staphylococci and those that does not. The media contains apart from mannitol agar a pH indicator named phenol red (the differential component) which turns yellow due to pH change when Mannitol is fermented, for the non-producers the media remains red. The reason behind colour change is that when mannitol is fermented acidic by-product is produced, thus lowering down the pH.