Question

why inoculating an EMB plate and an MSA plate potentially useful?

Answer 1

EMB plate contains Eosin and Methylene blue. This medium is selective as well as differential medium. The toxic component of EMB medium is toxic to Gram Positive bacteria and therefore only Gram negative bacteria can grow over this medium. This medium is also selective for coliforms. This medium distinguish ferment lactose bacteria from lactose non fermentors. Lactose fermentor bacteria , such as E. coli produce acids during their growth which lowers the pH of the medium and the colonies appear purple black colonies. These colonies are often called nucleated colonies as there are dark spot in the centre. Lactose non fermenters causes deamination of proteins and thus pH of the medium increases and colonies appear colourless.

Mannitol salt agar (MSA) is also a selective as well as differential medium. This medium contains high amount of salt, so halophilic bacteria can grow on this medium. The bacteria which can ferment mannitol can grow on this medium. It is is selective medium as it only permit salt loving bacteria to grow on it and other organisms die. This medium contain mannitol and phenol red as pH indicator. Staphylococcus can grow on this medium but other bacteria such as Streptocoocus is unable to grow on this MSA medium.