Something About Something

Microbiological Media Table of results: Media| E. coli| Salmonella| Serratia| Pseudomonas| Proteus| EMB(Colour: Wine)| Metallic Green sheen on streak. Media: Pink| ColourlessMedia: Wine| Black dot in centre of colonies. Media: Wine| Similar colour to the media. Media: Wine| ColourlessMedia: Wine | XLD(Colour: Red)| ColourlessMedia: Yellow| Black dot in centre of colonies. Media: Pink| Pink/OrangeMedia: Pink| ColourlessMedia: Pink| Black dot in centre of colonies. (smaller dots than Salmonella)| BGA(Colour: Orange)| ColourlessMedia: Yellow| ColourlessMedia: Pink| ColourlessMedia: Yellow| Pink-RedMedia: Pink| ColourlessMedia: Pink|

E. coli grown in EMB agar: As we can see from the table above, the streak of E. coli on a plate with EMB agar showed a metallic green sheen where E. coli was present. EMB stain is selective for gram-negative bacteria. It is made using 6:1 Eosin and Methylene Blue. EMB agar is a differential media and inhibits the growth of gram-positive bacteria while also using a colour indicator to tell the difference between organisms that ferment lactose and those that don’t. E. coli will give off a metallic green sheen when grown in EMB due to metachromatic properties in the dyes used, E. oli movement using flagella and the strong acid end products associated with fermentation. Salmonella grown in XLD agar: XLD can commonly be used as a selective growth medium for Salmonella and Shigella. Salmonella will ferment xylose to produce acid. This turns the media yellow due to the lowering of the pH which turns the phenol red indicator in XLD agar to yellow. Shigella colonies do not turn yellow as they cannot ferment xylose and therefore don’t produce acid in XLD. Once Salmonella has exhausted the xylose in XLD it decarboxylates lysine which increases the pH again and turns back to a similar colour of the Shigella colonies.

Salmonella will metabolise thiosulfate to produce hydrogen sulphide and this is what leads to the formation of colonies with black centers, this allows for differentiation to the similarly coloured Shigella colonies. Salmonella grown in BGA agar: BGA agar is selective and differential for Salmonella. When grown in BGA agar, Salmonella will remain colourless as they do not ferment sucrose or lactose therefore they do not increase the pH level and no indicating colour can be seen. Brilliant Green inhibits the growth of gram-positive and most of gram-negative organisms.