Question

what is the best the best kind of media (agar or broth medium) to use for identification of the following bacteria:

1. Staphylococcus aureus
2. Escherichia coli
3. Streptococcus pneumoniae

Examples of possible media choices may include: TSA, TSIA, MSA, MacConkey, Blood Agar, EMB, SIM, MR-VP, etc.

Also, provide reasoning behind each selection. Also list if each of the media used are selective and/or differential. What observable changes would you expect to see from the media after a proper incubation period with each bacterial species?

Answer 1

For proper identification of the organism, we use agar- solid medium because we can find the colony morphology shown by the bacteria only on a solid surface. According to the colony morphology and in case of selective/differential medium, the colour change shown by te bacteria is very useful for identifying it.

Staphyococcus aureus- MSA- Mannitol Salt Agar is the best option for identification for this particular bacterium.

MSA is a selective medium which selectively permits growth of some bacteria and inhibits the growth of some bacteria due to the high NaCl concentration in it. It is a differential media too. Because it helps in differenitiating coagulase positive Staphylococci from coogulase negative Staphylococci

As Staphyococcus aureus can grow in high salt concentration (6.5% of NaCl) and it can ferment the sugar mannitol, MSA is routinely used for identifying Staphylococcus aureus. It ferments the sugar and so the phenol rd indicator in the medium, coverts thered colour of the medium too yellow colour.

So Staphyococcus aureus produces yellow colonies on MSA.

Eschericia coli- EMB or MAcConkey agar

EMB- Eosin Methylene BLUE and MacConkey agar are both selective for Gram negative bacteria and inhibitory for Gram positive bacteria. It is differential too because it differentiates lactose fermenting bacteria from non lactose fermenting bacteria.

EMB- Eosin and Methylene blue inhibits Gram positive bacteria. The lactose sugar in the medium makes the media differential. The bacteria that ferments lactose changes the pH of the medium acidic and thus the colour of the medium. Lactose fermentors produce metallic sheen/ bluish black colonies and non fermentors produce pink colonies

MacConkey agar - Crystal violet inhibits the growth of Gram positive bacteria. Lactose sugar makes it differential. Bacteria those ferments lactose, chnages the pH of the medium acidic and changes the colour through the pH indicator- neutral red. lactose fermentors produce pink coloured colonies, Lactose non fermentors prodice colourless colonies.

So Eschericia coli is both Gramnegative and lactose fermentor. So it produces metallic sheen in EMB and pink coloured colony in Macconkey

Streptococcus pneumoniae- Blood agar can be used for identifying Streptococcus pneumoniae.

Blood agar is Nutrient agar supplemented with blood. The bacteria that grow in this agar is differentiated by observing their hemolytic morphology. Three type of hemolytic patterns are possible.

alpha hemolytic- partial hemolysis-green colour colonies

beta hemolytic - complete hemolysis- clear zone

gamma hemolytic- poor hemolysis or non hemolytic- brownish

S pneumoniae shows green, alpha hemolysis