Question

given are mixed of broth cluture of S. aureus and E.coli. Explain the procedure? The goal is to have a plate of S. aureus with isolated colonies and a plate for E.coli? include the temperatute and time for oxygen required?

given is a mixed broth culture of Staphylococcus aureus and Escherichia coli. We have to isolate and purify each bacterial species. what are the steps? the goal is to have a plate of pure S. aureus with isolated colonies and a plate of pure E. coli with isolated colonies.

Answer 1

The task mentioned above is to make independent cultures of E. coli and S. aureus from a mixed culture of these bacteria. E. coli is a gram-negative bacteria while S. aureus is a gram-positive bacteria. It is easier to suppress the growth of gram-positive bacteria by using inhibitors like methylene blue. Thus, a differential media called EMB Agar (Ethylene Methylene Blue Agar) can be used to obtain E. coli. The methylene blue in this agar inhibits the growth of gram-positive bacteria like S. aureus. E. coli can utilize the saccharose present in the media and form dark-green colonies.

On the other hand, to grow the S. aureus and suppress the growth of E. coli, Mannitol Salt Agar (MSA) selective media can be used. The high salt concentration in the agar inhibits the growth of most bacteria except Staphylococcus. The pathogenic S. aureus produces small colonies. They ferment the mannitol sugar which leads to production of acid and thereby a decrease in pH. The pH decrease leads to color change of the inidicator from red to yellow.