McGraw Hill Campus Connect 2 Saved Help Save& Exit CKathy Park Talaro Mannitol s
ID: 187052 • Letter: M
Question
McGraw Hill Campus Connect 2 Saved Help Save& Exit CKathy Park Talaro Mannitol salt agar contains 7.5% sodium chloride, the carbohydrate mannitol and a pH indicator, phenol red, which turns yellow in the presence of an acid. Three different species of Staphylococcus are inoculated onto a mannitol salt agar plate. They are incubated overnight and the plate shown represents the resulting growth. One of the species grows and the plate remains pink, the other two species grow and the agar turns yellow. Answer the folowing questions regarding these observations. E. coll is an intestinal organism, and Staphylococcus epidermidis resides on the skin What do you hypothesize w occur when both are inoculated onto mannitol salt agar? Multiple Choice Both organisms will grow since they are both ubiquitous of 20… Next>Explanation / Answer
The answer is,
The salt inhibits the growth of E.coli which is not a halophyte, but S.epidermidis grows.
It is because, MSA (Mannitol Salt Agar) medium is a differential and selective medium that promotes the growth of certain bacteria while it inhibits the growth of other bacteria. MSA is usually used to identify pathogenic strains from non-pathogenic because, many of the pathogenic organisms are able to convert mannitol in the medium to an acidic byproduct as a result of fermentation of manitol. So, the presence of this acidic byproduct changes pink colour of phenol red to an yellow colour and thus organisms can be differentiated.
In addition, the speciality of MSA medium is its high salt content which is about 7.5 %. It is this high salt content which makes the medium selective. Many of the organisms especially gram negative bacteria ( which has a thin peptidoglycan layer as compared with gram positive) is not able to withstand this high salt content. HHence they get killed. E.coli which is gram negative is one such example.
But, S. epidermidis grows as it it is gram positive ( having thick cell wall made of protein peptidoglycan) and is able to withstand this high concentation of NaCl.