<P> Mannitol salt agar or MSA is a commonly used selective and differential growth medium in microbiology . It encourages the growth of a group of certain bacteria while inhibiting the growth of others . This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time . It contains a high concentration ( about 7.5 % - 10 % ) of salt ( NaCl ) , making it selective for Gram - positive bacteria ( Staphylococcus and Micrococcaceae ) since this level of salt is inhibitory to most other bacteria . It is also a differential medium for mannitol - fermenting staphylococci , containing carbohydrate mannitol and the indicator phenol red , a pH indicator for detecting acid produced by mannitol - fermenting staphylococci . Staphylococcus aureus produces yellow colonies with yellow zones , whereas other coagulase - negative staphylococci produce small pink or red colonies with no colour change to the medium . If an organism can ferment mannitol , an acidic byproduct is formed that causes the phenol red in the agar to turn yellow . It is used for the selective isolation of presumptive pathogenic ( pp ) Staphylococcus species . </P>
<P> Mannitol salt agar or MSA is a commonly used selective and differential growth medium in microbiology . It encourages the growth of a group of certain bacteria while inhibiting the growth of others . This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time . It contains a high concentration ( about 7.5 % - 10 % ) of salt ( NaCl ) , making it selective for Gram - positive bacteria ( Staphylococcus and Micrococcaceae ) since this level of salt is inhibitory to most other bacteria . It is also a differential medium for mannitol - fermenting staphylococci , containing carbohydrate mannitol and the indicator phenol red , a pH indicator for detecting acid produced by mannitol - fermenting staphylococci . Staphylococcus aureus produces yellow colonies with yellow zones , whereas other coagulase - negative staphylococci produce small pink or red colonies with no colour change to the medium . If an organism can ferment mannitol , an acidic byproduct is formed that causes the phenol red in the agar to turn yellow . It is used for the selective isolation of presumptive pathogenic ( pp ) Staphylococcus species . </P>
what is the role of phenol red in the mannitol salt medium
what is the role of phenol red in the mannitol salt medium
a pH indicator for detecting acid
a pH indicator for detecting acid