Types of Growth Media to Culture Bacteria - P3

Mannitol Salt Agar 
(MSA)  Microbiology Lab Tutorial

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Another specialized medium, Blood Agar (BAP) contains sheep’s blood. If bacteria growing on the medium produce exotoxins that hemolyze (cut up) the red blood cells, the medium changes color. 

BAP medium that has changed from red to transparent (completely clear) indicates beta-hemolysis, and possibly the strep throat pathogen Staphylococcus pyogenes. BAP that is bruised (alpha-hemolysis) or unaffected by bacterial growth present (gamma hemolysis) indicate normal flora, non-pathogenic bacteria.

Blood Agar 
(BAP)  Microbiology Lab Tutorial

Mannitol Salt Agar grows halophilic (salt-loving) bacteria. Pathogenic Staph is growing on the left (yellow side) and normal flora Staph are growing on the right (pink). ​
Blood Agar is a nutrient-rich medium containing sheep's blood. Beta-hemolytic bacteria, such as pathogen Streptococcus pyogenes change this medium from solid red to clear.
Blood Agar, a nutrient-rich medium containing sheep's blood. Beta-hemolytic bacteria, such as pathogen 
Streptococcus pyogenes change this medium from solid red to clear.

Bacteria that grow on 
Mannitol Salt Agar (MSA) are all salt-loving halophiles. Salt is what makes this medium selective. 

MSA also contains the sugar-alcohol mannitol (food) and a pH sensitive dye. This is the differential aspect of the medium. 

If bacteria that cannot eat mannitol are growing on MSA (such as the normal flora species Staphylococcus 
epidermidis) the medium remains pink. If halophilic bacteria that are able to eat mannitol are growing, (such as the pathogen Staphylococcus aureus), the pH sensitive dye is triggered by bacterial waste and the medium changes to bright yellow.

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​Page last updated: 8/2016
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