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Identification of Staphylococcus and Streptococcus Using Mannitol Salt Agar

This laboratory exercise focuses on identifying Staphylococcus and Streptococcus bacteria through their growth on Mannitol Salt Agar (MSA). MSA is a selective medium with a high concentration of sodium chloride (7.5%), making it inhospitable for most bacteria while allowing Staphylococcus species to thrive. It serves as a differential medium by utilizing a pH indicator to signal mannitol fermentation. Pathogenic Staphylococcus aureus ferments mannitol, turning MSA bright yellow, while nonpathogenic species like Staphylococcus epidermidis do not. Understanding these differences aids in bacterial identification.

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Identification of Staphylococcus and Streptococcus Using Mannitol Salt Agar

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  1. Lab 14 IDENTIFICATION OF STAPHYLOCOCCUS AND STREPTOCOCCUS

  2. Staphylococcus • Mannitol Salt Agar (MSA) • Mannitol Salt is a selective bacterial growth medium because it has a very high concentration of NaCl (7.5%). Most bacteria cannot survive in this highly saline, hypertonic environment. But the genus Staphylococcus is well adapted to saline environments and grows well in this media. • This growing medium is also differentialbecause it contains a dye that identifies types of Staphylococcus that produce an organic acids from mannitol fermentation (eating mannitol, a type of alcohol). • The bacterial waste products generated, organic acid metabolites, change the pH indicator in MSA from red to bright yellow. Pathogenic staph, such as Staphylococcus aureus, are mannitol fermenters, and when growing on Mannitol Salt Agar, their wastes turn the MSA a bright yellow color. • In contrast, nonpathogenic Staph such as Staphylococcus epidermidis (aka Staph epi), the normal flora that grows on human skin, does not ferment mannitol.

  3. Staphylococcus Staphylococcus strains on MSA Staphylococcus aureus on MSA

  4. Staphylococcus aureus on SM 110 medium • Staphylococcus medium 110 (SM110) contains mannitol, 7.5% sodium chloride and does not contain phenol red • Colonies of S. aureus appear yellow

  5. Streptococcus

  6. Streptococcus hemolysis

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