Selective and Differential Media of Gram Positive Bacteria

Selective Media

Media that suppresses the growth of certain organisms while allowing others to grow.

Differential Media

Media that differentiates between different types of organisms based on their biological characteristics.

Pathogenic

Microorganisms that can cause disease.

Non-pathogenic

Microorganisms that do not cause disease.

Enterics

Bacteria that inhabit the intestinal tract.

Sensitivity

The ability of an organism to be inhibited or killed by an antibiotic.

Specificity

The ability of a test to correctly identify those without the disease (true negative rate).

Hemolysins

Exotoxins produced by several species of gram (+) cocci, which are able to destroy RBCs and hemoglobin.

Streptolysins

Hemolysins produced by Streptococci.

Streptolysin O

A type of streptolysin that is oxygen labile and expresses maximum activity under anaerobic conditions.

Streptolysin S

A type of streptolysin that is oxygen stable and is responsible for the hemolytic zones around streptococcal colonies growing on the surface.

Streak and Stab Technique

A method that allows isolation of colonies and encourages streptolysin activity due to reduced oxygen concentration in the subsurface environment.

Zone of Inhibition

A clear area showing antibiotic susceptibility or no growth.

Mannitol Salts Agar

Both selective and differential media that selects for gram (+) Staphylococci and differentiates between pathogenic and non-pathogenic Staphylococci.

Blood Agar

A differential media that differentiates among Streptococcus members by their hemolytic ability.

Alpha (α) Hemolysis

Partial destruction of RBCs resulting in a green halo.

Beta (β) Hemolysis

Full destruction of RBCs resulting in a clear halo.

Gamma (γ) Hemolysis

No hemolysis resulting in no change in media.

Bacitracin Susceptibility Test

A differential media that differentiates between resistant Staphylococcus and susceptible Micrococcus using bacitracin.

Zone of Inhibition (Bacitracin)

Indicates bacitracin susceptibility.

Mannitol Fermenters (MF)

Organisms that can ferment mannitol, such as pathogenic Staphylococcus.

Non-Mannitol Fermenters (NMF)

Organisms that cannot ferment mannitol, such as non-pathogenic Staphylococcus.

Escherichia coli

A gram (-) bacterium that does not grow on Mannitol Salts Agar.

Staphylococcus aureus

A pathogenic mannitol fermenter that grows with a yellow halo on Mannitol Salts Agar.

Staphylococcus epidermidis

A non-pathogenic non-mannitol fermenter that grows with no color change on Mannitol Salts Agar.