Staph and Strep

Microbiology Review: Staphylococcus and Streptococcus Tests

Slide and Tube Coagulase Test for Staph Species

• Principle:

  • The slide coagulase test identifies the presence of coagulase enzymes produced by Staphylococcus species, particularly Staphylococcus aureus.

  • Coagulase binds to prothrombin, causing plasma to clot, which can be observed microscopically in the slide test or as a tube test.

Catalase Test

• Principle:

  • The catalase test distinguishes between catalase-positive bacteria (e.g., Staphylococcus) and catalase-negative bacteria (e.g., Streptococcus).

  • Reagents:

  • Hydrogen peroxide (H₂O₂).

  • Procedure:

  • A drop of hydrogen peroxide is applied to the bacterial colony. If oxygen bubbles form, the organism is catalase positive (indicating the presence of the catalase enzyme).

Staph Latex Test for Confirmation of Staph aureus

• Principle:

  • The Staph latex test detects the presence of clumping factor and protein A on the surface of Staphylococcus aureus.

  • Latex beads coated with anti-Staphylococcus antibodies agglutinate in the presence of Staph aureus.

Mannitol Salt Agar (MSA) Differentiation

• Principle:

  • Mannitol Salt Agar is a selective medium for Staphylococcus species, with a high salt concentration inhibiting most other organisms.

  • Differentiation:

  • Staphylococcus aureus ferments mannitol, producing acid, which changes the pH indicator in the medium (phenol red) from red to yellow.

  • Other Staphylococcus species do not ferment mannitol, and thus no color change occurs (remains red).

A Disk Test

• Principle:

  • The A disk test assesses sensitivity to bacitracin in beta-hemolytic streptococci (Streptococcus pyogenes vs. Streptococcus agalactiae).

  • Positive Result:

  • If sensitivity is observed (zone of inhibition), it indicates the organism is Streptococcus pyogenes.

  • Negative Result:

  • No zone of inhibition suggests the organism is Streptococcus agalactiae.

CAMP Reaction

• Principle:

  • The CAMP test is used to identify Streptococcus agalactiae by its ability to produce a diffusible extracellular factor that enhances the hemolytic activity of Staphylococcus aureus.

  • Positive Result:

  • An arrowhead zone of hemolysis is observed in the agar where the two organisms meet.

  • Negative Result:

  • No enhanced hemolysis indicates a negative result.

BEA Test for Strep faecalis

• Principle:

  • The Bile Esculin Agar test differentiates Enterococcus faecalis (which hydrolyzes esculin in the presence of bile) from other streptococci.

  • Positive Result:

  • Blackening of the medium indicates esculin hydrolysis.

  • Negative Result:

  • No discoloration of the medium indicates no hydrolysis.

6.5% NaCl Test for Strep faecalis

• Principle:

  • The high salt concentration in the medium tests for the ability of Enterococcus faecalis to grow in salty conditions.

  • Positive Result:

  • Growth in the medium indicates that the organism is tolerant to high salt concentrations.

  • Negative Result:

  • No growth indicates sensitivity to high salt concentrations.

Optochin Test for Strep pneumoniae

• Principle:

  • The Optochin test identifies Streptococcus pneumoniae based on its sensitivity to optochin in a disc diffusion method.

  • Reagents:

  • Optochin discs placed on an agar plate of the organism.

  • Positive Result:

  • Zone of inhibition around the disc indicates sensitivity (S. pneumoniae).

  • Negative Result:

  • No zone of inhibition indicates resistance.

Bile Solubility Test for Strep pneumoniae

• Principle:

  • This test measures the ability of Streptococcus pneumoniae to lyse in the presence of bile salts, distinguishing it from other alpha-hemolytic streptococci.

  • Reagents:

  • Bile salts (e.g., sodium deoxycholate) added to a culture.

  • Positive Result:

  • Clearing of the culture indicates bile solubility (presence of S. pneumoniae).

  • Negative Result:

  • No clearing indicates resistance to bile salts, suggesting the organism is not S. pneumoniae.