Gram Positive Facultative Cocci

Properties that help distinquish among Staphylococcus speicies

• Coagulase has a prothrombin-like activity which, through the conversion of fibrinogen to fibrin, results in the clotting of blood plasma.

  Coagulase is present in Staphylococcus aureus but lacking in most other staphylococci

  

  The test depicted above actually identifies clumping factor, a related product to coagulase, which is present on the surface of the Staphylococcus aureus. There is a very high correlation between the presence of clumping factor and coagulase.

  The interaction of clumping factor with plasma results in aggregation of the organisms which is observed in the well containing Staphylococcus aureus.

  

• Novobiocin is a drug whose major inhibitory effect is on DNA replication. Its' mode of action is to prevent supercoiling of DNA by the enzyme DNA gyrase.

  Differential sensitivities to this antimicrobial help to distinquish between Staphylococcus epidermidis and Staphylococcus saprophyticus; most strains of Staphylococcus saprophyticus are resistant whereas most strains of Staphylococcus epidermidis are sensitive

  

Properties that help to distinquish among Streptococcus species

Hemolysis of red blood cells is useful in differentiating among streptococcal species; three terms are commonly used: alpha, beta, and gamma.

NOTE:

It is important to always specify the animal source of the red cells used as sensitivity to lysis of a particular population of erythrocytes by a given organism may be species specific

• Alpha hemolysis - a zone of greening surrounding a colony of bacteria due to partial lysis of red cells and subsequent oxidation of porphorin compounds retained in the red cell membrane

  

  Properties that help to distinquish among alpha streptococci

  • bile solubility - rapid lysis of the pneumococcus (Streptococcus pneumoniae) in the presence bile caused by the activation of cell wall autolytic enzyme as a consequence of the detergent action of bile.

    

  • optochin - pneumococci are sensitive (i.e. growth inhibition) to low concentrations of the chemical, ethylhydroxycupreine; other alpha streptococci are not.

    

• Beta hemolysis - a zone of clearing surrounding a colony of bacteria due to complete lysis of the red blood cells

  

  Properties that help to distinquish among beta streptococci

  • Bacitracin sensitivity - Bacitracin is a topically useful antimicrobial produced by Bacillus subtilis. It inhibits cell wall synthesis through its' blockage of the recycling of the lipid carrier of the peptidoglycan synthetic pathway.

    Streptococcus pyogenes (serological group A) strains are almost uniformally sensitive but streptococci of other serological groups are not.

    

• Gamma hemolysis - The term is really misleading. It describes a condition in which no hemolysis takes place; therefore, there is no change in the media surrounding a colony

  

  Properties that help to distinquish among gamma streptococci

  • Growth on Bile esculin agar - Esculin is a glycoside composed of glucose and a dihydroxycoumarin compound .

    Certain bacteria, such as the enterococci, are able to grow in the presence of bile and hydrolyze the esculin; the liberated diphydroxycourmarin complexes with ferric citrate present in the media to form a dark brown/black soluble compound.

    

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