Tryptic Soy Agar is based on the soybean casein digest agar formula as described in the U.S. Pharmacopeia.1 Blood is incorporated to facilitate the growth of more fastidious bacteria and to observe haemolytic reactions. The absence of carbohydrates and reducing sugars permits the demonstration of haemolysis which is an important differentiating characteristic for bacteria, especially Streptococcus species.
This medium contains casein and soy peptones which provide nitrogen, amino acids, and peptides necessary for bacterial growth. Sodium chloride supplies essential electrolytes which maintain osmotic equilibrium. Sheep blood enriches the medium by providing essential growth factors and allows haemolytic reactions to be demonstrated. Group B streptococci produce a protein-like compound called the CAMP factor which acts synergistically with beta toxin, produced by some strains of Staphylococcus aureus.2 This reaction occurs when a streak of betalysin-producing S. aureus is inoculated perpendicular to a streak of group B Streptococcus resulting in an area of complete lysis in the shape of an arrowhead or crescent. Blood Agar (TSA w/ 5% Sheep Blood) is suitable for performing the CAMP test.
1. The United States Pharmacopeia. 2006. 24th ed., Sup. 2., 29th rev. United States Pharmacopeial Convention, Rockville, MD.
2. Christie, R., N.E. Atkins, and E. Munch-Peterson. 1944. Aust. J. Exp. Biol. 22:197-200.
3. Clinical and Laboratory Standards Institute (CLSI). 2004. Quality Control for Commercially Prepared Microbiological Culture Media; Approved Standard, 3rd ed. M22-A3. CLSI, Wayne, PA.