Columbia Agar Base is a nutrient-rich formula containing 5% defibrinated blood, which provides more nutrients and the ability to exhibit hemolytic reactions. Columbia Blood Agar Base is used as a base for the preparation of blood-containing media and in selective media preparations where various combinations of antimicrobial agents are used as additives. Columbia C.N.A. Agar Base is prepared with the same formula as Columbia Agar Base with the addition of 10 mg/L colistin and 15 mg/L nalidixic acid to inhibit the growth of gram-negative bacteria and to support the growth of staphylococci, hemolytic streptococci and enterococci when supplemented with 5% blood. Biopeptone and tryptose B promote the luxuriant growth of microorganisms and the visualization of good hemolytic reactions. Sheep blood allows the detection of hemolytic reactions and provides the factor X necessary for the growth of many bacterial species. Horse blood provides both factor X and factor V, which is why it is preferred in most laboratories. Yeast extract and corn starch serve as energy source and neutralizer respectively. It should be noted that this medium has a relatively high carbohydrate content and therefore beta-hemolytic streptococci can produce a greenish hemolytic reaction that can be confused with alpha hemolysis. The addition of antimicrobial agents, colistin (or polymyxin B) and nalidixic acid, makes the medium selective for gram-positive microorganisms. Colistin and nalidixic acid disrupt the cell membrane of gram-negative organisms, while nalidixic acid blocks DNA replication in susceptible gram-negative bacteria. Columbia C.N.A. Agar Base with added blood allows the selective isolation of gram-positive cocci, staphylococci and streptococci, especially when gram-negative bacilli are present and tend to multiply on conventional blood agar plates. Also used for the selective isolation of Gardnerella vaginalis. This medium promotes the growth of Brucella abortus, Yersinia pestis, Clostridium perfringens and all common Enterobacteriaceae without the addition of blood.