Green

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  • DNase agar is used primarily in clinical laboratories to differentiate pathogenic Staphylococcus aureus from other staphylococci based on deoxyribonuclease (DNase) activity. The tryptone provides the required carbon, nitrogen and vitamins. Sodium chloride maintains the osmotic balance. The addition of DNA to the base medium provides a simple method to check for DNase activity. The methyl green fades into a colourless compound if the DNA in the medium is depolymerised. Organisms that can produce sufficient quantity of a DNase enzyme will hydrolyse the DNA resulting in a clear area around the colonies. Whereas DNase negative organisms will not produce clearing. NB: As with most tests of this type a positive result should not be taken in isolation and other appropriate tests, e.g. coagulase test, latex agglutination etc., should be carried out.  
  • Edwardsiella ictaluri medium is a selective medium for the isolation of Edwardsiella spp. based on the formulation by Shotts et al. (1) Edwardsiella spp. may be differentiated from other microorganisms on this medium due to its colony morphology. The peptones provide the required carbon, nitrogen and vitamins. Sodium chloride maintains the osmotic balance. Colistin and bile salts are selective agents to inhibit most Gram-negative and Gram- positive organisms. Mannitol is a fermentable carbohydrate and bromthymol blue is a pH indicator. Phenylalanine and ferric citrate are used to detect phenylalanine deaminase activity in Proteus spp. Agar is a solidifying agent. Shotts et al. (1) noted Proteus spp. swarming can overgrow on a mixed cultured sample. Therefore, a selective supplement (LS0021) may be added to the medium to restrict Proteus spp. swarming
  • E.E. Broth is recommended as an enrichment medium when examining food and feedstuffs for Enterobacteriaceae. It is a modification of Brilliant Green Bile Broth, with an improved buffering capacity to encourage early growth and prevent autosterilization. E.E. Broth uses glucose instead of lactose to make the medium a test for all enterobacteria including non-lactose fermenting organisms. This formulation complies with the Harmonized USP/EP/JP. Nitrogen is supplied by the gelatin peptone whilst glucose serves as the fermentable carbohydrate source. Oxbile and brilliant green are the selective agents helping to suppress Gram-positive non- target organisms. Auto sterilisation is prevented through the buffer system composed of potassium dihydrogen phosphate and disodium hydrogen phosphate.
  • Hektoen enteric agar was developed by King and Metzger as a differential selective medium for the isolation of Shigella spp. and Salmonella spp. species from enteric pathological samples. The meat peptone and yeast extract provide the required nitrogen, carbon and vitamins. Lactose, sucrose and salicin are fermentable carbohydrates. Bromothymol blue is added as a pH indicator in order to identify carbohydrate fermenting organisms. The combination of ferric ammonium citrate and sodium thiosulfate allows the production of hydrogen sulphide. Hydrogen sulphide positive colonies produce black centred colonies. Sodium chloride maintains the osmotic balance. The bile salts and acid fuchsin inhibit Gram-positive organisms.
  • Trichomonas medium is based on the formula described by Kupferburg, Johnson and Sprince for the selective isolation of Trichomonas spp. (1) The tryptone and liver extract act as carbon, nitrogen and vitamin sources in this medium. Maltose is a fermentable carbohydrate. The agar and cysteine HCl reduce the oxygen tension in the medium which aids the growth of trichmonads. Methylene blue is a redox indictor and allows for the visualisation of any significant oxygen diffusion in the medium. The addition of selective agents such as chloramphenicol is recommended to inhibit bacterial species that may be present in specimens. References 1) Kupferberg, A.B. Johnson, G., and Sprince, H. 1948. Proc. Soc. Exper. Biol. Med., 67:304-308