Bacterial Culture Media, their PH Indicators and color of bacterial colonies

Colony color or pigmentation is one of the mostly used characteristics which aids in the differentiation and identification of the isolates. Except in those cases where there is growth of pigmented bacteria, the characteristics pigmentation observed in the culture media is because of the changes in the pH of the medium and the development of the characteristics color of the pH indicator used.

pH indicators gives characteristics color in different pH (alkaline, neutral and acidic) e.g., phenol red gives yellow color in acidic pH so whenever fermenter grow in sugar containing media, they produce acid and give yellow colored colonies (e.g. Mannitol fermenter S. aureus in Mannitol Salt Agar, sugar fermenter in XLD agar etc).   In this post, we are grouping Bacteriological media on the basis of pH indicator used.

pH indicator: Bromothymol blue, it is a color indicator which turns yellow at acidic pH. At a neutral pH, bromthymol blue is green. At pH 7.5 or above, bromthymol blue turns royal blue.

  1. Cystine Lactose Electrolyte Deficient Agar (CLED): In  CLED media, lactose fermenting colonies of Escherichia coli gives pale yellow colonies whereas non lactose fermenting colonies of Proteus, Salmonella etc gives blue colored colony.

    Lactose fermenting (Yellow colonies) and Lactose Non fermenting colonies in CLED

  2. Simmons Citrate Agar: Simmons citrate agar tests the ability of organisms to utilize citrate as a sole carbon source and ammonium dihydrogen phosphate as the sole source of nitrogen. Organisms which can utilize citrate as their sole carbon source also utilize ammonium dihydrogen phosphate creating alkaline environment in the medium, turning the medium blue.
  3. Hugh and Leifsons Medium (OF Medium): OF medium (oxidative-fermentative test) is used to determine if gram-negative bacteria metabolize carbohydrates oxidatively (producing weak acid), by fermentation (producing mixed acid), or are nonsacchrolytic and therefore have no ability to use the carbohydrate in the media (no acid production). The high concentration of acid produced during fermentation will turn the bromthymol blue indicator from green to yellow in the presence or absence of oxygen.
  4. Thiosulfate Citrate bile salts sucrose (TCBS) agar is used for the selective isolation of Vibrio spp. TCBS has very high pH (8.5 to 9.5) to supress other intestinal flora. pH indicator Bromothymol blue is used in this media, so if the organism utilizes the sucrose, it lowers the pH of the media and yellow colored colonies are seen in TCBS.

pH indicator: Neutral Red, red at pH 6.8 and below, yellow at pH 8 and above.

Mixed growth of mucoid Lactose fermenting colonies and NLF colonies in MacConkey Agar
  1. MacConkey Agar: It is selective and differential medium. The pH indicator helps to differentiate between lactose fermenting and Lactose non-fermenter. E coli and other lactose fermenting bacteria give pink colored colonies in MacConkey agar whereas Non-lactose fermenter gram negative bacilli produces pale yellow colonies.
  2. Deoxycholate Citrate Agar (DCA): Lactose non-fermenter produces colorless colonies. Coliform bacteria if present form pink colonies.
  3. Salmonella-Shigella Agar: It is a selective media used to isolate Salmonella and Shigella Lactose fermenter produces red pigmented colonies where as Non lactose fermenter (e.g., Salmonella) grows as translucent colonies (colorless) with or without black centers (Neutral red do not have any role in black color formation, it’s because of action of Sodium thiosulphate and ferric citrate which are also other constituent of SS Agar).

pH indicator: Methylene Blue and Eosin dyes,

  1. Eosin Methylene Blue (EMB) Agar: EMB agar is used for the isolation and differentiation of lactose fermenting and non-lactose fermenting enteric bacilli.
    • Coliform bacteria; Purplish black colonies
    • Non Coliform Bacteria: Colorless colonies.

pH indicator: Malachite Green,

  1. Lowenstein Jensen Medium: Lowenstein Jensen medium is used to isolate Mycobacterium tuberculosis.

pH indicator: Phenol red,  it  is yellow at pH 6.8 and  red at pH 8.4 and above.

Yellow colonies of S. aureus in Mannitol Salt Agar (MSA).
Image source: ASM
  1. Mannitol Salt Agar (MSA): Staphylococcus aureus grows in Mannitol Salt Agar, ferments Mannitol and produces (acid) yellow colonies with yellow zones where as most coagulase negative staphylococci (CONS) and micrococci do not ferment mannitol and grow as small red colonies surrounded by red or purple zones.
  2. Triple Sugar Iron Agar (TSI) Test: TSI contains three sugar (glucose, sucrose and lactose) along with iron source. It helps in the identification of members of family enterobacteriaceae on the basis of their fermentative capability, gas production and H2S gas production.
    1. Lactose fermenters: yellow/yellow
    2. Non-lactose fermenter: red/yellow
    3. Non-fermenter: red/red
  3. Urea Agar base: Urease producing organisms gives pink red color in the media as they utilize urea with the formation of ammonia the pH of the media changed to alkaline condition.
  4. Xylose Lysine Deoxycholate (XLD) Agar: It is used for the isolation and differentiation of Salmonella and Shigella spp which appear as pink colored colonies (as they do not ferment carbohydrate) from other non pathogenic gram negative enteric bacilli which produce yellow colonies (as they ferment one or more of the sugars present in the media).