stains – General application of stain

Introduction

• Any chemical that utilized for coloring purposes are known as dye
• In biological terms, coloring agents are known as stain
• Dye may be defined as an organic compound containing both chromophore and auxochrome groups linked to benzene ring
• Stains are widely utilized for the staining of semitransparent bacterial cells in order to make easy visualization of bacterial cell and it’s cellular structure
• But it’s not necessary that stains are always utilised for the visualisation of bacteria. Following section describes general application of the stains other than staining of bacteria

Staining for the isolation and identification of bacteria

1. In medical microbiology and many other branches of microbiology, dyes are widely utilised in the selective and differential media for the isolation and identification of bacteria
   1. The most widely used media in microbiology laboratory is Eosin-methylene blue agar
   2. EMB agar is a type of selective and differential media which is selective for the coliforms and differential for E.coli and E. aerogens
   3. Medium contains following ingredients for the selective growth coliforms
      1. Lactose- for the growth of lactose fermenting organisms
      2. Bile salts- inhibit growth of organism other than coliforms
      3. Dye – inhibits growth of gram positive bacteria
   4. While medium is differential for E.coli and E. aerogens because following reasons
      1. Medium contains two dyes esosin yellow and methylene blue.
      2. As medium contains lactose, utilisation of lactose produces acid that changes color of dye
      3. E. coli  produces higher amount of acid and changes the color of dye to green and thus produces green metallic sheen colonies
      4. E. aerogens produces comparatively low amount of acid and produces pinkish, mucoid colony 
   5. Other examples of selective medium includes MacConkey’s agar

Use of staining as PH indicator in microbial media

1. Use of staining as ph indicator in microbial media
   1. In industrial microbiology , screening of organic acid producers is an important technique in order to isolate organisms that produces organic acid
   2. In the procedure, soil suspension is serially diluted and plated on the medium containing added dyes
   3. After incubation, due to growth of organism and metabolism, if organism produces organic acid then it would diffuse into surrounding area on agar plate
   4. Thus, after incubation the colony showing zone of color change might be considered as organic acid producers.