Staining Techniques in Microbiology PDF

Dyes and Stains Dr. Siddhi Shah  Importance of Staining Improves visibiltiy by greater contrast between the organism and the background, differentiate various morphological types (by shape, size, arrangement, etc.), determine the staining characteristic of o...

Dyes and Stains Dr. Siddhi Shah  Importance of Staining Improves visibiltiy by greater contrast between the organism and the background, differentiate various morphological types (by shape, size, arrangement, etc.), determine the staining characteristic of organism and, at times, direct diagnosis of disease, and demonstrate the purity of culture. observe certain structures (flagella, capsules, endospores, etc.),   A dye is a general-purpose coloring agent, whereas a stain is used for coloring biological material.  A stain is an organic compound containing a benzene ring plus a chromophore and an auxochrome group.  chromophore is a chemical group that imparts color to benzene.  auxochrome group is a chemical compound that conveys the property of ionization of chromogen (ability to form salts) and bind to fibers or tissues. Stain – Majority of the stains used for staining bacteria are of the basic type as nucleic acid of bacterial cells attract the positive ions, e.g. methylene blue, crystal violet. Acidic stains are used for background staining. Mordant – It is a chemical that forms an insoluble complex with the stain and fixes it or causes the stain to penetrate more deeply into the cell. These are used in indirect staining. For example, Gram’s iodine in Gram staining and phenol in Ziehl Neelson’s staining. Decolorizer – It is a chemical used to remove the excess stain in indirect regressive staining. For example, ethanol in Gram’s staining.  Types of staining techniques Simple staining Differential staining (Use of of single stain) (Use of two contrasting stains) Direct Indirect Separation Visualization (Positive) (Negative) into groups of structures 1. Gram stain 1. Flagella stain 2. Acid fast 2. Capsule stain stain 3. Spore stain SIMPLE STAINING  SIMPLE STAINING  A staining method that uses only a single dye that which does not differentiate between different types of organisms  There is only a single staining step and everything is stained with the same color.  Simple stains are used to stain whole cells or to stain specific cellular components.  Types of simple staining: 1. Direct / Positive staining : stain object 2. Indirect / Negative staining: stain background  Direct staining (Positive staining) A simple staining technique that stains the bacterial cells in a single color.  Many of the bacterial stains are basic chemicals Methylene blue, crystal violet, Malachite green  basic dyes react with negatively charged cytoplasm  Indirect staining (Negative staining)  In this staining process, instead of cells background is stained.  Here, an acidic dye like nigrosin or Indian ink is used. Acidic stain carries a negative charge and repelled by the bacteria, which also carry a negative charge on their surface. Hence, an acidic dye do not stain bacteria,  Instead, it forms a deposit around the organism, leaving the organism itself colorless or transparent upon examination.  STAINING: GENERAL TECHNIQUE SMEAR AIR DRY HEAT FIX STAIN OBSERVE  Importance of fixing the smears “Fixation accomplishes three things: (1) it kills the organisms; (2) it causes the organisms to adhere to the slide; and (3) it alters the organisms so that they more readily accept stai (dyes). DIFFERENTIAL STAINING GRAM’S STAINING  The Gram Stain In the late 1800’s, Christian Gram observed that some genera of bacteria retained a dye-Iodine complex when rinsed with alcohol, while other genera were easily decolorized with alcohol and could be then visualized by a contrasting counter stain. This staining procedure defines two bacterial groups: those which retain the primary dyes (“Positive by Gram’s Method” or “Gram-Positive”) and those which are easily decolorized (“Negative by Gram’s Method” or “Gram- Negative”). This is the starting point for bacterial identification procedures.  The Gram Stain  Gram-Positive bacteria have thick, dense, relatively non- porous walls, while Gram-Negative bacteria have thin walls surrounded by lipid-rich membranes.  Some non-bacterial organisms with thick cell walls (e.g., some yeasts) also stain Gram-Positive.  Gram-Positive bacteria which have lost wall integrity through aging or physical or chemical damage may stain Gram-Negative.  GRAM POSITIVE Lipoteichoic acid Peptidoglycan-teichoic acid Cytoplasmic membrane Cytoplasm GRAM NEGATIVE Porin Lipopolysaccharide Outer Membrane Braun lipoprotein Inner (cytoplasmic) membrane Cytoplasm  Gram staining – Requirements The specimen is mounted and heat fixed on a slide before proceeding to stain it. The reagents required are:  Crystal Violet (the Primary Stain)  Iodine Solution (the Mordant)  Decolorizer (ethanol)  Safranin (the Counter stain)  Gram staining – Procedure 1 The bacteria are first stained with the basic dye crystal violet (primary stain Both gram-positive and gram-negative bacteria become directly stained an appear purple after this step. 2 The bacteria are then treated with gram's iodine solution (mordant). Th allows the stain to be retained better by forming an insoluble crystal viole iodine complex, called as ‘iodine lake’. Both gram-positive and gram-negativ bacteria remain purple after this step. 3 Gram's decolorizer, a mixture of ethyl alcohol and acetone, is then adde This is the differential step. Gram-positive bacteria retain the crystal viole iodine complex while gram-negative are decolorized. 4 Finally, the counter stain safranin (also a basic dye) is applied. Since th gram-positive bacteria are already stained purple, they are not affected by th counter stain. Gram-negative bacteria, that are now colorless, becom directly stained by the safranin. Thus, gram-positive appear purple, an gram-negative appear pink. ACID FAST STAINING Structure of an Acid-Fast Cell Wall  Acid fast staining - theory  Once stained the acid fast bacterial cells resist decolorization with acidified organic solvents, e.g acid alcohol and are therefore called ACID FAST.  Acid fast staining property of the genus, Mycobacteria, depends upon their lipid-rich cell walls which are relatively impermeable to various basic dyes unless the dyes are combined with phenol.  The exact method by which the stain is retained is unclear but it is thought that some of the stain becomes trapped within the cell and some forms a complex with the mycolic acids. This is supported by the finding that shorter chain mycolic acids or mycobacterial cells with disrupted cell walls stain weakly acid-fast, e.g. Nocardia Acid-Fast Stain of Mycobacterium tuberculosis in Sputum Note the reddish acid-fast bacilli among the blue normal flora and white blood cells in the sputum that are not acid-fast. SPECIAL STAINING CAPSULE STAINING Capsule staining is diagnostically useful since it is a virulent factor(e.g. pneumococci). Bacterial capsules are non-ionic, so neither acidic nor basic stains will adhere to their surfaces. Capsules are demonstrated either by negative staining (Nigrosin or India ink) or by special staining, e.g. Hiss’ method, Anthony’s method  Hiss Method The capsule is non-ionic in nature so it doesn't get stain by a acidic stain but a basic stain, such as crystal violet, stains the cell as well as the capsule. This is followed by treatment with hypertonic solution - 20% Copper sulphate solution, which serves dual role of both the decolorizing agent and counter stain. Copper sulphate solution, being hypertonic, causes diffusion of stain towards outer surface of cell. After drying of slide, the stain which is not passed from the capsular layer during diffusion retains in the capsular layer. Copper sulphate then decolorizes the capsule. Capsule appears as a faint blue halo around a purple cell. ENDOSPORE STAINING  Endospore staining  Spores are normally impervious to stains. Under the light microscope endospores have a high light refractivity indicative of high protein content.  Endospores can be stained by:  Modified Zeihl-Nelson's method using 0.25-0.5% sulphuric acid as decolorizing agent,  Barthelomew-Mittwar’s method  Schaeffer-Fulton stain technique.  Schaeffer-Fulton method Malachite green is used to stain the endospores (primary stain) The malachite green is forced to permeate the spore wall by heating (mordant). Washing with water remove stain from vegetative cells, but not from spore wall. The endospores thus retain the primary dye while the vegetative cells lose the primary stain and take the red color of secondary stain (safranin). Endospore stain of Bacillus megaterium Note green endospores within pink bacilli