Sudan Black B Staining – Principle, Procedure and Results

Sudan Black B staining is a technique used to detect lipid-containing materials in cells and tissues. Sudan Black B (SBB) is a lipophilic dye. In the Sudan Black B staining procedure, a suitable specimen is prepared on a glass slide and stained with the dye. The excess stain is then removed. The slide is examined under a microscope. The materials which retain the stain appear blue-black in colour.

The staining can be carried out on frozen tissue sections and fixed bacterial smears, but the preparation methods are different. Frozen tissue sections are used to prevent the loss of lipids during tissue processing. In bacterial smears, the staining is used to examine intracellular storage granules such as polyhydroxybutyrate (PHB). The blue-black colour indicates that the material has retained the dye under the staining conditions. It does not confirm the exact lipid composition or the presence of a particular compound.

Purpose of Sudan Black B Staining

The main purpose of Sudan Black B staining is to detect lipid-containing materials and certain lipid-rich inclusions in cells and tissues. The following are the purposes of Sudan Black B staining:

1. It is used to detect neutral fats present in the form of lipid droplets.
2. Phospholipids present in the specimen can also be stained, provided they are not removed during specimen preparation.
3. It is used to demonstrate lipids associated with lipoproteins in certain specimens.
4. The intracellular lipid inclusions can be detected using Sudan Black B staining.
5. Sudan Black B stains different types of lipid-containing materials. It is not specific for a single lipid class and cannot identify the exact type of lipid present in the specimen.

Principle of Sudan Black B Staining

The principle of Sudan Black B staining is based on the greater solubility of the dye in lipids than in the staining solvent. When the stain is added, Sudan Black B passes from the dye solution into the lipid-containing parts of the specimen. This process is known as partitioning of the dye. Adsorption of the dye onto the lipid-containing material may also take place. The dye retained in these regions gives a blue-black colour.

The staining depends upon the preparation of the specimen and the lipids present in it. Some lipids may be dissolved and removed by the solvents used during tissue processing. The type of fixative and staining solvent also affects the amount of dye taken up by the specimen. The blue-black staining is not due to a specific chemical reaction between Sudan Black B and acidic lipid groups.

Materials Required for Sudan Black B Staining

The following are the materials required for Sudan Black B staining. The reagents used for frozen tissue sections and bacterial smears are different.

Instruments and Equipment

1. Microscope Slides and Coverslips – Clean glass slides are used for preparing tissue sections and bacterial smears. Coverslips are required for mounting the stained tissue sections.
2. Staining Jars or Racks – These are used to hold the slides during staining and washing.
3. Micropipettes and Droppers – Used to add the staining solutions and other reagents onto the slides.
4. Light Microscope – It is used to examine the stained specimens. An oil-immersion objective is required for examining bacterial smears at high magnification.
5. Cryostat – It is used to prepare thin frozen sections of tissue.
6. Inoculating Loop – Used to transfer bacterial cultures for smear preparation.
7. Other Equipment – Forceps, beakers, measuring cylinders, analytical balance, magnetic stirrer, hot plate, filter paper, glass filters and reagent bottles are required for preparing the specimens and staining solutions.

Reagents and Staining Solution Preparation

The reagents required for Sudan Black B staining depend upon the type of specimen. The following are the reagents used in the frozen-section and bacterial-smear methods:

1. Specimen – Fresh tissue is used to prepare frozen sections. For bacterial staining, a suitable bacterial culture is used to prepare a fixed smear.
2. Sudan Black B Solution for Frozen Sections – A 0.7% Sudan Black B solution is prepared by dissolving 0.7 g of Sudan Black B in 100 mL of propylene glycol. The solution is heated to about 100°C without exceeding 110°C. It is filtered while warm and again after cooling to remove the undissolved dye particles.
3. Fixative – A 10% formalin solution is used for fixing frozen tissue sections. It helps to preserve the tissue structure during staining. Some methods use formal-calcium fixative.
4. Propylene Glycol – Pure propylene glycol is used before staining the frozen sections. An 85% propylene glycol solution is prepared by mixing 85 mL of propylene glycol with 15 mL of distilled water. It is used to remove the excess Sudan Black B stain.
5. Nuclear Fast Red – It is used as a counterstain for frozen tissue sections. The nuclei appear red after staining.
6. Distilled Water – It is used for preparing the staining solutions and washing the tissue sections. Tap water is also used in some methods.
7. Aqueous Mounting Medium – Glycerin jelly or another suitable aqueous mounting medium is used to mount the stained tissue sections. Organic solvents used in some mounting media may remove the stained lipids.
8. Sudan Black B Solution for Bacterial Smears – A 0.3% Sudan Black B solution is prepared by dissolving 0.3 g of Sudan Black B in 100 mL of 70% ethanol. The solution is shaken at intervals and allowed to stand overnight before use. It is stored in a well-stoppered bottle.
9. Washing or Differentiating Solution – Tap water is used to wash the bacterial smears in some methods. Other methods use xylene to remove the excess Sudan Black B stain before counterstaining.
10. Safranin – A 0.5% aqueous safranin solution is prepared by dissolving 0.5 g of safranin in 100 mL of distilled water. It is used as a counterstain for bacterial smears. The bacterial cytoplasm appears pink, while the Sudan Black B-positive granules remain blue-black.
11. Storage and Safety – The preparation, filtration and storage of Sudan Black B solutions depend upon the method used. Some frozen-section methods require the stain to be kept warm, while others use the filtered solution at room temperature. Ethanol and xylene are flammable, while formalin is hazardous. Gloves, laboratory coat and protective goggles should be worn. Heating and handling of solvents should be carried out in a suitable fume hood, away from open flames.

Procedure of Sudan Black B Staining

The Sudan Black B staining procedure is different for frozen tissue sections and bacterial smears. The following are the steps involved in both methods:

Sudan Black B Staining of Frozen Tissue Sections

The following are the steps involved in Sudan Black B staining of frozen tissue sections:

1. At first, prepare 10–16 µm thick frozen sections of tissue using a cryostat. Attach the sections to clean glass coverslips.
2. Fix the sections in Baker’s fixative (modified calcium-formol) for 5 minutes. The fixative contains 0.75 g of anhydrous calcium chloride and 7.5 mL of 37% formaldehyde, made up to 250 mL with deionized water.
3. Wash the fixed sections three times with tap water or deionized water.
4. Place the sections in 100% propylene glycol for 5 minutes to remove water.
5. Remove the propylene glycol and add 0.7% Sudan Black B solution prepared in propylene glycol. Allow the sections to stain for at least 2 hours. Overnight staining is preferred.
6. Wash the stained sections three times with tap water or deionized water. The back of the coverslips is cleaned using cotton swabs.
7. Transfer the sections into 85% propylene glycol for 3 minutes to remove the excess stain.
8. Rinse the sections several times with deionized water. No counterstain is used in this method.
9. Mount the coverslips onto clean glass slides using an aqueous mounting medium such as glycerin jelly.
10. Examine the stained sections under a light microscope. The lipid deposits appear blue-black in colour.

Sudan Black B Staining of Bacterial PHB Granules

The following are the steps involved in Sudan Black B staining of bacterial PHB granules:

1. Prepare a thin bacterial smear on a clean glass slide using a suitable bacterial culture.
2. Allow the smear to air-dry completely and fix it by gentle heating.
3. Flood the smear with 0.3% Sudan Black B solution prepared in 70% ethanol. Leave the stain for 10–15 minutes at room temperature.
4. Drain off the excess staining solution and blot the slide dry.
5. Treat the smear with xylene to remove the excess Sudan Black B stain. The slide is then blotted dry.
6. Counterstain the smear with 0.5% aqueous safranin for 5–10 seconds.
7. Wash the slide with water, blot and allow it to dry.
8. Examine the stained smear under the oil-immersion objective of a light microscope. The Sudan Black B-positive granules appear dark blue-black, while the bacterial cytoplasm appears pink.

The staining time and washing solvents may vary in other published methods. The presence of blue-black intracellular granules gives a presumptive indication of PHB accumulation, but does not confirm PHB alone.

Results and Interpretation of Sudan Black B Staining

After Sudan Black B staining, the lipid-containing materials appear dark blue to black under a light microscope. The areas without visible lipid deposits do not show distinct blue-black staining. The surrounding cells may appear in different colours depending upon the counterstain used.

In frozen tissue sections, the lipid droplets are stained blue-black. The nuclei appear red when Nuclear Fast Red is used as a counterstain. In bacterial smears, the intracellular lipid granules appear dark blue-black, while the cytoplasm appears pink after safranin staining.

The following are the results and interpretation of Sudan Black B staining in different specimens:

A negative result is indicated by the absence of distinct blue-black lipid deposits or granules. However, a negative result does not always indicate the absence of lipids. Some lipids may be removed during specimen preparation or treatment with organic solvents.

The positive and negative control slides should be examined along with the test specimen. A known lipid-containing specimen is used as a positive control. The negative control helps to identify unwanted background staining.

Sometimes, the undissolved dye particles may appear as dark deposits on the slide. Thick smears, excess staining and improper washing can also affect the results. These should not be confused with the lipid granules present inside the cells.

The intensity of staining depends upon the amount of dye retained, the type of lipid and the staining conditions. It cannot be used to determine the exact amount of lipid present in the specimen. Similarly, a positive bacterial smear does not confirm PHB alone. In hematological smears, Sudan Black B staining is not sufficient for the diagnosis of leukemia without other laboratory tests.

Applications of Sudan Black B Staining

The following are the applications of Sudan Black B staining:

• It is used to detect lipid droplets present in frozen tissue sections, such as liver and adipose tissue.
• Sudan Black B staining is used to examine intracellular polyhydroxybutyrate (PHB) and other polyhydroxyalkanoate (PHA) storage granules in bacterial cells. It is a screening method and cannot identify the exact type of polymer.
• In hematology, it is used to stain the cytoplasmic granules of myeloid cells in blood and bone marrow smears. It was earlier used to help differentiate acute myeloid leukemia (AML) from acute lymphoblastic leukemia (ALL). Other tests, such as immunophenotyping, are required for diagnosis.
• Modified Sudan Black B staining is used to detect lipofuscin present in senescent cells. Lipofuscin contains oxidized lipids and proteins. Other markers are also required for the identification of cellular senescence.
• It is used to examine lipid accumulation in different cells, such as hepatocytes and adipocytes. The staining gives qualitative results and cannot determine the exact amount of lipid present in the cells.

Limitations of Sudan Black B Staining

The following are the limitations of Sudan Black B staining:

• Sudan Black B stains different types of lipids. It cannot identify the exact type of lipid present in the specimen.
• Some lipids, such as crystalline cholesterol, may remain unstained. Certain phospholipids and free fatty acids may also dissolve in the staining solution.
• Alcohol and other organic solvents used during tissue processing can remove the lipids. Frozen sections are generally used for staining neutral fats.
• The staining result depends upon the fixative, solvent and staining time. Improper staining may produce weak or uneven colour.
• Undissolved dye particles and excess stain may produce dark deposits on the slide, which can be confused with lipid granules.
• The staining gives qualitative results and cannot be used to measure the exact amount of lipid present.
• In bacterial smears, the blue-black granules do not confirm the presence of PHB or a particular PHA. Other chemical tests are required for identification.
• Some cases of acute myeloid leukemia (AML) may show weak or negative staining. Sudan Black B staining alone cannot be used for the diagnosis of leukemia.
• Lipofuscin is also found in cells which are not senescent. Other markers are required to identify cellular senescence.

Advantages of Sudan Black B Staining

The following are the advantages of Sudan Black B staining:

• Sudan Black B stains lipid-containing materials dark blue-black, which can be easily observed under a light microscope.
• It is used to detect different types of lipids, including neutral fats and some phospholipids present in tissue sections.
• The staining procedure is simple and requires commonly available laboratory reagents. A fluorescence microscope is not required for routine examination.
• The lipid droplets can be examined in their original position in properly prepared frozen tissue sections. It helps in studying the distribution of lipids in tissues.
• It is used for preliminary screening of bacterial cultures containing PHB and other PHA storage granules. The positive samples can be further examined using other methods.
• Sudan Black B staining helps in identifying myeloid cell differentiation in blood and bone marrow smears. It provides additional information along with other hematological tests.
• Modified Sudan Black B staining can be used to detect lipofuscin present in senescent cells. Some methods can also be carried out on preserved tissue samples.

Precautions of Sudan Black B Staining

The following are the precautions of Sudan Black B staining:

• Clean glass slides should be used for preparing tissue sections and bacterial smears. Dust and grease on the slides may produce unwanted staining.
• Frozen tissue sections should be used for staining neutral fats. The lipids may be removed by alcohol and other organic solvents used during paraffin processing.
• The tissue sections should be fixed using the specified fixative, as improper fixation may affect the staining result.
• The Sudan Black B solution should be filtered before use to remove undissolved dye particles. These particles may appear as dark deposits on the slide.
• The staining solution should be prepared according to the specimen used, as frozen tissue sections and bacterial smears require different staining methods.
• The slides should be stained for the required time. A shorter staining time may produce weak staining.
• The excess stain should be removed using the specified differentiating solution. Excessive differentiation may remove the stain from the lipid deposits.
• Bacterial smears should be completely air-dried before fixation. Excessive heating should be avoided.
• The stained slides should be washed carefully to remove excess dye without damaging the specimen.
• An aqueous mounting medium should be used for lipid-stained frozen sections. Organic solvents present in some mounting media may dissolve the lipids.
• Positive and negative control slides should be examined along with the test specimen to check the staining result.
• Ethanol and xylene are flammable, while formalin is hazardous. Gloves, laboratory coat and protective goggles should be worn. The solvents should be handled in a fume hood, away from open flames.

References

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