Introduction
India ink staining, also known as negative staining, is a simple yet effective approach for visualizing the exterior structures of bacteria, fungi, and other microbes in microbiology. The dye is used to stain the background in this staining process, while the microbe shows as a clear patch against the dark background. We will explore the principle, technique, benefits, restrictions, comparison with other stains, and quality control of India ink staining in this article.
Cryptococcal meningitis occurs in immunocompromised patients and when meningitis is clinically suspected, such as in HIV patients, or when yeast cells with lymphocytes are detected when performing a C.S.F. cell count or examining a Gram smear, or when India ink preparation for encapsulated yeasts is examined.
India ink is used as a negative stain in ion-negative staining, which allows visualization of the normally transparent and unstainable capsules of various microorganisms such as Cryptococcus neoformans (most commonly), Klebsiella pneumoniae, Streptococcus pneumoniae, and others.
Principle of India ink staining
The India ink staining technique is based on the negative staining principle, which states that the dye is used to stain the background while the microbe remains unstained, resulting in a clear image against a dark background. The India ink stain is made up of microscopic, opaque carbon black particles floating in an aqueous solution. Because the carbon black particles do not penetrate the microbe, it remain unstained and visible under the microscope. Because the capsule is non-ionic, the India ink used will not bind to it. As a result, the capsule appears as a clear halo around the yeast cells.
India Ink Composition
Deionized Water, Thimerosal, and Black Pelican Drawing Ink No. 17
Requirements for India Ink Preparation
- Nigrosine stain or India ink,
- Slides and coverslips that are clean and free of grease.
- The C.S.F. specimen,
- Droppers or a loop for inoculation,
- Bunsen burner
- waste disposal container,
- Centrifuge,
- Tubes for testing,
- Microscope and
- Cryptococcus neoformans (for positive control)
- Candida albicans is used for negative control.
Ink preparation
- CSF should be centrifuged for 5 to 10 minutes.
- Remove the supernatant fluid and combine it with the sediment.
- Transfer an equal amount of sediment and India ink to a slide, i.e. a drop of sediment and a drop of India ink.
- Cover with a coverslip after mixing. Examine the preparation using the 40 X objective under a microscope.
Observation
Look for oval or round cells that are irregular in size, measuring 2-10 m in diameter, and are surrounded by a large unstained capsule.
Result interpretations
Positive control: The presence of encapsulated yeasts was observed.
Negative control: The absence of encapsulated yeasts was observed.
Advantages
There are various advantages of using India ink staining, including:
- It can be performed easily.
- Minimal equipment and materials are required.
- A vivid, high-contrast view of the microorganism’s exterior structures is provided.
- It is capable of displaying a vast variety of microorganisms, including bacteria, fungi, and parasites.
- Heat fixation is not required, which might harm fragile structures.
Limitations of test
- India ink preparation is only for presumptive identification of organisms; consequently, other tests such as biochemical, immunological, molecular, or mass spectrometry testing must be done on pure culture colonies for complete identification.
- Fat droplets, white blood cells, and tissue cells are frequently misidentified with Cryptococcus neoformans cells. A drop of 10% KOH can be used to disintegrate leukocytes and tissue cells.
- Some Cryptococcus neoformans and other cryptococci strains may not develop visible capsules in vitro.
- It cannot be used to visualize microbe internal structures.
- Very small or thin microorganisms may not have enough contrast.
- When the background is uneven or needs better it can be difficult to interpret.
Comparison with other staining techniques
Negative staining with India ink is a technique that is frequently used in conjunction with other staining procedures such as Gramme staining or acid-fast staining. Unlike these methods, India ink staining does not require heat fixation and does not reveal information about the microorganism’s internal structure. However, India ink staining is a useful tool for visualizing the exterior structures of microorganisms, which can help with identification and classification.
Quality control
Several quality control methods should be implemented to assure the reliability and precision of India ink staining outcomes:
- Use only fresh, high-quality India ink.
- Keeping the staining solution’s pH stable.
- Making certain that the culture is well-prepared and of high quality.
- To avoid contamination, use suitable sterile practices.
- Calibration and maintenance of the microscope.
References
- BAL, C. K., BHATIA, V., KHILLAN, V., RATHOR, N., SAINI, D., DAMAN, R. & SARIN, S. K. J. I. J. O. C. C. M. P.-R., OFFICIAL PUBLICATION OF INDIAN SOCIETY OF CRITICAL CARE MEDICINE 2014. Spontaneous cryptococcal peritonitis with fungemia in patients with decompensated cirrhosis: Report of two cases. 18, 536.
- SHALINI, D., NANDINI, D., HANS, C. & DUGGAL, A. J. E. O. A. 2014. Epidemiology of cryptococcal meningitis associated with HIV in an Indian hospital. 4.
- https://en.wikipedia.org/wiki/India_ink
