Negative-staining with nigrosin or India ink is a quick and easy method to gain information about the presence or absence of capsules or any other layers around bacteria. An organism with a capsule will show a halo around the cell.
• Place a drop (B25 μl) of a 7% nigrosin solution or Indian ink on a microscope slide; depending on the specimen, sometimes Indian ink has to be diluted with distilled water.
• Resuspend a small quantity of biomass in the staining solution.
• Apply a cover slip.
• Soak up redundant staining solution with filter paper until a thin layer of staining solution is left under the cover slip and examine.
• Alternatively, place a drop (B15 μl) of the bacterial suspension on a microscope slide.
• Cover with a cover slip.
• Place 7% nigrosin or the water-diluted Indian ink at one edge of the cover slip.
• Soak the staining solution through the suspension by applying a piece of a filter paper on the opposite side of the cover slip.
• This will result in an uneven distribution of the dye solution, and the best area of dye thickness can then be examined.