Fimbriae and flagella are both filamentous proteinaceous structures found on the surface of some bacterial cells, but they have different functions and structures. Fimbriae are bristle-like short fibers that occur on the surface of some Gram-positive and Gram-negative bacteria. Their main function is surface attachment, which enables the bacterial cell to stick to the surface of host cells. They also help in the formation of pellicles or biofilms.

Flagella, on the other hand, are long whip-like filamentous structures present in some bacteria. The most important function of flagella is to assist in locomotion. Flagella can also act as a sensory organ to detect temperature and the presence of certain chemicals in the external environment of the cell.

In summary, fimbriae are used for attachment while flagella are used for movement.

Gram-positive bacilli are rod-shaped bacteria that stain blue or purple on Gram’s staining. They can be divided into five medically important genera:
1. Bacillus,
2. Clostridium,
3. Corynebacterium,
4. Listeria, and
5. Gardnerella.
Some examples of gram-positive bacilli are
1. Bacillus anthracis,
2. Bacillus cereus,
3. Corynebacterium diphtheriae,
4. Listeria monocytogenes, and
5. Clostridium species.

Gram staining is a laboratory technique used to differentiate bacterial species into two groups based on the characteristics of their cell walls. The technique is named after its inventor, Hans Christian Gram, who developed the method in 1884.

In the Gram staining process, bacterial cells are first stained with crystal violet, then treated with iodine, which forms a complex with the crystal violet. The cells are then washed with alcohol or acetone, which causes the cells to either retain or lose the crystal violet-iodine complex. Finally, the cells are counterstained with safranin, which stains the cells that have lost the crystal violet-iodine complex.

Bacteria that retain the crystal violet-iodine complex after washing with alcohol or acetone are called gram-positive bacteria and appear purple under a microscope. Bacteria that lose the crystal violet-iodine complex and take up the safranin counterstain are called gram-negative bacteria and appear pink or red under a microscope.

It is the differential staining techniques which was first developed by Ziehl and later on modified by Neelsen. So this method is also called Ziehl-Neelsen staining techniques. Neelsen in 1883 used Ziehl’s carbol-fuchsin and heat then decolorized with an acid alcohol, and counter stained with methylene blue. Thus Ziehl-Neelsen staining techniques was developed.

The main aim of this staining is to differentiate bacteria into acid fast group and non-acid fast groups.

This method is used for those microorganisms which are not staining by simple or Gram staining method, particularly the member of genus Mycobacterium, are resistant and can only be visualized by acid-fast staining.

Principle of Acid-Fast Stain
When the smear is stained with carbol fuchsin, it solubilizes the lipoidal material present in the Mycobacterial cell wall but by the application of heat, carbol fuchsin further penetrates through lipoidal wall and enters into cytoplasm. Then after all cell appears red. Then the smear is decolorized with decolorizing agent (3% HCL in 95% alcohol) but the acid fast cells are resistant due to the presence of large amount of lipoidal material in their cell wall which prevents the penetration of decolorizing solution. The non-acid fast organism lack the lipoidal material in their cell wall due to which they are easily decolorized, leaving the cells colorless. Then the smear is stained with counterstain, methylene blue. Only decolorized cells absorb the counter stain and take its color and appears blue while acid-fast cells retain the red color.