Fimbriae and pili are extra-cytoplasmic appendages found on bacterial cell walls. Fimbriae are short, bristle-like fibers found on both gram-positive and gram-negative bacteria that aid in attachment and aggregation. Pili are longer hair-like structures found mainly on gram-negative bacteria that can facilitate attachment and conjugation. Key differences are that fimbriae are shorter and solid, while pili are hollow tubes that can confer twitching motility or act as viral receptors. Both structures aid in bacterial pathogenesis but through different mechanisms of attachment and gene transfer.

1. [B]. FIMBRIAE and PILI
 Flagella like appendages on bacterial cell wall, but smaller in size.
2. EXTRA-CYTOPLASMIC APPENDAGES
 They originate from cytoplasmic membrane that protrudes outside after
penetrating the peptidoglycan layer of cell wall.
 Fimbriae- bristle-like short fibers occurring on the bacterial surface in both
gram +ve and gram –ve bacteria.
 Pili- longer hair-like tubular microfibers on the surface of gram –ve bacteria.

2.  The shaft having pilin consists of adhesive tip structures towards its
terminal with a shape that corresponds to that of particular glycolipid
receptors on the cell of the host.
 Both differ from flagella in being shorter and thinner, straight and less ligid.
 But they are in large numbers either at the poles of bacterial cell or evenly
distributed over the entire surface of the cell.
 Composed of 100% protein called “fimbrilin or pilin” which consists of
about 163 amino acids.

3.  Long conjugation pili are helical tubes having a hollow core, forming a
cylinder structures of repeating protein units- with diameter of 65-135 Å and
length close to 20 µm which is greater in comparison to fimbriae.
Role of Fimbriae:
 Adhesive in nature attaching the cell to the substrate.
 Cause ‘agglutination’ of blood cells- leucocytes, erythrocytes, etc.
 Operate as aggregation organelles, forming stellate aggregation on a static
liquid medium- like ‘biofilms’ (e.g, pneumoniae type 3 fimbriae form biofilms on
plastic or HECM-coated surfaces).
 Armed with antigenic traits- serving as thermolabile
agglutinogen.

4. Role of Pili:
Common Pili (Adhesiveness):
 They mediate the attachment of bacteria to specific
receptors on the cell surface (e.g. human cell), which is the
first step in establishing infection in some organisms.
 They contribute to the pathogenicity of certain bacteria by
enhancing colonization on the surfaces of the cells of other
organisms. E.g. Mutants of Neisseria gonorrhoeae that do
not form pili are non-pathogenic.
Sex pili (conjugation tube):
Pili that form attachment between (donor) and the
(recipient) bacteria during conjugation and acts as a
conduit for the passage of DNA. E.g. in gram negative
bacillus Escherichia coli.
 Some pili are involved in a special form of cell movement- ‘twitching motility’
(through type IV pili- in Pseudomonas aeruginosa).

5.  Size: 0.03-0.14 μm
 Number: 200-400 (per cell)
 Less rigid structures
 These are solid structures made of
fimbrilin protein
 Present on both gram +ve and
gram -ve bacteria
 Evenly distributed on the entire
surface of the cell
DIFFERENCES BETWEEN FIMBRIAE AND PILI
 Size: 0.5-2 μm
 Number: 1-10 (per cell)
 More rigid as compared to fimbriae
 Hollow tubular structures made of
pilin protein
 Present mostly on some gram -ve
bacteria…!!! (in gram +ve covalent
polymerization of pilin subunits using a
transpeptidase enzyme- Sortase.
 Randomly distributed on the cell
surface
Fimbriae Pili

6.  Formation is controlled by the
genes present in the nucleoid region
of bacteria
 Basic function- surface attachment
 No role in bacterial mobility
 Not reported to act as receptors
for viruses
 Examples of Fimbriate bacteria-
Salmonella typhimurium, Shigella
dysenteriea, etc.
DIFFERENCES BETWEEN FIMBRIAE AND PILI (cont…..)
 Formation is controlled by the
genes present in plasmids
 Basic function- gene transfer (by
conjugation) and attachment
 Some type IV pili show a special
twitching type of mobility
 Majority of pili can act as receptors
for some viruses
 Examples of bacteria having pili-
Escherichia coli, Neisseria
gonorrhoeae, Pseudomonas, etc.
Fimbriae Pili