Pili: Pili are hair-like structures on the surface of the cell that help attach to other bacterial cells. Shorter pili called fimbriae help bacteria attach to various surfaces. A pilus is typically 6 to 7 nm in diameter. The types of pili are Conjugative pili and Type IV pili. Conjugative pili allow the transfer of DNA between bacteria, in the process of bacterial conjugation. Some pili, called type IV pili, generate motile forces.

Morphology of prokaryotic cells

Prokaryotic cells have various shapes; the four basic shapes are (Figure 3):

• Cocci - spherical
• Bacilli - rod-shaped
• Spirochaete - spiral-shaped
• Vibrio - comma-shaped

Figure 3: Morphology of prokaryotic cells

Milieu

Prokaryotes live in nearly all environments on Earth. Some archaea and bacteria thrive in extreme conditions, such as high temperatures (thermophiles) or high salinity (halophiles). Organisms such as these are referred to as extremophiles. Many archaea grow as plankton in the oceans. Symbiotic prokaryotes live in or on the bodies of other organisms, including humans.

Sociability
Prokaryotes are believed to be strictly unicellular though most can form stable aggregated communities in a stabilizing polymer matrix called “biofilms”. Cells in biofilms often show distinct patterns of gene expression (phenotypic differentiation) in time and space. Also, as with multicellular eukaryotes, these changes in expression appear as a result of quorum sensing or cell to cell signal transduction. Bacterial biofilms are often made up of approximately dome-shaped masses of bacteria and matrix separated by “voids” through which the medium (water) may flow relatively uninhibited and such system are termed as microcolonies. The microcolonies may join together above the substratum to form a continuous layer, closing the network of channels separating microcolonies. Bacterial biofilms may be 100 times more resistant to antibiotics than free-living unicells and may be difficult to remove from surfaces once they have colonized them. Other aspects of bacterial cooperation like bacterial conjugation and quorum-sensing-mediated pathogenicity provide additional challenges to researchers and medical professionals seeking to treat the associated diseases.

Colony of bacteria
Most bacteria represent themselves in colonies. By colony we mean individual organisms of the same species living closely together in mutualism. All species in a colony are genetically equivalent. The shape of the colony can be circular and irregular. Bacterial colonies are frequently shiny and smooth in appearance. In microbiology, colony-forming unit (CFU) is a measure of viable bacteria in such colonies.  If a bacterial cell like Escherichia coli divides every 20 minutes then after 30 cell divisions there will be 2³⁰ or 1048576 cells in a colony.