Flagella are rigid protein structures aboutnanometres in diameter and up tomicrometres in length that are used for motility. Flagella are driven by the energy released by the transfer of ions down an electrochemical gradient across the cell membrane.

Fimbriae are fine filaments of protein just – nanometres in diameter and up to several micrometers in length. They are distributed over the surface of the cell and resemble fine hairs when seen under the electron microscope. Fimbriae are believed to be involved in attachment to solid surfaces or to other cells and are essential for the virulence of some bacterial pathogens. Pili sing. pilus are cellular appendages slightly larger than fimbriae that can transfer genetic material between bacterial cells in a process called conjugation see bacterial genetics below.

Capsules or slime layers are produced by many bacteria to surround their cells and vary in structural complexity ranging from a disorganised slime layer of extracellular polymer to a highly structured capsule or glycocalyx. These structures can protect cells from engulfment by eukaryotic cells such as macrophages. They can also act as antigens and be involved in cell recognition as well as aiding attachment to surfaces and the formation of biofilms.

The assembly of these extracellular structures is dependent on bacterial secretion systems. These transfer proteins from the cytoplasm into the periplasm or into the environment around the cell. Many types of secretion systems are known and these structures are often essential for the virulence of pathogens so are intensively studied.

Endospores

Further information Endospores

Bacillus anthracis stained purple growing in cerebrospinal fluid
Bacillus anthracis stained purple growing in cerebrospinal fluid

Certain genera of Grampositive bacteria such as Bacillus Clostridium Sporohalobacter Anaerobacter and Heliobacterium can form highly resistant dormant structures called endospores. In almost all cases one endospore is formed and this is not a reproductive process although Anaerobacter can make up to seven endospores in a single cell. Endospores have a central core of cytoplasm containing DNA and ribosomes surrounded by a cortex layer and protected by an impermeable and rigid coat.

Endospores show no detectable metabolism and can survive extreme physical and chemical stresses such as high levels of UV light gamma radiation detergents disinfectants heat pressure and desiccation. In this dormant state these organisms may remain viable for millions of years and endospores even allow bacteria to survive exposure to the vacuum and radiation in space. Endosporeforming bacteria can also cause disease for example anthrax can be contracted by the inhalation of Bacillus anthracis endospores and contamination of deep puncture wounds with Clostridium tetani endospores causes tetanus.