BACTERIAL CELLBACTERIAL CELL

BACTERIAL CELL

The illustration shows a generalised bacterium with many of the main components illustrated. No real organism would have all of these features.

The top half of the diagram shows a Gram +ve bacterium with its thick peptidoglycan outer wall closely apposed to the inner plasma membrane. The lower half of the cell shows a Gram -ve bacterium with its inner plasma membrane, thin intermediate peptidoglycan layer and external membrane. The outer portion of the external membrane is a lipopolysaccharide layer. This layer comprises core components that form the outer layer of the membrane and (usually) side chains that radiate off. These side chains are called O-antigens. They are absent in Yersinia pestis. In this diagram, parts of the two types of wall are covered in a regular arrangment of proteins called an S-layer. A good example of an S-layer is shown in our diagram of anthrax.

DETAILED BACTERIAL CELL WALL PICTURES:

Gram positive Bacterial Cell Wall Gram negative Bacterial Cell Wall

CAPSULEA generalised capsule is shown covering part of the Gram +ve and Gram -ve regions. Capsules can prevent drying, help in adhesion and help to ward off attack from viruses and host cells.

SURFACE FEATURESSeveral flagella are shown. These are spiralised protein tubes that have a motor at their base. This motor anchors the flagella into the cell wall and its rotation causes the flagella to propel the bacterium along, like the propeller of a boat. Various flagellar arrangements are possible, from a single flagellum at one pole to numerous flagella radiating from all over the bacterial surface. Straighter protein tubes called pili are also shown. These can help in attachment of bacteria to host cells or other substrates. Pili are also used for gene transfer.

GENOMEThe genetic material is a skein of circular DNA localised as the nucleoid. The nucleoid lacks a nuclear membrane (a defining characteristic of prokaryotic cells). Peeping out from the upper right part of the nucleoid is a plasmid - a separate piece of DNA. Plasmids replicate independently of the nucleoid DNA and can be exchanged between cells (the means of passing on, for example, drug resistance).

CYTOPLASMThe bacterial cytoplasm is shown filled with ribosomes. These are somewhat smaller than their eukaryotic counterparts and many are shown linked into polysomes. Infoldings of the Plasma membrane are common and are often associated with the nucleoid. In this picture such an infolding can be seen around the middle of the cell and it is termed a mesosome.

HELICAL FILAMENTSHelically arranged protein filaments are shown winding around just beneath the plasma membrane. They are thought to guide the production of the cell wall to create the shape of the bacterium. They are absent in spherical forms. The purple helix (MreB) controls cell width and the bluish one (Mbl) controls length. They are composed of actin like molecules. This form of prokaryotic cytoskeleton was recently described in the journals CELL and NATURE (references: CELL, Vol. 104, 913-922, March 23, 2001. Nature, Vol. 413, September 2001).

PLAGUE - Yersinia pestis

ANTHRAX - Bacillus anthracis

Bacteria Cell StructureThey are as unrelated to human beings as living things can be, but bacteria are essential to human life and life on planet Earth. Although they are notorious for their role in causing human diseases, from tooth decay to the Black Plague, there are beneficial species that are essential to good health.

For example, one species that lives symbiotically in the large intestine manufactures vitamin K, an essential blood clotting factor. Other species are beneficial indirectly. Bacteria give yogurt its tangy flavor and sourdough bread its sour taste. They make it possible for ruminant animals (cows, sheep, goats) to digest plant cellulose and for some plants, (soybean, peas, alfalfa) to convert nitrogen to a more usable form.Bacteria are prokaryotes, lacking well-defined nuclei and membrane-bound organelles, and with chromosomes composed of a single closed DNA circle. They come in many shapes and sizes, from minute spheres, cylinders and spiral threads, to flagellated rods, and filamentous chains. They are found practically everywhere on Earth and live in some of the most unusual and seemingly inhospitable places.

• In the late 1600s, Antoni van Leeuwenhoek became the first to In the late 1600s, Antoni van Leeuwenhoek became the first to study bacteria under the microscope. During the 19th century, study bacteria under the microscope. During the 19th century, the French scientist Louis Pasteur and the German physician the French scientist Louis Pasteur and the German physician Robert Koch demonstrated the role of bacteria as pathogens Robert Koch demonstrated the role of bacteria as pathogens (causing disease). The 20th century saw numerous advances in (causing disease). The 20th century saw numerous advances in bacteriology, indicating their diversity, ancient lineage, and bacteriology, indicating their diversity, ancient lineage, and general importance. Most notably, a number of scientists around general importance. Most notably, a number of scientists around the world made contributions to the field of microbial ecology, the world made contributions to the field of microbial ecology, showing that bacteria were essential to food webs and for the showing that bacteria were essential to food webs and for the overall health of the Earth's ecosystems. The discovery that overall health of the Earth's ecosystems. The discovery that some bacteria produced compounds lethal to other bacteria led some bacteria produced compounds lethal to other bacteria led to the development of antibiotics, which revolutionized the field to the development of antibiotics, which revolutionized the field of medicine.of medicine.

• There are two different ways of grouping bacteria. They can be There are two different ways of grouping bacteria. They can be divided into three types based on their response to gaseous divided into three types based on their response to gaseous oxygen. Aerobic bacteria require oxygen for their health and oxygen. Aerobic bacteria require oxygen for their health and existence and will die without it. Anerobic bacteria can't tolerate existence and will die without it. Anerobic bacteria can't tolerate gaseous oxygen at all and die when exposed to it. Facultative gaseous oxygen at all and die when exposed to it. Facultative aneraobes prefer oxygen, but can live without it.aneraobes prefer oxygen, but can live without it.

• The second way of grouping them is by how they obtain their The second way of grouping them is by how they obtain their energy. Bacteria that have to consume and break down complex energy. Bacteria that have to consume and break down complex organic compounds are heterotrophs. This includes species that organic compounds are heterotrophs. This includes species that are found in decaying material as well as those that utilize are found in decaying material as well as those that utilize fermentation or respiration. Bacteria that create their own fermentation or respiration. Bacteria that create their own energy, fueled by light or through chemical reactions, are energy, fueled by light or through chemical reactions, are autotrophs.autotrophs.