Bacteria (advanced)

Bacteria (advanced)

Bacteria are unicellular organisms that have no cell walls and are a few micrometers in length

Biology

Keywords

bacillus, bacterium, Gram positive, Gram staining, Gram negative, lactic acid bacterium, gene, polysaccharide, antibiotic, flagellum, pilus, DNA, cytoplasm, cilium, cell wall, plasmid, binary fission, prokaryote, cosmopolitan, endotoxin, peptidoglycan, tok, endospore, cholera, plague, Salmonella, gonococcal infection, diphtheria, Lyme disease, gastric ulcer, pneumonia, Escherichia coli, meningitis, intestines, syphilis, infection, biology, Microbiology, living organism

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Questions

  • What is the size of most bacteria?
  • Is it true that the term 'bacteria', in its strict sense, refers to Archaea?
  • Is it true that similarly to the DNA of eukaryotes, histones are attached to the DNA of bacteria too?
  • Is it true that while plasmids are physically separate from bacterial DNA, they are functionally related?
  • What shape are bacilli?
  • What is the shape of the pathogens of Lyme disease?
  • What is the shape of the pathogen of tetanus?
  • What can be found in the cell wall of bacteria?
  • What colour are Gram-positive bacteria stained during Gram staining?
  • Is it true that Gram staining is important in deciding the treatment of a bacterial infection?
  • What is the motive force that drives the movement of the flagellum?
  • What is the purpose of the endospore that is produced by certain bacteria?

Scenes

Bacterium

Bacteria are single-celled, prokaryotic microorganisms, that is, they do not have nuclei. Most bacteria measure between 0.5 to 10 micrometers.

The two major groups of prokaryotes are Eubacteria and Archaea. The term 'bacteria', in its strict sense, refers to Eubacteria.

The ancestors of modern bacteria appeared on Earth about 3.5 billion years ago. Bacteria are cosmopolitan, they are found in almost all environmental conditions. These microorganisms reproduce mainly asexually, through binary fission.

Anatomy

  • outer membrane
  • cell wall - Based on the type of their cell walls, bacteria are either Gram-negative or Gram-positive. The basis of differentiation is that Gram staining produces different results in the cell walls of different types of bacteria because of their different structure. Gram-positive bacteria retain the dye, while Gram-negative ones do not.
  • cytoplasmic membrane (cell membrane) - A double lipid layer.
  • cytoplasm
  • DNA - The DNA of bacteria forms a closed loop and is not surrounded by a nuclear envelope (or membrane), bacteria are therefore prokaryotic cells ('before nucleus'). In the cells of eukaryotic organisms, however, DNA does not form a closed loop and is surrounded by a nuclear envelope. Humans are made up of eukaryotic cells.
  • plasmid - A small DNA molecule within cells that is physically separated from the chromosomal DNA and can replicate independently. It is most commonly found in bacteria as ring-like DNA molecules.
  • cilium - A thin, tubular protein structure. Some cells have a few hundred cilia. Their primary function is to provide adhesion to solid surfaces or other cells. Some types of cilia also serve locomotion.
  • flagellum - A long protein structure that serves the locomotion of the bacterium.
  • pilus - An appendage on certain Gram-negative bacteria, longer than a cilium. They serve bacterial conjugation, that is, connect two bacteria for the transfer of plasmids.

The cytoplasm forms the main mass of a bacterial cell. The circular bacterial DNA is embedded in the cytoplasm and is connected to the cytoplasmic membrane. Only proteins that are involved in replication and transcription are connected to the bacterial DNA. Histones, which can form nucleosomes together with DNA, are not present in bacteria.

Short, ring-like DNA molecules, called plasmids, are also present in the cytoplasm but are physically separated from the bacterial DNA and function differently.

Almost all bacteria are enclosed by a cell wall. It provides the cells with structural support and protection. Most bacteria have shorter or longer tubular formations called cilia, pili, or flagella outside the cell wall; these serve locomotion.

Morphology

  • coccus - An example of such bacteria is Micrococcus luteus, a non-pathogenic bacterium that is part of the normal flora of the skin.
  • diplococcus - One such bacterium is Neisseria gonorrhoeae, which may cause infection of the genitals.
  • streptococcus - Streptococcus pneumoniae, a bacterium responsible mainly for pneumonia, has this shape.
  • staphylococcus - One such bacterium is Staphylococcus aureus, which may cause skin infections.

A common way to identify bacteria is by their shape. Spherical bacteria, which are also called cocci, can occur as single cells, pairs, chains or clusters. Certain bacteria that cause suppurative infections, pneumonia or gonorrhea are spherical in shape.

There are rod-shaped bacteria as well, one group of which are called bacilli. Some of these are harmless, such as the lactobacilli used in the food industry. Many bacilli, however, cause diseases; for example, the pathogens of plague, tuberculosis, salmonellosis, and tetanus. Escherichia coli, which is commonly found in the human intestinal system, is also a bacillus.

Spiral-shaped bacteria exist as well. The pathogens of Syphilis, Lyme disease, and leptospirosis are all spiral-shaped bacteria. Spirochetes are spiral-shaped, flexible bacteria. Some bacteria, such as Vibrio cholerae, are similar to commas used in writing, while others, like Corynebacterium diphtheriae, resemble a club.

Cell wall

  • Gram negative
  • Gram positive
  • lipopolysaccharide (LPS, endotoxin) - A polymer molecule, consisting of lipids and polysaccharides, that is an important component of the outer membrane of Gram-negative bacteria. It contributes to the structural integrity of the cell and protects against some chemical attacks. It can trigger an intense immune response in the host.
  • cytoplasmic membrane (cell membrane)
  • thin peptidoglycan layer - It is a heteropolymer of N-Acetylglucosamine (GlcNAc) and N-Acetylmuraminic acid (MurNAc) that are bound by peptides.
  • outer membrane
  • thick peptidoglycan layer - It consists of polysaccharide chains that are attached to one another by peptides.
  • protein

The cell walls of bacteria are resistant, rigid structures made up of peptidoglycan. Peptidoglycan is a polysaccharide that consists of N-acetylglucosamine and N-acetylmuramic acid alternating in long chains and connected by peptides. In laboratories, a dye is added to bacteria to differentiate those that contain an outer membrane (or lipid bilayer). This method is called Gram staining. Bacteria with a thick layer of peptidoglycan are stained dark blue and are called Gram-positive bacteria. Those bacteria whose peptidoglycan layer is thinner and have a lipid bilayer are stained pink and are called Gram-negative bacteria. This difference is important because these two types of bacteria react to antibiotics differently, so treating different bacterial infections requires different antibiotics.

Flagellum

  • outer membrane
  • peptidoglycan layer
  • rod
  • cytoplasmic membrane (cell membrane)
  • rings
  • rotor
  • stator
  • filament - A large protein molecule that can reach 30 micrometers in length.
  • hook - Bent tube-like protein molecule.
  • basal body

Several types of bacteria have flagella. A single bacterial cell can have up to thirty flagella. The flagellum helps locomotion in an aqueous environment, such as inside the bodies of animals.

A flagellum consists of three main parts: a basal body, a hook, and a filament. The basal body, the motor of the flagellum, is enclosed in the cell wall. In the middle, there is a rod connected to the hook. There are rings around the rod that stabilize and rotate it. The filament, the largest part of the flagellum, comprises flagellin proteins. It works as a helical screw that creates the torque necessary for locomotion of bacteria to occur. It is the proton motive force that propels the rotary motion. Protons start moving because of a concentration gradient between the inside and the outside of the bacterial cell. This makes it possible for the flagellum to rotate hundreds of times per second.

Plasmid

  • origin of replication - The enzymes of the host cell can duplicate plasmid through this sequence.
  • antibiotic resistance gene - Resistance against certain antibiotics (e.g. ampicillin) is encoded in it.

The length of plasmids is measured in kilobase pairs (kbp). Their length varies between 1 and 200 kbp. Plasmids carry genes that are not vital under normal circumstances. There are plasmids, however, that may benefit the survival of the bacterium. Such plasmids are those carrying antibiotic resistance genes or bacterial toxin genes. Plasmids can replicate independently and spread through different bacteria, even of another species. This process is called horizontal gene transfer.

Capsule and slime layer

  • microcapsule - It consists of exopolysaccharides and has an organized structure. It hides the bacteria from the immune system of the host.
  • slime - It can be made up of exopolysaccharides. Its structure is less organized than that of the microcapsule.

The cell wall of certain bacteria is surrounded by a rigid or a slimy structure. It is called a capsule or microcapsule when it has a well-organized structure and a slime layer when its structure is less organized. The function of these structures is to hide the bacteria from the immune system or to attach them to surfaces. There are also bacteria which, under unfavorable conditions, form an extremely resistant structure called endospore. Such bacteria include rod-shaped Clostridium and Bacillus bacteria.

Animation

  • outer membrane
  • cell wall - Based on the type of their cell walls, bacteria are either Gram-negative or Gram-positive. The basis of differentiation is that Gram staining produces different results in the cell walls of different types of bacteria because of their different structure. Gram-positive bacteria retain the dye, while Gram-negative ones do not.
  • cytoplasmic membrane (cell membrane) - A double lipid layer.
  • cytoplasm
  • DNA - The DNA of bacteria forms a closed loop and is not surrounded by a nuclear envelope (or membrane), bacteria are therefore prokaryotic cells ('before nucleus'). In the cells of eukaryotic organisms, however, DNA does not form a closed loop and is surrounded by a nuclear envelope. Humans are made up of eukaryotic cells.
  • plasmid - A small DNA molecule within cells that is physically separated from the chromosomal DNA and can replicate independently. It is most commonly found in bacteria as ring-like DNA molecules.
  • cilium - A thin, tubular protein structure. Some cells have a few hundred cilia. Their primary function is to provide adhesion to solid surfaces or other cells. Some types of cilia also serve locomotion.
  • flagellum - A long protein structure that serves the locomotion of the bacterium.
  • pilus - An appendage on certain Gram-negative bacteria, longer than a cilium. They serve bacterial conjugation, that is, connect two bacteria for the transfer of plasmids.
  • coccus - An example of such bacteria is Micrococcus luteus, a non-pathogenic bacterium that is part of the normal flora of the skin.
  • diplococcus - One such bacterium is Neisseria gonorrhoeae, which may cause infection of the genitals.
  • streptococcus - Streptococcus pneumoniae, a bacterium responsible mainly for pneumonia, has this shape.
  • staphylococcus - One such bacterium is Staphylococcus aureus, which may cause skin infections.
  • bacillus - Escherichia coli, a bacterium commonly found in our intestines has such shape.
  • diplobacillus - Such bacterium is Moraxella lacunata, which is commonly associated with eye infections.
  • coccobacillus - One such bacterium is Haemophilus influenzae, which, contrary to its name, is not associated with influenza. In children, it causes meningitis.
  • helical shape - Helicobacter pylori, a bacterium associated with gastric ulcer, is also of this shape.
  • spirillum - One of the largest bacteria, the 8-10 micrometers long Spirillum volutans has such shape.
  • spirochaeta - Examples are the pathogens of Lyme disease (Borellia burgdorferi) and of syphilis (Treponema pallidum).
  • comma - Vibrio cholerae, the pathogen of cholera, is an example.
  • club - Corynebacterium diphtheriae, the pathogen of Diphtheria, has this shape.
  • Gram negative
  • Gram positive
  • lipopolysaccharide (LPS, endotoxin) - A polymer molecule, consisting of lipids and polysaccharides, that is an important component of the outer membrane of Gram-negative bacteria. It contributes to the structural integrity of the cell and protects against some chemical attacks. It can trigger an intense immune response in the host.
  • cytoplasmic membrane (cell membrane)
  • thin peptidoglycan layer - It is a heteropolymer of N-Acetylglucosamine (GlcNAc) and N-Acetylmuraminic acid (MurNAc) that are bound by peptides.
  • outer membrane
  • thick peptidoglycan layer - It consists of polysaccharide chains that are attached to one another by peptides.
  • protein
  • outer membrane
  • peptidoglycan layer
  • cytoplasmic membrane (cell membrane)
  • filament - A large protein molecule that can reach 30 micrometers in length.
  • microcapsule - It consists of exopolysaccharides and has an organized structure. It hides the bacteria from the immune system of the host.
  • slime - It can be made up of exopolysaccharides. Its structure is less organized than that of the microcapsule.

Narration

Bacteria are single-celled, prokaryotic organisms. They are cosmopolitan, they are found in almost all environmental conditions on Earth. Most bacteria measure just a few micrometres.

The bacterial cell is enclosed by a cell wall. Cilia, pili and flagella may be found on the surface of the cell wall. The cytoplasm, which is surrounded by a membrane, constitutes the main mass of a bacterial cell and the circular bacterial DNA is embedded in it. The cytoplasm also contains short, ring-like DNA molecules, called plasmids, which are both physically and functionally separate from the bacterial DNA.

Bacteria can be categorised by their shape. Spherical bacteria are called cocci. Cocci can occur as single cells, pairs, chains or clusters. Certain bacteria that cause suppurative infections, pneumonia or gonorrhoea are spherical in shape.
Rod-shaped bacteria are called bacilli. Some of these are harmless, such as lactobacilli, used in the food industry. Many bacilli, however, cause diseases; for example, the pathogens of plague, tuberculosis, salmonellosis, and tetanus.
The pathogens of Syphilis, Lyme disease, and leptospirosis are spiral-shaped bacteria. Some bacteria, such as Vibrio cholerae, that causes cholera, are similar to commas, while others, like the pathogen of diphtheria, Corynebacterium diphtheriae, resemble a club.

Bacteria can also be classified according to their cell walls' reaction to a test called Gram staining. Those bacteria in which the cell wall contains a thin peptidoglycan layer and a lipid bilayer are stained pink and are called Gram-negative. However, bacteria with a thick peptidoglycan layer in their cell walls are stained dark blue and are called Gram-positive.

Several types of bacteria have flagella which play an important role in the locomotion of bacterial cells. The filament, the largest part of the flagellum, comprises flagellin proteins. It works as a helical screw that creates the torque necessary for locomotion of bacteria to occur. The cell wall of certain bacteria is surrounded by a rigid or slimy structure. The function of these structures is to hide the bacteria from the immune system or to attach them to different surfaces.

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