Anatomy of Bacteria | Organisms | Microbiology

The anatomy of bacteria or any organism refers to its size and shape and the structural features that make it distinctive. Anatomy is an inherited trait derived from information stored in the chromosomal DNA. This trait is passed from generation to generation in the genes.

Viewed under the light microscope, most bacteria appear in variations of three different shapes – the rod, the sphere, and the spiral. As suggested by Muller, the rod is known as a bacillus (pl., bacilli). In various species of bacteria, a bacillus may be as long as 20 µm or as short as 0.5 µm.

Certain rods such as those of typhoid fever are slender; others such as the agents of anthrax are rectangular with squared ends; still others such as diphtheria bacilli are club shaped. Most rods occur singly, but some form long chains called streptobacilli. It should be noted that the word bacillus is used two ways in microbiology – to denote a rod form, and as a genus name. The organism of anthrax, for instance, is a bacillus having the name Bacillus anthracis.

A bacterial sphere is known as a coccus (pl., cocci), a term derived from the Greek kokkos, for berry. Cocci are approximately 0.5 µm in diameter. They are usually round, but they may also be oval, elongated, or indented on one side. Those cocci remaining together in pairs after reproducing are called diplococci.

The organisms of gonorrhea and one type of bacterial meningitis are examples. Those cocci consisting of chains of diplococci are called streptococci. Certain streptococci are involved in strep throat and tooth decay, but many are harmless enough to be used for producing dairy products such as yogurt.

Another variation of cocci is the sarcina. The sarcina is a cube-like packet of eight cocci (sarcina is Latin for bundle). One species, Micrococcus luteus, is a common inhabitant of the skin. Certain cocci divide randomly and form an irregular grapelike cluster of cells called a staphylococcus), from staphyle, the Greek word for grape.

A well-known example. Staphylococcus aureus, is a widespread cause of food poisoning, as well as toxic shock syndrome and numerous skin infections. The latter are known in the modern vernacular as “staph” infections.

The third important shape of bacterial organisms is the spiral. Certain spiral bacteria called vibrios are curved rods that resemble commas. The cholera organism is typical. Other spiral bacteria called spirilla (sing., spirillum) have a corkscrew shape with a rigid cell wall and hair-like projections called flagella that assist movement.

Still others, known as spirochetes, have a flexible cell wall but no flagella in the traditional sense. Movement in these organisms occurs by contractions of long filaments (endoflagella) that run the length of the cell. The organism of syphilis typifies a spirochete.

Variations in bacterial anatomy are readily visible when the organisms are magnified a thousand times under the light microscope. When the electron microscope is used, however, a magnification of a million times or more is possible and scientists can observe a world of fine bacterial details not otherwise seen by the casual observer.

i. Flagella:

Numerous species of bacterial rods and spirilla and a limited number of species of cocci are capable of independent motion. To achieve motion, they utilize structures called flagella (sing., flagellum). Flagella are composed of long, rigid strands of a protein called flagellin.

Within the strands, the protein exists in ultrathin fibers permanently bent like a coil or helix. This structure permits the flagellum to rotate. (By contrast, the flagella whip about in eukaryotic cells such as protozoa [flagellum is Latin for whip]. Here the strands are flexible, and the fibers are elongated and slide past one another.)

Electron microscopy reveals that the flagellum of Gram-negative bacteria is anchored to a hook-like shaft, which penetrates the cell wall and attaches to two ring-shaped bases in the cell membrane. Gram-positive bacteria have only the inner ring. The inner ring rotates while the outer ring, when present, remains in place.

This activity creates a propeller-like rotation that drives the bacterium forward much as a motor propels a boat. The boat, however, remains upright while the bacterium rotates in a direction opposite to the flagellar rotation.

Flagella can vary in number and placement. A monotrichous bacterium (a montrichaete) possesses a single flagellum, while a lophotrichous organism (a lophotrichaete) has a group of two or more flagella at one pole of the cell. An amphitrichous bacterium (an amphitrichaete) has groups of flagella at both ends, and a peritrichous organism (a peritrichaete) is covered with flagella. The arrangement of flagella is characteristic of a species and is used in classifying the species in taxonomic schemes.

The flagellum ranges in length from 10 µm to 20 µm and is therefore many times the length of the cell. However, the flagellum is only about 0.2 µm thick and cannot be seen under the light microscope unless coated with dye. In the human body, flagella enable bacteria such as cholera bacilli to move among the tissues and colonize various areas. Some bacteria are known to travel up to 2000 times their own length in an hour.

ii. Pili:

Pili (sing., pilus) are bacterial appendages that appear as short flagella but have no function in motility. Instead, certain pili aid the transfer of genetic material among bacteria, while other pili anchor bacteria to surfaces such as living tissue. By doing so, pili enhance an organism’s ability to cause disease.

Pili are primarily found on Gram-negative bacteria such as Neisseria gonorrhoeae, the cause of gonorrhea. Because pili are composed of protein, the body’s immune system responds to their presence by producing antipili antibodies. Therefore, an important avenue of gonorrhea research has been to develop anti-pili antibodies to neutralize the pili and prevent attachment to the patient’s tissues.

This would limit the need for antibiotics, especially since drug resistance is increasing among gonorrhea organisms. It should be noted that some microbiologists use the word fimbriae (sing., fimbria) to refer to bacterial structures of attachment and reserve the word pili for structures that function in genetic transfers.

iii. Capsule:

Many species of bacteria secrete a layer of polysaccharides and small proteins that adheres to the bacterial surface. Commonly known as a capsule, this layer is a very sticky, gelatinous structure formed by various species of bacilli and cocci, but not by spiral bacteria.

The capsule serves as a buffer between the cell and its external environment. Because of its high water content, the capsule protects the cell against dehydration while preventing nutrients from flowing away. In the body, it also contributes to the establishment of disease because white blood cells that normally engulf and destroy bacteria by phagocytosis cannot perform this function on encapsulated bacteria. For example, a principal cause of bacterial pneumonia. Streptococcus pneumoniae, is deadly in its encapsulated form but harmless when the capsule has been experimentally removed.

When the capsule has a looser consistency and is less tightly bound to the cell, it is commonly referred to as a slime layer, or glycocalyx. This structure usually contains a mass of tangled fibers of a polysaccharide called dextran. The fibers attach the bacterium to tissue surfaces.

A case in point is Streptococcus mutans, an important cause of dental caries. This bacterium attaches itself to the surface of the teeth using dextran it synthesizes from sucrose (table sugar). Soon a layer of dental plaque has formed and the streptococci begin breaking down dietary carbohydrates to acids that dissolve the enamel.

In food products, the slime-producing bacteria may cause an unsightly and distasteful experience. For instance, the glue like slime of Alcaligenes viscolactis accumulates in milk, causing it to become thick and stringy. The result is ropy milk. Bread may also become ropy if contaminated with capsule-producing Bacillus subtilis.

Cell Wall:

With the notable exception of mycoplasmas, all bacteria have a cell wall. This structure protects the cell and, to a large extent, determines its shape. A century ago, taxonomists classified bacteria as plants because of the presence of a cell wall, but modern biochemists have established that the chemical composition of the bacterial cell wall differs from that in plants.

The important chemical constituent of the bacterial cell wall is peptidoglycan. This is a very large molecule composed of alternating units of two amino-containing carbohydrates, N-acetylglucosamine and N-acetylmuramic acid, joined by cross-bridges of amino acids. Peptidoglycan occurs in multiple layers connected by side chains of four amino acids. Therefore, the many layers comprise one extremely large molecule.

The cell walls of Gram-positive and Gram-negative bacteria differ considerably. In Gram-positive bacteria, the peptidoglycan layer is about 25 nm wide and contains an additional polysaccharide called teichoic acid. About 60 to 90 percent of the cell wall is peptidoglycan, and the material is so abundant that Gram-positive bacteria are able to retain the crystal violet-iodine complex in Gram staining.

By contrast, Gram-negative bacteria have a peptidoglycan layer only 3 nm wide without any evidence of teichoic acid. The cell wall in these bacteria contains various polysaccharides, proteins, and lipids and so is much more complex than the cell wall of Gram-positive bacteria. Also, the cell wall is surrounded by an outer membrane barely separated from the cell wall by a so called periplasmic space containing a gel-like material called periplasm.

On the inner side of the cell wall the periplasmic space is wider. Bacterial toxins and enzymes apparently remain in this space and destroy antibacterial substances before they can affect the cell membrane, and other proteins, facilitate passage through the cell membrane.

The multiple layers of the Gram-negative cell also afford protection by restricting the passage of chemicals such as antibiotics, salts, and dyes to the cell. The crystal violet-iodine complex in Gram staining is lost partly because of the thinness of the cell wall in Gram-negative bacteria.

The cell wall holds the cell together. It also prevents the cell from bursting because the internal pressure may be up to 20 times the external pressure due to the high concentration of inorganic salts, carbohydrates, amino acids, and other small molecules within the cell.

The antibiotic penicillin prevents the construction of the cell wall in new cells, and they quickly burst. Where penicillin acts on new cells, lysozyme destroys existing cells. Lysozyme is an enzyme in human tears and saliva. It attacks the linkages between carbohydrates in the peptidoglycan layer, thus causing the cell wall to break down and the cell to explode.

In both cases, the effect is more dramatic in Gram-positive bacteria because these organisms have more peptidoglycan. Moreover the lipopolysaccharide layer plays a protective role in Gram-negative organisms.

Cell Membrane:

The cell membrane (also called the plasma membrane) is the boundary layer of the bacterial cell. It exists inside the cell wall and functions in transporting nutrients into the cell and waste materials out of the cell. It also anchors the DNA during replication and is a site for enzymes that function in cell wall synthesis.

Moreover, it is the location of enzymes used in energy production by the cell, a factor that makes it the equivalent of the membranes in mitochondria of a eukaryotic cell. Some microbiologists combine the cell membrane, cell wall, and capsule (if present) together as a group and term them the cell envelope.

Approximately 60 percent of the cell membrane is composed of protein, and about 40 percent of lipid, mainly phospholipid. The phospholipid molecules are arranged in two parallel layers (a phospholipid bilayer), one at the outside, the other at the inside of the membrane. In contrast, the proteins are arranged as globules floating like icebergs at or near the inner and outer surfaces of the membrane, and some globules extend from one surface of the membrane to the other.

This model of the membrane, called the fluid mosaic model, accounts for the membrane’s appearance under the electron microscope and helps explain how it allows passage of certain substances. For example, lipid-soluble materials dissolve in the phospholipid layer and pass through the membrane, while amino acids and nitrogenous bases, which do not dissolve in lipids, move through the protein passageways.

When antimicrobial substances act on the cell membrane, bacterial death usually follows. Certain detergents, for instance, dissolve the phospholipid layers and cause the cytoplasmic contents to leak out. Ethyl alcohol and some antibiotics such as polymyxin work similarly.

Cytoplasm:

Inside the cell membrane lies the cytoplasm, a gelatinous mass of proteins, carbohydrates, lipids, nucleic acids, salts, and inorganic ions, all dissolved in water. Cytoplasm is the foundation substance of a cell and the center of its growth and biochemistry. It is thick, semitransparent, and elastic.

Several cytoplasmic bodies are of interest. Ribosomes are bodies of RNA and protein associated with the synthesis of protein. Other bodies found in various bacteria include globules of starch, glycogen, or lipid. Often referred to collectively as inclusion bodies, these globules store nutrients for later use during periods of starvation. Certain other bodies serve as phosphate depots.

Commonly known as metachromatic granules, or volutin, these bodies stain deeply with dyes such as methylene blue. Their presence in diphtheria bacilli assists identification procedures. A recently discovered body, the magnetosome, helps certain bacteria orient themselves to the environment.

Crystals of an iron-containing compound called magnetite fill the magnetosome and align themselves with the local magnetic field. Scientists believe that the magnetite directs bacteria toward their preferred habitat.

The cytoplasm is also the site of the bacterial chromosome. This closed loop of DNA contains the hereditary information of the cell. It is suspended in the cytoplasm without a covering or membrane and is not associated with protein. These factors are important in classifying bacteria as prokaryotes. The term nucleoid is often applied to the chromosome region.

Smaller molecules of DNA exist apart from the chromosome in closed loops called plasmids. Although they contain few genes and are not essential for bacterial growth, plasmids are significant because many carry genes for drug resistance. For this reason they are often called R factors (“R” for resistance).

Plasmids may be transferred between cells during recombination processes and are known to multiply during cell reproduction. They are a focus of attention in industrial technologies that utilize genetic engineering. The morphological features of bacterial cells.

Spores:

Certain Gram-positive bacteria are able to produce highly resistant structures called endospores or simply, spores. Members of the genera Bacillus and Clostri­dium are among the best-known spore-formers. These bacteria grow, mature, and reproduce for several hours as vegetative cells. Spore formation then begins.

The bacterial chromosome repli­cates, a small amount of cytoplasm gathers with it, and the cell membrane grows in to seal off the developing spore. Thick layers of peptidoglycan form and a series of coats are synthesized to protect the contents further. The cell wall of the vegetative cell then disintegrates and the spore is freed.

Endospores may develop at the end of the cell, near the end, or at the center of the cell, depending on the species. They contain little water and exhibit very few chemical reactions. However, they do have a large amount of dipicolinic acid, an organic substance that helps stabilize their proteins.

When the external environment is favourable, the protective layers break down and the spores germinate to vegetative cells. It should be noted that spore formation is not a reproductive process – a vegetative cell forms a single spore, and later the spore germinates to one vegetative cell.

Bacterial spores are probably the most resistant living things known. For example, most vegetative bacteria die quickly in water over 80°C, but bacterial spores may remain alive in boiling water (100°C) for 2 hours or more. When placed in 70 percent ethyl alcohol, spores have survived for 20 years.

Humans can barely withstand 500 rems of radiation, but spores can survive a million rems. Drying has little effect on the spores, and living spores have been recovered from the intestines of Egyptian mummies. In 1983, archaeologists found spores alive in sediment lining Minnesota’s Elk Lake. The sediment was 7518 years old.

Only four serious diseases in humans are known to be caused by spore-formers. The first, anthrax, is due to Bacillus anthracis. This deadly blood disease was studied by Koch and Pasteur. The periodic recurrence of anthrax in the countryside was due to spores remaining alive in the soil where they could be ingested by animals.

The other three diseases are botulism, gas gangrene, and tetanus. These diseases are caused by species of Clostridium. Clostridial spores are often found in soil as well as in human and animal intestines. For the spores to germinate to vegetative cells, the environment must be free of oxygen.

The dead tissue in a wound provides such an environment for the tetanus and gas gangrene spores, and a vacuum-sealed can of food is suitable for the spores of botulism. Some imaginative scientists envision spores as visitors to Earth from a distant galaxy.