Bacteria cells are the smallest living cells that are known; even though viruses are smaller than bacteria, viruses are not living cells; hence bacteria are the smallest of living cells. There are different types of bacteria with various sizes, shapes, and structures. The types of bacteria and their labeled diagrams are shown below. The sizes of bacteria cells that can infect humans beings range from 0.1 to 10 micrometers. Some larger types of bacteria such as the rickettsias, mycoplasmas, and chlamydias have similar sizes as the largest types of viruses, the poxviruses.
Because of the small sizes of bacteria cells, they are difficult to identify even with good light microscopes. In fact, they are almost colorless and looks transparent, taking the similarity of their environment. Because of all these, some biochemical tests are often carried out in the laboratories to identify and group bacteria cells into different types.
Shapes of Bacteria cells
Bacteria have four basic types of shapes. They include:
- Spherical cells or cocci
- Rod-shaped cells or bacilli
- Spiral-shaped cells or spirilla
- Comma-shaped cells or vibrios
Coccal cells (spherical bacteria cells) may be arranged into pairs, known as diplococci; or maybe arranged into chains known as streptococci, or could be arranged into clusters called staphylococci.
The Rod-shaped bacteria cells may somewhat be shorter and are called coccobacilli or may have club-or dumbbell-shapes and are called coryneform.
Structure of Bacteria cells
Most bacteria structures are made up of the envelope and its associated structures, the cytosol, and the nucleoid. The cytosol together with the nucleoid (also called the Nuclear body) forms the inner core of the general structure of bacteria cells. The genetic material of bacteria known as the nucleoid or nuclear body is not separated from the cytosol as in eukaryotic cells (such as that of human cells). For the human cells, the nuclear materials are separated from the cytoplasm by a membrane known as the nuclear membrane. Hence the bacterial cell structure lacks a nucleus. Because the bacteria cell structure lacks a nuclear membrane, they are group as Prokaryotic cells different from the cells with nuclear membranes known as Eukaryotic cells.
Components that make up the structures of a bacterial cell
- The Envelope, which consists of the bacteria cell wall, capsule, and cell membrane
- Appendages which could be Pili or flagella
- Core components such as the cytosol, nucleoids, and plasmids
- Endospores – this consists of all bacteria cells components with dipicolinate and some special envelope components
The structures listed above are not present in all types of bacteria. Some bacteria cells have some structures that are not available in others. The types of bacteria with their associated structures would be mentioned when discussing each structure below.
Bacterial Cell Envelope
The envelope of the bacterial cell comprises of complex structures that vary among different major groups of bacteria. The bacterial cell envelope carries out many cellular
processes such as protecting the bacterial cell against chemical and biological attacks in its environment while also aiding the colonization of surfaces by bacteria.
Bacterial Capsule or Slime layer
Many bacteria cells are surrounded by a thick layer made up of polysaccharides, polypeptides, or proteins that are hydrophilic in nature and gel-like. This layer is called the Capsule when it is thick and firmly bound to the cell but referred to as a Slime layer when it is loosely bound and thinner. Capsule-containing bacteria give rise to smooth, mucus-like
colonies when cultured compared to the unencapsulated bacteria that have nonmucoid colonies that are rough.
Bacterial capsules play protective roles by limiting their ingestion by white blood cells (leukocytes). The capsules of bacteria also aid colonization by enhancing the attachment of bacteria cells to surfaces.
Bacteria Cell Wall
The cell wall of bacteria is located at the inner side of the capsule. It is usually rigid and surrounds all eubacterial cells. Most bacteria have a cell wall; there are some that have no cell walls such as mycoplasmas bacteria and Chlamydia.
Functions of the bacteria cell wall
- Bacteria cell wall prevents osmotic lysis
- The cell wall helps to give shape to the bacteria
- It protects against toxins and phagocytosis
- The bacteria cell wall also helps in colonization
- Bacteria cell walls are used to group bacteria into Gram-positive and Gram-negative bacteria
The shape of bacteria is determined by its cell wall. The bacterial cell protects it any disruption caused by mechanical forces. It also protects the bacterial cell from bursting as a result of turgor pressure caused by osmosis; this occurs when the bacteria is in a hypotonic environment.
The bacterial cell wall also serves as a barrier that prevents some toxic chemical and biological agents. Some species of bacteria such as Streptococcus are protected from being eaten up (phagocytose) with the help of their cell walls.
Bacterial binding to the cell wall of the host is enabled with the help of its cell wall.
Types of Bacteria Cell walls
The cell walls of bacteria are used for grouping bacteria into two major types. Some chemicals are first added to the bacteria-containing medium. After that, they are again stained using a certain dye and then viewed with a light microscope. This procedure is called Gram staining. When the bacteria retain the color of the stain, it is called Gram-negative bacteria; when the bacteria fail to retain the color of the stain (becomes violet) it is called Gram-positive bacteria.
The gram-Positive Cell wall of Bacteria
Bacterial cell wall that is gram-positive contains peptidoglycan and teichoic
acids with some species having additional carbohydrates and proteins. The murein component is what gives shape to the gram-positive bacterial cell wall; it also helps the bacteria cells to resist osmotic pressure.
The Teichoic acids help to anchor the bacteria cell wall to the cell membrane and have antigenic properties.
The gram- Negative Cell wall of Bacteria
Cell walls of Gram-negative bacteria also have murein, but very few, together with an additional membrane known as the outer membrane that is different from the cell membrane. There is an intermembrane structure known as the periplasm in the gram-negative bacteria cell wall; it lies between the cell membrane and the outer membrane and holds digestive enzymes, protective enzymes, and some proteins that are important in transport and chemotaxis.
The outer surface layer of the gram-negative bacteria cell wall has a strong negative charge that helps bacteria to evade phagocytosis and also prevents complement proteins from destroying the bacteria cells.
Bacteria Cell membrane
The cell membrane of bacteria has similarities as that of eukaryotic cells in the sense that, they both contain phospholipids and proteins. The differences between eukaryotic cell membranes and that of bacteria are many.
The bacterial cell membrane is highly rich in proteins and these proteins form about 70% of the weight of the bacteria. With the exception of the mycoplasma, the cell wall of bacteria does not contain sterols. The bacterial chromosome is attached to the cell membrane and carries out the role of the segregation of daughter chromosomes during cell division.
The cell membrane is also the site of synthesis of DNA, membrane lipids, and cell wall polymers. It contains the entire electron trans-
port system of the cell (and, hence, is functionally analogous to the mitochondria of
eukaryotes). Other functions of bacterial cell membranes include the secretion of exoproteins to the exterior.
Some bacterial cells have long hair-like projections on the surface of the bacterial envelope. These projections are called Flagella. They help in the motility of the bacteria cells. When the flagella are arranged around the cell, this type of distribution is called peritrichous flagella (derived from the Greek word trichos for “hair”). If the flagella occur only at one pole, they are called polar or monotrichous Flagella; when they occur at both ends of the cell, they are called lophotrichous flagella.
These helical flagella rotate at the point of their insertion in the envelope of the bacterial cell in order to propel the bacterial cell.
The Pili are similar to flagella but are shorter than flagella. Pili are hair-like projections made up of molecules of proteins known as pilin that are arranged to form a tubular hollow structure. There are two major types of pili: common pili (or the fimbriae)and sex pili. The common pili help the bacterial cells to attach and colonize surfaces. The sex pilus is only found in male bacteria and plays a major role in the exchange of genetic material between some Gram-negative bacteria. There is only one sex pilus per bacterial cell.
Bacterial Cell Cytosol
The bacterial cell contains an area bounded by the cell membrane in which are many particles including densely packed ribosomes that make it appear granular. These numerous ribosomes are needed because of the higher growth rate of bacteria cells; the more the growth rate, the higher the number of ribosomes in the cytosol.
The cytosol plays a major role in a bacterial cell because most of the metabolic reactions of the cell take place in the cytosol. Some species of bacteria contain nutritional storage granules known as reserve granules in their cytosol.
Nucleoid of Bacterial cells
The genetic materials of bacteria cells are stored in a single chromosome with a few exceptions. These genetic materials consist of about 4000 genes encoded in one molecule of double-stranded DNA that is large and circular; containing about 5 million nucleotide base pairs. This genetic material is not surrounded by a membrane, hence, it is not proper to call it a nucleus but rather called a nucleoid or nuclear body. Each nuclear body corresponds to a DNA molecule. The more the growth rate of bacteria cells, the more the number of nuclear bodies; resting cells have a single nuclear body while rapidly growing cells could have up to four nuclear bodies.
Plasmids of Bacteria cells
Bacterial plasmids are small, often circular, and covalently closed, double-stranded DNA
molecules that are separate from the chromosome. There could be more than one type of plasmid in a bacterial cell or many copies of the same plasmid may be found in the cell. Plasmids carry genes that code for the production of enzymes that protect bacterial cells from toxic substances. The resistance to antibiotics is caused by plasmids in the bacteria cells. The genes in plasmids are also responsible for the production of some pili and some exotoxins.
Bacterial Spores or Endospores are produced by bacterial cells in times of nutrient lack; for survival. These small and dehydrated structures are the metabolically inactive forms of bacteria cells produced in the presence of nutrient limitation.
A bacterial endospore is not a reproductive structure but rather an inactive form of the bacteria produced through the process of sporulation to overcome a harsh environment or condition pending when a suitable environment or condition occurs; in suitable conditions, an endospore then gives rise to a bacterium through a process known as germination. Some spores may persist for centuries and can still give rise to bacteria cells in the right environment or condition.
Bacterial endospores layers
The endospores consist of multiple protective layers, that help the bacteria withstand extremes of heat or boiling, cold, drying, or chemical agents. These multiple layers consist of:
- The first cell membrane
- A thick layer of peptidoglycan mesh
- Another cell membrane
- A wall of keratin-like protein
- An outer layer called the exosporium