Bacteria Cell

Bacteria are disease-causing, microscopic, single-celled organisms with prokaryotic cell structures. They do not have membrane-bound organelles, including a true nucleus. Being the lowest and simplest form of life, they are found almost everywhere on earth and thus, are the most dominant living creature. Bacteria are thought to have been the first living organism to appear on earth about 4 billion years ago.

Dutch scientist Antonie van Leeuwenhoek first observed bacteria in 1676, using an ordinary microscope. In 1838, the German naturalist Christian Gottfried Ehrenberg called them ‘bacteria’, from the Greek word ‘baktḗria’, meaning ‘little stick’. Later, Robert Koch’s research, famously dubbed ‘Koch’s postulates’, demonstrated that microorganisms such as bacteria cause infectious diseases.

Traditionally, all prokaryotic cells were called bacteria and were classified under Kingdom Monera. Later in the late 1970s, American microbiologist Carl Woese pioneered a major change in their classification by placing bacteria under two separate domains, Bacteria and Archaea. While the domain Bacteria contains the group of microorganisms originally called Eubacteria, the other domain contains organisms originally termed Archaebacteria.

How Big is a Bacteria

Bacteria cells are typically 0.5–5.0 µm in length. Among the smallest bacteria are members of the genus Mycoplasma, which measure only 0.2-0.3 µm, while a few others are so big that they are visible even to the naked eye. For example, Thiomargarita namibiensis is the largest and longest bacteria with a diameter of 100–300 µm (0.1–0.3 mm).

Shapes of Bacteria

Based on their shape and arrangement, bacteria can be classified into the following four main types:

• Cocci: spherical, round or oval-shaped, example – Staphylococcus group
• Bacilli: rod-like or cylindrical shaped, example – Bacillus group,
• Spirilla: spiral or coil-shaped, example – Spirillum group
• Vibrios: curved or comma-shaped, example – Vibrio group

Parts and Their Morphology

1. Capsule: Made up of complex polysaccharides, it forms the outermost covering of the bacterial cell. Capsules are an important structural component that keeps the bacterium from drying out, also protecting them from being engulfed by larger microbes. It provides the bacterial cell the ability to cause disease.
2. Cell Wall: It is a rigid covering composed of peptidoglycan, a polymer of protein and sugar molecules that provides shape and structural support to the cell. Based on their staining properties and composition of cell wall, bacteria can be classified into gram-positive and gram-negative group.
3. Cytoplasmic Membrane: It is a semi-permeable membrane composed of lipids and proteins that separates the inside of the cell from its external environment. The cytoplasmic membrane carries out many necessary cellular functions such as energy generation, protein secretion, cell division, and transport of nutrients across the cell.
4. Cytoplasm: Present below the cytoplasmic membrane and distributed throughout the cell, it is a gel-like matrix mainly composed of water along with dissolved salts and minerals. Along with some cellular parts and components such as nucleoid, ribosomes, and inclusion bodies, the cytoplasm of a bacterial cell also contains a circular (or sometimes linear) auxiliary DNA molecule called a plasmid. This extra-chromosomal piece of DNA participates in cell division and confers the bacterial cell with properties such as antibiotic resistance and the ability to infect other cells known as pathogenicity.
5. Nucleoid: It is the region where the single, circular DNA of bacteria is found. The bacterial DNA usually ranges in size from 160,000 base pairs to 12,200,000 base pairs (12.2 Mbp). The nucleoid is not membrane-bound but is simply an area of the cytoplasm where the strands of DNA are located.
6. Ribosomes: They are granular structures that are found scattered unevenly throughout the cytoplasm, which helps bacteria to synthesize proteins.
7. Inclusion Bodies: They are non-living structures present in the cytoplasm. Based on their nature, the inclusion bodies are of three types— gas vacuoles, inorganic inclusions, and food reserve. While the gas vacuoles allow bacteria to float in waters, the inorganic inclusions serve to store certain hydrolytic enzymes capable of destroying the host cell.
8. Flagella: They are long hair-like structures attached to the cell surface that helps in bacterial movement. They can be found at either or both ends of a bacterial cell and sometimes all over its surface. The flagella beat in a propeller-like motion to help the bacterium move toward nutrients and light, as well as away from toxic chemicals. The presence of flagella, their number, and arrangement on their cell surface also forms a basis of classification in bacteria.
9. Pili: They are short hair-like projections emerging from outside the cell. These outgrowths assist the bacteria in attaching to other cells and surfaces, such as teeth, intestines, and rocks. Bacteria use specialized pili, called sex pili, for reproduction.

In nature, bacteria do not experience perfect environmental conditions for growth. Under optimal growth conditions in a laboratory, the pattern of bacterial growth can be represented in the form of a curve called the growth curve. It has the following four phases:

Lag Phase: This is the initial phase during which the cells acclimatize to the new environment. During this phase, the cells increase in size due to the accumulation of proteins and other molecules necessary for cell division, without any increase in cell number.

Log Phase: Also known as the exponential phase, this is when the metabolic activity of the cell is high, and the cell undergoes cell division to rapidly increase in number. 

Stationary Phase: Eventually, the population growth declines due to the depletion of available nutrients and the accumulation of waste products in culture. During this phase, bacterial cell growth stops and reaches a plateau since the number of dividing cells equals the number of dying cells.

Death Phase: As nutrients become less available and waste products start to accumulate, the number of dying cells continues to rise. In the death phase, the number of living cells decreases exponentially, and population growth experiences a sharp decline.

Being a diverse group, bacteria are also sometimes classified based on their growth parameters such as temperature, pressure, modes of nutrition, respiration, pH, and osmotic pressure.

Asexual Modes

a) Binary Fission: It is the most common mode of reproduction in bacteria. Under favorable conditions of growth, the parent cell grows in size and duplicates its genetic material, which then undergoes division to form two identical daughter cells. Bacteria can grow and divide extremely rapidly, and a fixed bacterial population doubles itself in just 9.8 minutes.

b) Budding: A small bud forms at one end of the mother cell, gradually growing in size while the mother cell remains the same. When the bud is about the same size as the mother cell, it separates to form an independent bacterial cell.

c) Sporulation: Under unfavorable conditions of growth such as adverse temperature or shortage of nutrients, some bacterial cells convert into spores and cysts by forming a thick-walled covering around the cell. This is basically an adaptation to survive the harsh environmental conditions. On return of favorable conditions, the spores’ returns back into reproductive cells.

Exchanging Genetic Material

Although bacteria do not have an obligate sexual reproductive stage in their life cycle, they can be very active in exchanging their genetic material, the DNA. It occurs by three different processes:

a) Conjugation: Transfer of DNA through direct contact between two bacterial cells. Conjugation occurs with the help of sex pili that forms a bridge between the two participating cells. 

b) Transformation: Bacteria cell taking up free fragments of DNA that are floating in the medium.

c) Transduction: Transfer of DNA from one bacterium to the other using viruses that are capable of infecting a bacterial cell.

• Soil bacteriahelping in the decay and decomposition of dead organic matter, thus contributing to environmental cleaning.
• Continuing the nitrogen cycle in nature by performing nitrogen fixation in the soil with the help of leguminous plants such as pea, beans and clover. Nitrogen fixation also increases soil fertility.
• Helping in biological pest control in place of using chemicals in agriculture.
• Helping in industrial fermentation that produces food products used commercially for human consumption such as bread, cheese, yogurt, vinegar, vitamins, alcohol, and many others.
• Producing antibiotics such as streptomycin, aureomycin, tetracycline, tyrothricin, subtilin, polymyxin B, and bacitracin that are used to cure human diseases.
• Preparing serum and vaccines that are commonly used to develop immunity against various diseases in humans.
• Helping in the tanning industry to prepare leather from animal hides.
• Helping in curing and ripening of tea and tobacco leaves, coffee, and cocoa beans to remove their bitterness.

Of course, all activities of bacteria are not beneficial. Some parasitic bacteria that live inside the body of another organism are found to cause diseases in plants and animals, including humans. Some bacteria act as the agent of food poisoning by releasing toxins, thus making the food unpalatable and can be a cause of food poisoning in humans.