Bacteria, microscopic & unicellular organisms, are often coccus- (spherical) or rod-shaped and 0.5-5 µm in the longest dimension, although the wide diversity of bacteria can display a huge variety of morphologies. The study of bacteria is known as bacteriology, a branch of microbiology.
Bacteria are ubiquitous in the environment, living in every possible habitat on the planet including soil, underwater, deep in the earth’s crust, and even such environments as sulfuric acid and nuclear waste. There are typically ten billion bacterial cells in a gram of soil, and one hundred thousand bacterial cells in a millilitre of sea water. Bacteria play an important role in the cycling of nutrients in the environment. They play many important steps in the nutrient cycle as catalysts, such as the fixation of nitrogen from the atmosphere.
There are more bacterial cells on each of our bodies than there are cells of our own and bacteria are a natural component of the human body, particularly on the skin and in the mouth and intestinal tract. Bacteria are important to human health, as they are the causative agent of many infectious diseases, including cholera and tuberculosis. Historically, bacteria have been responsible for such diseases as bubonic plague and leprosy, but after the discovery of antibiotics many bacterial diseases are able to be controlled. Bacteria are also important to numerous industrial processes, such as wastewater treatment and more recently the industrial production of antibiotics
and other chemicals.
The first bacteria were observed by Anton van Leeuwenhoek in 1674 using a single-lens microscope of his own design. The name bacterium was introduced much later, by Ehrenberg in 1828, derived from the Greek word meaning “small stick”.
Cell Morphology and Arrangement of bacterial cells
Bacterial cells are typically 0.5-5 ìm in length, however some species, for example Thiomargarita namibiensis and Epulopiscium fishelsoni, may be up to 500 µm (0.5 mm) long and are visible to the unaided eye. Among the smallest bacteria are members of the genus Mycoplasma which measure just 0.2 µm; approximately the same size as the largest viruses.
Most bacteria are either spherical, called coccus or rod-shaped, called bacillus (pl. baccili, from Latin baculus, stick)
Some rod-shaped bacteria, called vibrio, are slightly curved or comma-shaped, while others, called spirilla, form twisted spirals. Many bacterial species exist simply as single cells, while others tend to associate in diploids / pairs (for example Neisseria), or chains (such as Streptococcus), while members of the genus Staphylococcus, form a “bunch of grapes” clusters. Bacteria can also be elongated to form filaments, for example the Actinomycetes.
The bacterial cell is bound by a lipid membrane, or plasma membrane, which encompasses the contents of the cell, or cytoplasm, and acts as a barrier that holds nutrients, proteins and other essential molecules within the cell. Bacteria do not have membrane-bound organelles in the cytoplasm and thus contain few intracellular structures. They lack mitochondria, chloroplasts and the other organelles present in eukaryotic cells, such as
the golgi apparatus and endoplasmic reticulum. Bacteria do not have a membrane-bound
Bacteria do not have a membrane-bound nucleus and their genetic material is typically
a single chromosome located in the cytoplasm as an irregularly-shaped body called the nucleoid. The nucleoid consists mainly of the chromosome but has also associated proteins
and RNA. Like all living organisms bacteria contain ribosomes for the production of proteins.
External to the cell membrane is the bacterial cell wall. Bacterial cell walls are composed of peptidoglycan, different from the cell walls of plants and fungi which have cell walls of cellulose and chitin, respectively. The cell wall is essential to the survival of bacteria; the antibiotic penicillin is able to effectively kill bacteria by inhibiting a step in the synthesis
of peptidoglycan and stopping the production of the cell wall.
Flagella are rigid protein structures, about 20 nm in diameter and up to 20 µm in length, that are used for motility.
Some bacteria also produce nutrient storage granules, such as glycogen, polyphosphate, sulphur or polyhydro- xyalkanoates. These storage compounds enable bacteria to store compounds for later use. Certain bacterial species, such as the photosynthetic Cyanobacteria, produce internal gas vesicles which they use to regulate their buoyancy to
regulate the optimal light intensity or nutrient levels.
Bacteria serve as a link between plants and animals. They are now regarded as the simplest plants because of the following reasons:
- They have a rigid cell wall made up of mucopolysacchrides.
- Some of the bacteria like chlorobium are autotrophic, like green plants.
- Some forms of bacteria are capable of sythesizing vitamins just like green plants.
- Bacteria take in their nutrition in the form of solution only.
Bacteria differ from the eukaryotic cells of other plants in the following respects:
- The mitochondria and endoplasmic reticulum are lacking in the cytoplasm.
- The nuclear body does not divide by mitosis.
- The photosynthetic lamellae when present are not organised into chloroplast.
- No protoplasmic streaming movements seen.
- Contains no vacuoles.
Respiration in Bacteria
Most bacteria make use of the free oxygen of the atmosphere or oxygen dissolved in the liquid environement, they are called aerobes or aerobic bactria. Free oxygen is necessary for their respiration. There are many bacteria which are able to live and multiply in the
absence of free oxygen. They obtain oxygen from oxygen containing compounds such as sugar. They are called the anaerobes or anaerobic bacteria, but the amount of energy available from anaerobic respiration is much less than that from aerobic respiration, Syphilis and Tetanus bacteria are e.g. of obligate anaerobes. There are some bacteria which
can live and grow whether oxygen is present or not. They are called facultative anaerobes.
Growth and Reproduction
All bacteria reproduce through asexual reproductio (one parent) binary fission, which results in cell division. Two identical clone daughter cells are produced. Some bacteria, while still reproducing asexually, form more complex reproductive structures that facilitate the dispersal of the newly-formed daughter cells.
Bacteria, as asexual organisms, inherit an identical copy of their parent’s genes (i.e. are clonal). All bacteria, however, have the ability to evolve through selection on changes to
their genetic material (DNA) which arise either through mutation or genetic recombination. Mutation occurs as a result of errors made during the replication of DNA. The most frequent genetic changes in bacterial genomes come from random mutation. Some bacteria can also
undergo genetic recombination. This can occur when bacteria take-up exogenous environmental DNA from closely related genera in a process called transformation. In the process of transduction, a virus can alter the DNA of a bacterium by becoming lysogenic and introducing foreign DNA into the host chromosome, which can then be transcribed and
Because of their ability to quickly grow, and the relative ease with which they can be manipulated, bacteria have historically been the workhorses for the fields of molecular biology, genetics and biochemistry. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists have been able to determine the function of many different genes and enzymes.
Benefits and Dangers Bacteria are both harmful and useful to the environment
Bacteria are both harmful and useful to the environment and animals, including humans. Some bacteria act as pathogens and cause tetanus, typhoid fever, pneumonia, syphilis, cholera, food-borne illness, leprosy, and tuberculosis(TB). Sepsis, a systemic infectious syndrome characterized by shock and massive vasodilation, or localized infection, can be caused by bacteria such as Streptococcus, Staphylococcus bacteria. Some bacterial infections can spread throughout the host’s body and become systemic. In plants, bacteria cause leaf spot, fireblight, and wilts. The mode of infection includes contact, air, food, water, and insect-borne microorganisms. The hosts infected with the pathogens may be treated with antibiotics, which can be classified as bacteriocidal and bacteriostatic, which at
concentrations that can be reached in bodily fluids either kill bacteria or hamper their growth, respectively. Antiseptic measures may be taken to prevent infection by bacteria, for example, by swabbing skin with alcohol prior to piercing the skin with the needle of a syringe. Sterilization of surgical and dental instruments is done to make them sterile or pathogen-free to prevent contamination and infection by bacteria. Sanitizers and disinfectants are used to kill bacteria or other pathogens to prevent contamination and risk of infection.
In soil, microorganisms which reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) help in the transformation of molecular dinitrogen gas as their source of nitrogen, converting it to nitrogenous compounds in a process known as nitrogen fixation.
This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of the gut flora in the large intestine can help prevent the growth of potentially harmful microbes. The ability of bacteria to degrade a variety of organic compounds is remarkable. Highly specialized groups of microorganisms play important roles in the mineralization of specific classes of organic compounds.
For example, the decomposition of cellulose, which is one of the most abundant constituents of plant tissues, is mainly brought about by aerobic bacteria that belong to the genus Cytophaga. This ability has also been utilized by humans in industry, waste processing, and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills.
Bacteria, often in combination with yeasts and molds, are used in the preparation of fermented foods such as cheese, pickles, soya sauce, sauerkraut, vinegar, wine, and yogurt. Using biotechnology techniques, bacteria can be bioengineered for the production of therapeutic drugs, such as insulin, or for the bioremediation of toxic wastes.
“Friendly bacteria” is a term used to refer to those bacteria that offer some benefit to human hosts, such as Lactobacillus species, which convert milk protein to lactic acid in the gut. The presence of such bacterial colonies also inhibits the growth of potentially pathogenic genic bacteria (usually through competitive exclusion). Other bacteria that are helpful inside the body are many strains of E. coli, which are harmless in healthy individuals and provide Vitamin K.