What is bacteria? Bacteria is an essential part of the environment. They are considered as the first life forms to appear on Earth. Biologists estimated that the first forms of life appeared on Earth about 4 billion years ago, which was a unicellular microorganism, the modern name is bacteria.

It is estimated that one gram of soil can contain approximately  40 million bacterial cells whereas one milliliter of fresh water can contain 1 million bacterial cells. The total number of bacterial cells on earth is approximately 5×10^30.

What is bacteria?

• Bacteria are referred to as microscopic, single-celled organisms which exist in their millions, in every environment, both inside and outside other organisms.
• In 1676, a Dutch microscopist Antonie van Leeuwenhoek first observed a bacterial cell by using a single-lens microscope of his own design. 
• In 1828, Christian Gottfried Ehrenberg first introduced the term “bacterium”
• Bacteria belong to the prokaryotic domain.
• They have various shapes such as spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios), or corkscrew (spirochaetes).
• They can be found in soil, water, acidic hot springs, radioactive waste, and the deep biosphere of the earth’s crust.
• Bacteria can be found in both symbiotic and parasitic relationships with plants and animals.
• The biology where we study about bacteria is called bacteriology, which is a branch of microbiology.
• All bacterial species are not harmful some of them are beneficial, for example, they help in the synthesis of vitamin B12 and also play an important role in the fixation of nitrogen from the atmosphere.
• In the animal body, the largest number of bacterial cells can be found in the gut and skin.

Classification of Bacteria Types

Bacteria are classified into three groups based on their shape such as;

1. Coccus: These are spherical or round in shape. Example: streptococcus group, responsible for “strep throat.”
2. Bacillus: These appear as rod shapes. Example: Bacillus anthracis (B. anthracis), or anthrax.
3. Spiral: These are curved or spiral shapes. Example:  Leptospirosis, Lyme disease, and syphilis are caused by bacteria of this shape.

Follow this article for more detail study: Size, Shape, and Arrangement of Bacterial Cells With Picture and Example

Bacterial Cell can be classified based on their Cell Wall Nature Such as;

1. Gram-Positive bacteria, contain thick cell walls.
2. Gram-negative bacteria, contain thin cell walls as compared to gram-positives.

Classification of bacteria based on the Mode of Nutrition

Type of Classification	Examples
Autotrophic Bacteria	Cyanobacteria
Heterotrophic Bacteria	All disease-causing bacteria

Classification of bacteria based on the Mode of Respiration

Type of Classification	Examples
Anaerobic Bacteria	Actinomyces
Aerobic Bacteria	Mycobacterium

Bacteria Morphology

• They appear in various sizes and shapes.
• Their size is one-tenth of eukaryotic cells and approximately 0.5–5.0 micrometres in length.
• The largest bacteria is Thiomargarita namibiensis is up to half a millimetre long whereas the Epulopiscium fishelsoni is 0.7 mm.)
• The smallest bacteria is Mycoplasma genitalium which is approximately 200 to 300 nm in size.
• Bacterial cells appear in three shapes such as curved or spiral-shaped (spirilla), spherical or round shape (cocci), and rod shape (bacilli).
• Tightly coiled spiral-shaped bacteria is known as the spirochaetes.
• Some bacteria appear in unusual shapes such as star-shaped bacteria.
• Variation also appears in the arrangement of bacterial cells. There are five types of bacterial cell arrangement such as 
  • Diplo: Bacterial cells remain in pairs after division.
  • Strepto: Bacterial cells remain in chains after division.
  • Tetrad:  Bacterial cells remain in groups of four and divide in two planes.
  • Sarcinae: Bacterial cells remain in groups of eight and divide in three planes.
  • Staphylo: Bacterial cells remain in clusters and divide in multiple planes.
• They formed fruiting bodies in certain conditions.
• They sometimes attach to surfaces and form dense aggregations called biofilms.

Bacteria structure

Intracellular structures

• They contain a cell membrane which is made of phospholipids. This membrane is surrounding the bacterial cell and acts as a barrier. It protects the other essential components of cytoplasm.
• They lack membrane-bounded organelles such as the nucleus, mitochondria, chloroplasts, etc.
• They contain a cytoskeleton that maintains the proteins and nucleic acid movements within the cell and also helps in cell division.
• Their genetic material is a single circular bacterial chromosome of DNA that is located in the cytoplasm and formed an irregularly shaped nucleoid.
• They contain ribosomes where they produce their protein. The ribosome of a bacterial cell is made of RNA-rich granules.
• Some bacteria cells produce glycogen, polyphosphate, sulfur or polyhydroxyalkanoates which are known as the intracellular nutrient storage granules.
• Some bacterial cells produce internal gas vacuoles such as photosynthetic cyanobacteria. They use this to control their buoyancy.

Extracellular structures

• They contain cell walls outside the cell membrane which are made of peptidoglycan.  Peptidoglycan is composed of polysaccharide chains cross-linked by peptides containing D-amino acids.
• Some bacterial cells contain a layer outside the cell wall known as Capsule.
• In gram-positive bacteria, the cell wall is made of many layers of peptidoglycan and teichoic acids. Whereas the gram-negative cell wall is made of few peptidoglycan layers which is surrounded by a second lipid membrane made of lipopolysaccharides and lipoproteins. 
• Bacteria cells contain a rigid protein structure called Flagella which is20 nanometres in diameter and up to 20 micrometers in length. This flagellum helps in the locomotion of bacterial cells.
• The bacterial cell contains a hair-like structure over the cell surface called Fimbriae, which is made of fine filaments of protein. The Fimbriae is 2–10 nanometres in diameter and up to several micrometers in length. It helps in the attachment to solid surfaces or to other cells that is why they are also called  “attachment pili”.
• The bacterial cell also contains pili which are slightly larger than the fimbriae it helps in transfer of genetic material between two bacterial cells during the conjugation process. The type IV pili generate movement.

Structure of Bacteria in detail

Capsule

• Some bacteria species have a third protective cover, a capsule that is made up of complex carbohydrates (polysaccharides). 
• The most important roles of capsules are to prevent the bacterium’s drying out and protect it against engulfing by larger microorganisms. 
• The capsule is a key virulence factor for major disease-causing bacteria such as Escherichiacoli and Streptococcus pneumoides. 
• These organisms can be found in non-encapsulated variants, which are also virulent. They don’t cause any disease.

Cell Envelope

• The cell envelope consists of two to three layers.
• These are the cell wall, the interior cytoplasmic membrane and the outer capsule.

Cell Wall

• A rigid cell wall is a structure made of peptidoglycan (a protein-sugar molecule) and surrounding the bacterium. 
• The cell’s shape is given by the wall, which surrounds the cytoplasmic plasma membrane and protects it from the outside world. 
• The wall also serves to anchor any appendages, such as the flagella and pili, that originate in the cytoplasm membrane.
• They protrude through it to the outside. 
• Cell wall strength is crucial for keeping cells from burst when the osmotic pressure in the environment is high. 
• A thick mesh-like structure, which allows you to distinguish between two types of bacteria, is an example. Hans Christian Gram, a Danish physician, devised a method to distinguish between the two types of bacteria. He used stains and washing techniques. Gram-positive bacteria retains the purple stain color when exposed to a “gram stain”. This is due to the structure of their cells that trap the dye. Gram-negative bacteria’s cell walls are thin and release the dye easily when they are washed with alcohol or acetone.

Cytoplasm

• This is the cellular area responsible for cell growth, metabolism and replication. 
• It is a gel-like structure made up of water, enzymes and nutrients.
• Also, it contains structures such as ribosomes and chromosomes. 
• The cell envelope contains the cytoplasm as well as all its components. 
• The bacteria does not have a membrane-enclosed nucleus, unlike the true eukaryotic cells. 
• The nucleoid, which is a single, continuous DNA strand, contains the chromosome. 
• The cytoplasm contains all other components of the cell.

Plasmids

• One component, plasmids are the small extrachromosomal genetic structures that can be carried by many bacteria strains. 
• A circular piece of DNA is what makes plasmids, which are similar to the chromosome.
• They are not involved with reproduction, unlike the chromosome. 
• Only the chromosome contains the genetic instructions to initiate and carry out cell division or binary fission. This is the primary method of reproduction in bacteria. 
• Although not necessary for survival, plasmids can replicate independently from the chromosome, giving bacteria a distinct advantage.
• Plasmids can be passed to bacteria by two methods. Binary fission is the most common way that plasmids are passed to other bacteria. Some plasmids form a tube-like structure on the surface, called a pilus. This allows for the passing of copies of the plasmid onto other bacteria through conjugation, which is a process in which bacteria exchange genetic information. 
• Plasmids are instrumental in transmitting special properties such as resistance to antibiotics and heavy metals, or virulence factors that allow for the infection of animals and plants. 
• They are extremely useful tools in molecular biology and genetics.

Cytoplasmic Membrane

• A membrane of phospholipids, proteins and other substances called the cytoplasmic Membrane encloses the bacterium’s interior and regulates the flow of materials into and out of it. 
• This barrier is a structural characteristic that bacteria share with all living cells.
• It allows them to interact with their environment in a selective way. 
• Membranes are highly symmetrical and organized, with two sides.
• Each side has a different surface and functions. 
• Membranes can also be dynamic and adapt to changing conditions.

Flagella

• Flagella (singular flagellum) is a hair-like structure that provides locomotion to bacteria that has them. 
• These structures can be found on either one or both ends or all over a bacterium’s surface. To help the bacterium move towards nutrients, away from toxic chemicals and toward the light (in the case of photosynthetic Cyanobacteria), the flagella moves in a propeller-like fashion.

Nucleoid

• The nucleoid refers to the area of the cytoplasm in which the chromosomal genetic DNA is found. 
• It is not a membrane-bound nucleus but merely an area in the cytoplasm that contains the DNA strands. 
• Although most bacteria only have one circular chromosome responsible for replication, some species have more. 
• The cytoplasm also contains smaller circular auxiliary DNA strands called plasmids.

Pili

• Many bacteria species have pili, which are small hair-like projections that emerge from the cell’s outside surface. 
• These outgrowths aid bacteria in attaching to other cells or surfaces such as teeth, intestines and rocks. 
• Many disease-causing bacteria can’t attach to host tissues without pili. 
• Conjugation is a process in which two bacteria share fragments of their plasmid DNA.

Ribosomes 

• Ribosomes, which are microscopic “factories”, can be found in all cells, even bacteria. 
• They convert the genetic code of a cell from the molecular language that is nucleic acids to the one of amino acids, which are the building blocks of protein. 
• Proteins are the molecules responsible for all functions in cells and living organisms. 
• Although ribosomes from bacteria are very similar to those found in eukaryotes they are much smaller and have a slightly modified molecular structure and composition. 
• Bacterial Ribosomes don’t attach to other organelles like they are in eukaryotes (bound to endoplasmic retinalum). They are independent structures that are distributed throughout the cytoplasm. 
• There are enough differences between bacterial and eukaryotic Ribosomes to warrant that antibiotics can be used to inhibit the functioning of bacterial Ribosomes but not those of eukaryotes. This will kill bacteria, but not the eukaryotic organisms they infect.

Nutrition of Bacteria

Bacteria get their nutrition in various ways such as; some bacteria get energy from light using photosynthesis known as phototrophy whereas a few bacterial cells get energy from chemical compounds by oxidizing them known as chemotrophic.

• Phototrophic bacteria or phototrophs get their energy from Sunlight, carbon from Organic compounds (photoheterotrophs) or carbon fixation (photoautotrophs). Some examples of phototrophs are Cyanobacteria, Green sulfur bacteria, Chloroflexi, or Purple bacteria.
• Lithotrophs get their energy from Inorganic compounds, carbon from  Organic compounds (lithoheterotrophs) or carbon fixation (lithoautotrophs). Examples of Lithotrophs are Thermodesulfobacteria, Hydrogenophilaceae, or Nitrospirae.
• Organotrophs get their energy from  Organic compounds (chemoheterotrophs) or carbon fixation (chemoautotrophs). Examples of Lithotrophs are Bacillus, Clostridium or Enterobacteriaceae.

Growth Factors

Bacterial cells required Growth factors for the biosynthesis. Bacterial cells required these following growth factors such as;

1. Purines and Pyrimidines: Purines and Pyrimidines are needed for the synthesis of DNA and RNA
2. Amino acids:  Bacterial cells require Amino acids for the synthesis of proteins.
3. Vitamins: Bacterial cells needed vitamins as coenzymes and functional groups of certain enzymes.

Habitat of Bacteria

• Bacteria can be found in or in the stratosphere which is between 6 and 30 miles up in the atmosphere.
• They can be found in ocean depths, down to 32,800 feet or 10,000 meters deep.
• Aerobic bacteria grow where oxygen is present such as soil, water, plants, animals, etc.
• Anaerobic bacteria grow where oxygen is absent such as the gastrointestinal tract.
• Facultative anaerobes, or facultative anaerobic bacteria, can thrive with or without oxygen, but most of them prefer environments where there is oxygen.
• Mesophiles or mesophilic bacteria live in moderate temperatures, around 37°C. These bacteria are responsible for human infections. They can be found in the human body.
• Extremophiles or extremophilic bacteria can be found in extreme environments such as hot springs, deep in the ocean, etc. Some example of Extremophiles are;
  • Thermophiles: Can thrive up to 75 to 80°C.
  • Hyperthermophiles: Can thrive up to 113°C.
  • Halophiles: Live in a salty environment.
  • Acidophiles: Found in environments as acidic as pH 0.
  • Alkaliphiles: can be found in alkaline environments up to pH 10.5.
  • Psychrophiles: Can live in cold temperatures, for example, in glaciers.

Growth and Reproduction of Bacteria

Bacteria can be reproduced by these following methods;

1. Binary fission

It is an asexual method, where the bacterial cell grows and then form a new cell wall between the center and divides into two identical daughter cell. Both of these two daughter cells contain the same genetic material.

2. Through the transfer of genetic material

Some Bacterial cells can reproduce by transferring genetic material through the processes of conjugation, transformation, or transduction

3. Budding

A group of bacteria follow this method for reproduction. In this method a bud or outer growth occurs at one end of the mother cell. Then the size of buds constantly increases, when the bud is about the same size as the parent cell, it separates from the parent cell. Then it grows and forms a fully formed bacterial cell. 

4. Spores:

In certain conditions when bacterial cells lack nutrients or resources they form spores and remain in a dormant stage for centuries until the right conditions occur. When they found an optimum environment they reactivated and formed bacteria.

Spores can thrive in different environmental stress such as ultraviolet (UV) and gamma radiation, desiccation, starvation, chemical exposure, and extremes of temperature.

Movement of Bacteria

• Bacteria show motility when they are attracted or repelled by certain stimuli such as chemotaxis, phototaxis, energy taxis, and magnetotaxis.
• Bacteria use different types of mechanisms for their motility. Most of these bacteria use  a long filament known as flagella for their movement.
• Flagella is attached with a motor at the base which rotates the flagella to generate propeller-like movement. Motor uses the electrochemical gradient across the membrane for power to rotate the flagella.
• Flagellum is composed of about 20 proteins, with another 30 proteins which is required for its regulation and assembly.
• Bacteria shows two types of movement: forward movement (swimming) and tumbling. In tumbling they reorient and create a three-dimensional random walk.
• Based on the number and arrangement of flagella bacteria is classified in different groups such as;
  • Monotrichous: Contain single flagella.
  • Amphitrichous:  a flagellum at each end of a bacterial cell.
  • Lophotriccus: contain clusters of flagella at the poles of the bacterial cell.
  • Peritrichous: They have flagella distributed over the entire surface of the cell.
• The second type of bacterial motion is called twitching motility which depends on type IV pilus, and gliding motility. In this motility the rod-like pilus extends out from the cell, binds some substrate, and then retracts, pulling the cell forward.
• Some bacterial species show motion inside the host cells by usurping the cytoskeleton such as Listeria and Shigella. This method is normally used by organelles to move inside the cell. 

Communication of Bacteria

• A few groups of bacteria generate light known as bioluminescence. They Often lived with fish, and generated light to attract fish or other large animals.
• Often bacterial cells generate multicellular aggregates known as biofilms, which exchange different types of molecular signals for inter-cell communication.
• Another type of intercellular communication is quorum sensing, it allows bacteria to coordinate gene expression, and also allows them to produce, release and detect autoinducers or pheromones.

Useful Bacteria

Some bacteria can be harmful, but not all. Some bacteria are beneficial in other ways. Below are some benefits of bacteria.

1. Convert milk to curd – Lactobacillus and lactic acid bacteria
2. Ferment foods products – Streptococcus & Bacillus
3. Actinobacteria and Bacteroidetes help digestion and improve the body’s immune system. Firmicutes, Proteobacteria
4. Production of antibiotics that are used in the prevention and treatment of bacterial infections. Soil bacteria

Harmful Bacteria

Many illnesses can be caused by bacteria. Many of the infections they cause include pneumonia, diphtheria and syphilis as well as tooth decay. You can treat their symptoms with antibiotics or prescribed medication.

Precaution is better than cure. These bacteria can most often be eradicated by disinfecting and sterilizing surfaces, instruments, tools, and other utility equipment. These include heat, pasteurization and boiling as well as UV radiations, UV radiations, heat, and disinfectants.

Application of Bacteria

• Many bacteria play an important role in the digestive system by breaking down nutrients, such as complex sugars, into forms the body can use.
• Some bacteria prevent pathogenic bacteria by competing with them for nutrient and attachment sites.
• Many bacteria help in atmospheric nitrogen fixation, so that plants can easily take up the nitrogen from soil.
• Bacteria is also used in food industries for the production of cheese, soy sauce, natto (fermented soy beans), vinegar, yogurt, and pickles.
• In industries they are used for production of amylases, proteases (thermolysin, subtilisin, aqualysin), glucose isomerase, beta-galactosidase, vinegar, cobalamin (cyanocobalamin), monosodium glutamate, dextran, etc.
• In medicine industries they are used for production of different types of antibiotics.

Bacteria Examples

• Escherichia coli 
• Lactobacillus acidophilus 
• Clostridium botulinum
• Staph aureus
• S. epidermidis
• S. saphrophyticus
• S. haemolyticus
• S. hominis
• S. capitis
• S. schleiferi
• S. warneri
• S. lugdenenis
• Strep pyrogenes (gr. A)
• C. diptheriae
• C. jeikenium
• C. urealyticum
• Lactobacillus sp.
• Bacillus anthracis
• B. cereus
• Listeria monocytogenes
• Erisipelothrix rhusiopathiae
• Arcanobacterium bemolyticum
• Escherichia coli
• Klebsiella pneumoniae

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