Bacteria: Definition, Characteristics, Structures and Types of Pathogenic Bacteria

As such, the bacterial structure has developed in such a way that it allows the cell to perform its activity efficiently.

Bacteria are so small that their presence was first recognized in 1677 when the Dutch naturalist Antonie van Leeuwenhoek saw microscopic organisms in various substances.

These organisms he saw with the help of early microscopes (more similar in design to modern magnifying glasses than modern microscopes), some of which were capable of more than 200 magnifications.

Bacteria are now generally examined under light microscopes capable of magnification of more than 1000 times; however, the details of its internal structure can be observed only with the aid of much more powerful transmission electron microscopes.

Unless particular phase contrast microscopes are used, bacteria must be stained with a colored dye to make them stand out from their background.

Bacteria are single-celled, which means that they consist of a single cell.

Everything that bacteria need to survive and reproduce, except for food, is contained within this single cell.


Bacteria characteristics


As they need oxygen to carry out the respiratory process, Bacteria are classified as anaerobic; they do not demand oxygen, and aerobic if they do.


Regarding nutrition, bacteria are classified into:

  • Autotrophic: they synthesize their food.
  • Heterotrophic: they cannot synthesize their food and depend on organic matter for their synthesis.
  • Symbiotic: the food is obtained from a symbiotic relationship with other organisms.
  • Parasitic: the food is obtained from a parasitic relationship with other organisms and causes damage to the host.


Bacteria reproduce primarily through cell division and binary fission.

Some types of bacteria are also usually produced through budding.

Form and arrangement

Individual bacteria can assume three forms and are classified by shape into three primary groups: cocci, bacilli, and spirochetes.

Individual bacteria can assume one of three basic shapes: spherical (coccus), rod or rod (bacillus), or curved (vibrio, spirillum, or spirochete).

Considerable variation is observed in the actual shapes of bacteria, and cells can be stretched or compressed in one dimension.

Bacteria that do not separate from each other after cell division form distinct groups useful for identification.

For example, some cocci are found primarily in pairs, including Streptococcus pneumonia, pneumococcus that causes bacterial lobar pneumonia, and Neisseria gonorrhoeae, a gonococcus that causes the sexually transmitted disease gonorrhea.

Most streptococci resemble a long string of pearls, while staphylococci form random clusters (the name “staphylococci” is derived from the Greek word staphyloma, meaning “a bunch of grapes”).

Also, some coccal bacteria appear as square or cubic packages.

Rod-shaped bacilli usually occur singly, but some strains form long chains, like rods of corynebacteria, normal mouth inhabitants that frequently attach at random angles.

Some bacilli have pointed ends, others have squares, and some rods bend in a comma shape.

These bent rods are often called vibrios and include Vibrio cholerae, which causes cholera.

Other forms of bacteria include the spirilla, which bends and refolds, and the spirochetes, which form a corkscrew-like helix. The cell body is wrapped around a central fiber called the axial filament.

Some bacteria are variable in shape and are said to be pleomorphic (in many forms).

The shape of a bacterium is determined by its rigid cell wall.

The microscopic appearance of a bacterium is one of the most important criteria used in its identification.

In addition to their characteristic shapes, the arrangement of the bacteria is essential. For example, certain cocci occur in pairs (diplococci), some in chains (streptococci), and others in staple-like groups (staphylococci).

These arrangements are determined by the bacteria’s orientation and degree of attachment during cell division.

The arrangement of rods and spirochetes is medically less critical.


Bacteria range in size from about 0.2 to 5 μm.

The smallest bacteria (Mycoplasma) are about the same size as the most significant viruses (poxviruses) and are the smallest organisms capable of existing outside of a host.

The most extended rods of bacteria are the size of some yeast and human red blood cells (7 μm).

Bacteria are the smallest living entities.

An average-sized bacterium, such as the rod-shaped Escherichia coli, a typical inhabitant of the intestinal tract of humans and animals, is about 2 micrometers (μm) of a millionth of a meter in length and 0.5 μm in diameter, and Spherical Staphylococcus aureus cells are up to 1 μm in diameter.

Some bacteria are even smaller, such as Mycoplasma pneumonia, one of the smallest bacteria, ranging from about 0.1 to 0.25 μm in width and about 1 to 1.5 μm in length, the shape of rod Bordetella pertussis.

Which is the causative agent of pertussis, which varies from 0.2 to 0.5 µm in diameter and 0.5 to 1 µm in length; and the corkscrew form Treponema pallidum, which is the causative agent of syphilis, averaging only 0.1 to 0.2 μm in diameter but 6 to 15 μm in length.

Some bacteria are relatively large, such as Azotobacter, which has diameters of 2 to 5 μm or more, and Achromatium, which has a minimum width of 5 μm and a maximum length of 100 μm, depending on the species.

Giant bacteria can be visible to the naked eye, such as Thiomargarita namibiensis, which measures an average of 750 μm in diameter, and the rod-shaped Epulopiscium fishelsoni, which ranges from 30 to over 600 μm in length.

Gram stain

One of the most helpful staining reactions for bacteria is the Gram stain; this staining procedure, developed in 1884 by the Danish physician Christian Gram, is crucial in microbiology.

Bacteria in suspension are fixed to a glass slide by brief heating and then exposed to two dyes that combine to form a sizeable blue dye complex within each cell.

When the device is rinsed with an organic solvent, such as acetone or ethanol, the blue dye complex is extracted from the lipid-rich, thin-walled gram-negative bacteria to a greater degree than from the thick-walled, lipid-poor gram-positive bacteria.

Gram-negative organisms appear colorless; gram-positive bacteria remain blue.

The Gram stain reacts to differences in the surface structure of bacterial cells, differences that are evident when the cells are viewed under an electron microscope.

However, not all bacteria can be seen on the Gram stain.

The Gram stain is helpful in two ways:

  1. In the identification of many bacteria.
  2. By influences the choice of antibiotics because, in general, gram-positive bacteria are more susceptible to penicillin G than gram-negative bacteria.

Morphological characteristics of bacteria

Cellular wall

The cell wall is the outermost component common to all bacteria, except Mycoplasma species, bounded by a cell membrane, not a cell wall.

Some bacteria have surface features external to the cell wall, such as a capsule, flagella, and pili.

The cell wall is located external to the cytoplasmic membrane and is composed of peptidoglycan.

Peptidoglycan provides structural support and maintains the characteristic shape of the cell.

Plasma membrane

Just within the peptidoglycan layer of the cell wall is the cytoplasmic membrane, which is composed of a phospholipid bilayer similar in microscopic appearance to that of eukaryotic cells.

They are chemically similar, but eukaryotic membranes contain sterols, while prokaryotes generally do not.

The only prokaryotes with sterols on their membranes are members of the genus Mycoplasma.

The membrane has four essential functions:

  • Active transport of molecules in the cell.
  • Generation of energy by oxidative phosphorylation.
  • Synthesis of cell wall precursors.
  • Secretion of enzymes and toxins.


The cytoplasm has two distinct areas when viewed under the electron microscope:

  • An amorphous matrix contains ribosomes, nutrient granules, metabolites, and plasmids.
  • An internal nucleoid region is made up of DNA.


Bacterial ribosomes are the site of protein synthesis as in eukaryotic cells but differ from eukaryotic ribosomes in size and chemical composition.

Bacterial ribosomes are 70S in size, with 50S and 30S subunits, while eukaryotic ribosomes are 80S in length, with 60S and 40S subunits.

Differences in ribosomal RNAs and proteins are the basis for the selective action of several antibiotics that inhibit bacterial, but not human, protein synthesis.


The cytoplasm contains several granules that serve as storage areas for nutrients and characteristically stain with certain dyes.

For example, volutin is a high-energy reserve stored in the form of polymerized metaphosphate.

It appears as a “metachromatic” granule as it is stained red with the methylene blue dye instead of blue, as expected.

Metachromatic granules are a characteristic feature of Corynebacterium diphtheriae, the cause of diphtheria.


The nucleoid is the area of ​​the cytoplasm in which DNA is found.

Prokaryotic DNA is a single circular molecule with a molecular weight of approximately 2 × 10 9 and contains about 2,000 genes.

In contrast, human DNA has approximately 100,000 genes.

Because the nucleoid contains no nuclear membrane, nucleolus, mitotic spindle, or histones, there is little resemblance to the eukaryotic nucleus.

An essential difference between bacterial and eukaryotic DNA is that bacterial DNA does not have introns, whereas eukaryotic DNA does.


Plasmids are double-stranded, extrachromosomal circular DNA molecules capable of replicating independently of the bacterial chromosome.

Although plasmids are generally extrachromosomal, they can integrate into the bacterial chromosome.

Plasmids appear in both gram-positive and gram-negative bacteria, and several different types of plasmids can exist in a cell:

  • Transmissible plasmids can be transferred from cell to cell. They are large, containing about a dozen genes responsible for synthesizing the sexual pilus and the enzymes necessary for transfer. They are generally present in a few (1 to 3) copies per cell.
  • Non-transmissible plasmids are small since they do not contain the transfer genes; they are often present in many (10 to 60) copies per cell.

Plasmids carry genes for the following medically essential functions and structures:

  • A variety of enzymes mediates antibiotic resistance.
  • Resistance to heavy metals, such as mercury, the active component of some antiseptics, and silver, mediated by a reductase enzyme.
  • DNA repair enzymes mediate resistance to ultraviolet light.
  • Pili (fimbriae) mediate the adherence of bacteria to epithelial cells.
  • Exotoxins, including various enterotoxins.

Cell wall structures

There are various extensions on the outside of the cell wall to help bacteria perform actions such as moving and attaching to other objects.

Flagella, protein filaments, are found outside the body and move bacteria through their environment.

Bacteria can have one or many flagella depending on the species.

Another feature of the surface of bacteria is the protein filaments used by the bacteria to bind to other structures.

They exist on the entire surface of the bacteria and are much smaller than flagella.

A third component is a thread-like extension of a pilus protein, which is also found outside the cell.

The pili help in bacterial conjugation and attachment.

Pathogenic bacteria

Bacteria are present in the air, water, and soil, the essential elements of life closely associated with all living organisms.

Prevailing both internal and external environments, bacteria constitute beneficial and pathogenic microbes.

The gut of humans has abundant microflora that plays a vital role in benefiting humans and are examples of beneficial bacteria.

Human pathogenic bacteria enter the body through various routes and cause disease.

The ability of bacteria to enter the human body and its potential to generate toxins (exotoxins and endotoxins) are the two primary factors that determine the pathogenicity of bacteria.

Some deadly bacterial diseases created a history of epidemics.

Cholera and plague are the best examples of bacterial epidemic diseases.

Deadly diseases associated with pathogenic bacteria led to the development of various antibiotics and vaccines.

With that knowledge of pathogenic bacteria, let us look at some of the common bacterial diseases caused by different pathogenic bacteria, the nature of the bacteria, the types of conditions, the route of entry and available treatment methods, and the best preventive measures.

Bacillus anthracis

This is a rod-shaped, Gram-positive endospore-forming bacterium.

It is transferred from animals such as sheep and goats to humans by coming into contact with such animals or entering the human body by inhaling air contaminated with Bacillus spores or physical contact with the body.

Bacillus anthracis is known to cause multiple diseases, including cutaneous anthrax, pulmonary anthrax, and gastrointestinal anthrax.

Treatment is guaranteed with medications such as penicillin, doxycycline, and ciprofloxacin.

Even the anthrax vaccine is available to protect against the disease.

There is also some evidence of using this deadly spore as a bioweapon.

Vibrio cholerae

Vibrio cholera is a causative agent of gram-negative, comma-shaped bacteria for the cholera epidemic.

The primary source of this bacteria is contaminated food and water, and it gets direct entry into the body of the person consuming the contaminated food or water.

It causes the loss of fluids from the body by inducing watery diarrhea and vomiting.

The best way to control cholera disease and antibiotics such as erythromycin, tetracycline, and chloramphenicol is to replace fluids through oral or intravenous rehydration.

Vaccination against cholera with Dukoral is the best preventive method.

Escherichia coli

This bacterium is gram-negative, rod-shaped, and is considered the indicator organism when checking for water contamination.

Water contaminated with fecal matter is known to have E. coli, and drinking contaminated water causes severe diarrhea.

Rehydration with electrolytes and antibiotics helps control the condition once it is acquired.

Proper handwashing with disinfectants after using washrooms and drinking purified water is the fundamental hygienic way to develop protection against infection.

One more strain of E Coli called enteropathogenic E coli is known to transfer from mother to fetus and cause diarrhea in the newborn.

Clostridium botulinum

Rod-shaped, gram-positive spore-forming bacteria enter the human body through contaminated foods such as vegetables and meat. They are known to produce a toxin that is vulnerable to paralysis-causing nerves.

The patient can be treated with an antitoxin derived from horse serum versus serum.

Ensuring standard food preservation methods is one way to prevent infection.

Chlamydia trachomatis

This Gram harmful bacteria is sexually transmitted and can cause diseases like urethritis, pelvic inflammation, ectopic pregnancy, newborn conjunctivitis, etc.

Antibiotics such as erythromycin, doxycycline, and azithromycin are prescribed for infected patients.

Safe and protective sex is the suggested way to prevent infection.

Salmonella typhi

The causative agent of typhoid fever is a gram-negative bacillus transferred to humans through contaminated food and water or human-to-human interaction.

Antibiotics and vaccination are available, and standard hygiene keeps bacteria away.

Streptococcus pneumonia

They are Gram-positive spherical bacteria that cause pneumonia and meningitis in the infected person.

It is transferred between humans through respiratory droplets.

Children infected with this bacteria develop sinusitis.

Penicillin G and vancomycin are the available antibiotics.

Yersinia pestis

Rod-shaped, gram-negative bacteria are transferred to humans from animals through flies or by consuming meat from infected animals, or through respiratory droplets.

A plague is a disease caused by this bacterium. Although antibiotics are available, developing a barrier between humans (rodents and human flies) and taking the vaccine is best to prevent infection.

Following standard preventative measures is the best way to protect the human race from these bacterial infections than relying on antibiotics after acquiring the infection. Some of these pathogenic bacteria have been identified with antibiotic resistance.