Bacteria and its various characteristic features


Bacteria are the most primitive type of organisms present in nature and these organisms have their own way of life. This article gives an account of their discovery, structure, various characteristic features of their bodily organization, various life processes carried about them, and some important features of their life cycle. This article also deals with the classification of these organisms. Some most frequently asked questions were also included along with this article.

Introduction

Bacteria are the most primitive simple single-celled prokaryotic organisms present in nature. They are present everywhere in nature and are cosmopolitan in their distribution. These organisms are so small in their structure and that can't be observed with our naked eye but can be observed only under a microscope. The singular form of bacteria is a bacterium.

Discovery of Bacteria

Bacteria were first discovered by a Dutch physician Antony Von Leeuwenhoek in 1675 when he observed the scrapings of teeth under the microscope constructed by himself. He named these tiny organisms as little 'animalcules'. These organisms were later coined as bacteria by Ehrenberg in 1838. Cohn classified bacteria as plants in 1856 and scientist Robert Koch proved bacteria cause diseases in 1876. Koch discovered Anthrax bacteria in 1876 and paved the way for the establishment of the Bacteriology branch in Science.

A France scientist Louis Pasteur established the branch of Science dealing with microscopic organisms like bacteria as Microbiology. He did exhaustive work on the fermentation and decay process connected with bacteria. He developed the first vaccine that cures hydrophobia disease caused by the bite of a rabid dog.

Occurrence of bacteria

At present, we have the knowledge of several thousand known species of bacteria that are there everywhere in nature. They are present in the air, water, soil, hot springs, glaciers, inside and outside the human body or animal body, the food we eat, as parasites in plants and animals, as saprophytes on dead and decaying matter, etc.

Size of bacteria

Bacteria are the smallest living unicellular Prokaryotic organisms present in nature and their size usually ranges from 0.1-10 microns (1 micron=1/1000 m.m.) in their length. The size of spherical bacteria may range from 0.5-2 microns and rod-shaped bacteria may range from 2 to 10 microns. The smallest known bacteria is Mycoplasma. The largest known bacteria is Bacillus butschlii and that measures 80 microns in length.

Shapes of Bacteria

Bacteria can be of four basic types basing on their shape. Under the high power of the microscope only we can observe the following different shapes.

Shapes of Bacteria
Shapes of Bacteria (By the Author)

Coccus bacteria

These bacteria are spherical or globular or oval in shape. The average size of spherical bacteria maybe 0.5-2 microns in size. The plural of coccus is cocci. If spherical bacteria exist as single individuals are known as Mono coccus, exist in pairs are known as Diplococcus, exist in clusters are known as Staphylococcus, and exist in chains are known as Streptococcus.

Ex:-Diplococcus pneumonia that causes Pneumonia disease is a diplococcus type of bacteria. Streptococcus pyogenes that cause scarlet fever and sore throat are Streptococcus bacteria. Staphylococcus aureus lives in nasal passages, different strains of it that causes boils, pneumonia, and food poisoning are staphylococcus type of bacteria.

Bacillus Bacteria

These are rod-shaped bacteria with or without flagella. Bacilli are the plural of Bacillus. Rod-shaped bacteria may measure around 2-10 microns. If rod-shaped bacteria occur in singly are known as Monobacillus, occur in pairs are known as Diplobacillus and if exist chains are known as Streptobacillus.

Ex:-Escherichia coli is a common gut living bacteria, Salmonella typhi that causes typhoid, Lactobacillus bacteria are single rod bacillus bacteria. Azotobacter, a nitrogen-fixing bacteria, Bacillus anthracis that causes anthrax disease are a streptobacillus type of bacteria.

Spirillum Bacteria

These bacteria are usually spirally twisted or cork-screw shaped mostly with flagella. Spirillum bacteria is this type of bacteria.

Vibrio Bacteria

These are small comma-shaped bacteria with a flagellum. Vibrio cholera, a bacterium that causes Cholera disease is this type of bacterium.

Classification of bacteria basing on affinity towards a stain

Bacteria can be classified into the Gram-positive and Gram-negative type of bacteria basing upon the affinity towards a particular stain. Hans Christian Gram introduced a violet stain called Gram stain that colors bacteria. Those bacteria that can be stained violet with Gram stain are known as Gram-positive bacteria while the other bacteria that cannot be stained with it are known as Gram-negative bacteria. Those bacteria with thick Peptidiglycon cell walls have the ability to retain the stain color while the other bacteria with different cell wall materials lack the ability to absorb the stain. Salmonella and Escherichia coli bacteria are gram-negative bacteria. Staphylococcus, Bacillus anthracis are gram-positive bacteria.

Structure of a typical generalized Bacterial cell

The following is the generalized structure of a bacterial cell under an electron microscope.

Structure of a Bacterial Cell
Ultrastructure of a Bacterial Cell (Courtesy:-wikipedia.org)

A bacterial cell is a typical prokaryotic cell lacking nucleus with the nuclear membrane and membrane-bound cell organelles. A bacterial cell will have a rigid cell wall like a plant cell but it is not made with cellulose and it is composed of a material called peptidoglycan (murein). The cell wall is non-living in nature. Some species of bacteria that are surrounded by a slimy mucilaginous sheath around their cell wall and that is considered to be secreted by the protoplasm of the bacterial cell. Bacterial species possessing such capsules on their outer surface are considered to be resistant varieties.

Below the cell wall, a thin semipermeable living plasma membrane or cell membrane is present. The plasma membrane surrounds the cytoplasm of the bacterial cell. The plasma membrane functions are similar to the plasma membrane functions of the eukaryotic cells. In addition to that plasma membrane also acts as a respiratory surface as it produces respiratory enzymes for respiration. In addition to this, the plasma membrane also forms small infoldings called mesosomes. These mesosomes help the bacteria in the division of cells during binary fission. In some bacteria, the cell membrane cut off sac-like tubular structures called photosynthetic membranes. These specialized structures formed from cell membranes include bacteriochlorophyll that aids in photosynthesis.

Inside the plasma membrane, a granular cytoplasm is present. The cytoplasm usually contains glycogen granules, lipid, and other food reserves. As it is a prokaryotic cell, the bacterial cell lacks a well-defined nucleus. The nuclear content is simply dispersed in the cytoplasm without a nuclear membrane. Such a primitive nucleus of a bacterial cell is known as the nucleoid. The genetic material of the bacterial cell can be found in the form of the single circular double-stranded DNA molecule. Such a single DNA molecule is known as a bacterial chromosome. Membrane-bound cell organelles like nucleolus, Golgi bodies, plastids, mitochondria, large vacuoles, and lysosomes are absent in a bacterial cell. But a bacterial cell includes ribosomes of smaller 70S type ribosomes. They help in protein synthesis of the bacterial cells. In addition to a DNA molecule, a bacterial cell may include small rings of DNA in the cytoplasm called plasmids.

Bacteria may also contain flagellum or flagella around its outer body surface. These flagella may be useful in certain bacteria for their motility or movement. Basing on the flagella the bacteria possess, the bacteria are classified into the following types. If the bacteria don't possess any flagella is said to be atrichous, one flagellum monotrichous, a tuft of flagella lophotrichous, two tufts of flagella amphitrichous, and flagella around its body surface peritrichous.

In certain species of bacteria, fine threads of a protein called sex pili or pili or fimbriae cover on their outer surface. These pili in these bacteria are useful in attaching the partner bacteria in the sexual conjugation process and also attaching themselves to some support.

Movements in bacteria

Most of the bacteria can't move on their own. They passively move with the help of air or water. But certain bacteria possess whip-like flagellum or flagella and with the help of their lashing movements bacteria can swiftly move in the liquid medium.

Nutrition in bacteria

Bacteria can be divisible into two groups basing on the mode of nutrition-Autotrophic and heterotrophic.

Autotrophic bacteria

These bacteria can prepare their own food by using materials available from nature. Autotrophic bacteria can again divisible into two kinds-Photosynthetic bacteria and Chemosynthetic bacteria.

Photosynthetic bacteria like Chlorobium and Rhodospirillum possess bacteriochlorophyll pigment by which they can carry photosynthesis. In this type of photosynthesis, food is prepared with the help of sunlight but no oxygen is released as in plants.

Chemosynthetic bacteria are a type of bacteria that oxidize inorganic compounds like nitrogen, sulfur, and iron to get chemical energy. By using this chemical energy, they reduce CO2 to make food. Thus these bacteria are also autotrophic in nature. Ex:-Nitrosomonas and Nitrobacter.

Heterotrophic bacteria

They can be divisible again into three groups-Saprophytic, Parasitic, and Symbiotic.

Saprophytic bacteria live on dead and decaying matter. They secrete enzymes on dead material and digest them. The digested material will be absorbed as their food by these organisms. Ex:-Clostridium spp.

Parasitic bacteria bacteria draw nourishment directly from the host body. Parasites that cause diseases to host are known as pathogens. Ex:- Vibrio cholerae.

Symbiotic bacteria are those bacteria that live in close association with other organisms by which both the organisms get benefited. Ex:-Rhizobium bacteria live in the root nodules of leguminous plants and they both mutually help each other in their existence.

Respiration in bacteria

Bacteria respire through their general body surface by simple diffusion method. Bacteria that need oxygen for their respiration are said to be aerobes and those bacteria that don't need oxygen are said to be anaerobes. Mycobacterium tuberculosis and Corynebacterium diphtheria are obligate aerobes and thus they can't live without oxygen. Similarly, Clostridium tetani and Clostridium septicum are obligate anaerobes and thus they cannot live in the presence of oxygen.

Reproduction in Bacteria

Bacteria reproduce most commonly by the Asexual method of reproduction called Binary fission under favorable conditions. Some species of bacteria rarely undergo sexual reproduction by the Conjugation method.

Asexual reproduction

The most common method of asexual reproduction in Bacteria is binary fission. Binary fission occurs under favorable conditions like the availability of food, air, water, and optimum temperature. The following are the steps in binary fission of bacteria:-
Binaryfission in Bacteria
Binary Fission in Bacteria (Courtesy:-wikipedia.org)

  1. The circular DNA in the bacterial cell duplicates
  2. The cell now expands and the two DNA get separated
  3. The cell constricts in the middle by the development of a membrane
  4. Two identical bacterial cells are formed due to the constriction formed at the middle of original bacterial cell.


Under favorable conditions, a single bacterium can divide into two bacterial cells within 20 minutes. If this rate continues for about 24 hours, 280 billion bacteria can be produced from one bacterium. But this rate never continues like this because as the number of bacteria increases enormously, there occurs a situation where food availability decreases, poisonous metabolic wastes accumulate and that will kill the bacterial population.

Sexual reproduction

A highly primitive type of sexual reproduction occurs in certain limited species of bacteria. Sexual reproduction occurs through a method called conjugation. In this method, bacteria of two different strains of the same species come close to each other and attached to each other by sex pili. Through the sex pili, DNA from donor bacterium passes into the recipient and changes the genetic nature of the recipient. Here in this sexual process, there is no increase in individuals but only a genetic transformation occurs.
Conjugation process in Bacteria
Sexual reproduction by Conjugation method in Bacteria (By the Author)

Spore formation of bacteria

Under severe conditions of climate like unavailability of water, high temperature, lack of food will make bacteria to undergo spore formation to overcome the conditions. The cell content of bacterium transforms into a spherical mass and develops a thick resistant wall around it during spore formation. This hard spherical body is contained inside the original bacterial cell wall and become dormant until favorable condition return. Once the favorable conditions return, the cell wall of bacterium ruptures, and the spore inside transform into a bacterium. This method is not considered to be a method of reproduction as it doesn't allow an increase in the number of individuals. This method is only helpful for the bacteria to overcome unfavorable conditions.
Spore formation in Bacteria
Spore formation in Bacteria (By the Author)

Plant characteristic features of Bacteria


  1. A bacterial cell is having a cell wall similar to that of a plant cell but it is not made with cellulose as in plants and is made with a chemical Peptidiglycan
  2. Like plants, bacteria also synthesize vitamins
  3. A few species of bacteria include bacteriochlorophyll that helps to carry photosynthesis like in plants


Classificatory position of bacteria

According to the Five Kingdom system of classification, bacteria were included under Monera kingdom for the following reasons:-
  1. Bacteria are unicellular prokaryotic organisms
  2. Bacterial cells lack nuclear membrane for its nucleus and lacks double membraned cell organelles in the cytoplasm of the cell
  3. Bacteria cells have a rigid cell wall
  4. Have various modes of nutrition, may be heterotrophic or autotrophic
  5. Follow mostly primitive type of asexual reproduction


Most frequently asked questions:-


  1. Why a bacterium is a prokaryotic cell?
    Ans. A bacterial cell lacks a membrane-bound nucleus, lacks double membraned cell organelles in cytoplasm, including ribosomes of 70S type.

  2. Is endospore formation in bacteria is a type of reproduction? why or why not?
    Ans. Endospore formation is not a type of reproduction in bacteria because it doesn't help to increase the number of individuals. The main purpose of endospore formation in bacteria is to tide over unfavorable conditions.

  3. What are mesosomes and plasmids in bacteria? State the functional role of those structures in bacteria.
    Ans. The infoldings in the plasma membrane of bacteria are known as mesosomes. They are rich in enzymes that help to perform cellular respiration, DNA duplication, and cell division.

    Plasmids are small circular extrachromosomal DNA molecules present in the cytoplasm of the cell. They include a small number of genes and helps the bacteria in survival ability and helps to develop antibiotic resistance.

  4. State two plant cell characteristic features of bacteria.
    Ans. Presence of a rigid cell wall like in plant cells and the ability to synthesize vitamins.

  5. Name a bacterium that makes symbiotic association with roots of leguminous plants.
    Ans. Rhizobium bacterium


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