Classification of Bacteria | by Shapes & Characteristics

Bacteria  are unicellular organims present in entire biosphere.

They have vital importance on earth and hence studied extensively in microbiology.

You can study about them in detail in microbiology courses in medicine, pharmacy and even basic biology.

These bacteria are unicellular (single-celled) organisms and are of microscopic in size invisible to naked eye.

The bacterial classification is one of the key factors to tackle them in disease.

The classification is done based on factors like their shape, nutrition requirement, cell wall staining, the cell appendages etc.

Of these bacteria, those harmful and useful to humans are widely studied in medicine and pharmacy. While those pathogenic bacteria which cause the disease to plants and animals are extensively studied in agriculture and animal husbandry sciences.

Some of the aspects of bacterial classification also help in identification bacteria.

Classification of bacteria|by cell wall

The cell wall is a characteristic plant cell. Bacterial cell wall feature has features useful for classification.

 check out: Differences between Bacterial cell and Human cell.

Rod-shaped bacteria, illustration-Getty images

Staining: Cell wall of bacteria differs based on the layers in it. Gram stain is used to classify these bacteria based on the variation in the layers.

This bacterial cell wall is made of 3 materials in general viz. carbohydrates, proteins (peptidoglycan) and lipids  (lipopolysaccharide).

But there is variation in the quantity of peptidoglycan and lipopolysaccharides ratio among them.

The grams staining helps in the classification of bacteria as gram positive & gram negative bacteria.

In gram +ve bacteria there is a thicker peptidoglycan layer while gram -ve has less peptidoglycan and more of glycolipid membrane.

So when stained with grams stain, crystal violet, peptidoglycan retains it giving violet color to gram +ve bacteria. Gram-ve bacteria cannot retain this crystal violet and instead retains saffron color.

The gram stain consists of coloring dyes like crystal violet and saffron. When a bacteria culture is added with Gram’s stain, gram-positive bacteria show violet color, while gram-negative bacteria show saffron color.

So, the bacterial species which take up the blue color on grams stain is termed gram +ve bacteria. The one which takes up orange color is called gram -ve bacteria.

Classification of Bacteria by shape or cell structure

Bacterial species are differentiated based on their shapes. They have different cell structure but most of them come under two basic shapes like. Check out the image below for an idea.

a) Bacillus (stick or filament or rod-like shape)- Among them if a bacterium is

♦ The single cell then it is Bacillus if more it is bacilli like

♦ Two-celled: Diplo bacilli, four-celled – tetrad etc.

♦ Palisade: Here two cells of Bacillus are arranged side by side like sticks in a matchbox

♦ Streptobacilli: Bacilli bacteria arranged like a chain.

b) Coccus (These are spherical or oval shaped)- Of Them

Single-celled is called Monococcus,

♦ Two-celled: Diplococci,

♦ A chain like an arrangement: Strepto bacilli.

♦ Bunch of grapes like Staphylococci.

c) Comma-shaped bacteria, Ex: Vibrio cholera bacteria causing cholera.

d) Spiral bacteria:  They are also called as spirochetes. These are spiral or hair like in shape and hence called as spiral-shaped bacteria. Ex: syphilis-causing bacteria.

Classification of bacteria based on flagella presence

Flagella are appendages of movement for the bacteria. They emerge from the cell membrane. Not all bacteria have flagella but motile bacteria have flagella. Based on the number of flagella and also the location bacterial classification is done as below.

See the figure above for details.

♦  Atrichous bacteriaWith no flagella on the bacterial cell wall. These are non-motile bacteria

Monotrichous bacteria: With one single flagellum.

Amphitrichous bacteria: Two flagella on both sides of the cell

Peritrichous bacteriaMany flagella at different points

Lophotrichous bacteria: Flagella at one pole or point of the cell

Flagella around the cell: Peritrichous bacteria. The flagella are present all over the cell wall.

Classification of bacteria based on nutrition requirements

Bacteria obtain nutrition in different forms like. Because of this character, they contribute greatly to humans and the environment.

check out: Use of Bacteria to humans and the environment.

Autotrophs: These are bacteria which prepare their own food. Due to the presence of chlorophyll like pigment they perform photosynthesis. They do this by using sunlight as the source of energy. Besides they take up CO2 and water from nature.  This photosynthesis helps in the formation of carbohydrates. These carbohydrates provide the energy. Ex: Chlorella.

Chemoautotrophs: As the name indicates they survive on chemicals. These are bacteria synthesize their own food by use of energy obtained from chemical sources. They differ from autotrophs in that they do not need sunlight.

Heterotrophs: These are bacteria which do not synthesize their own food but obtain it from others They can feed on food materials as animals do.

Symbiotic bacteria: (Sym + Biosis = Living together) These are bacteria which obtain food by living together with other organisms. They reside in mutual beneficial support with others.

Ex:  Rhizobium bacteria in leguminous plants. Here the bacteria fix nitrogen in roots by absorbing it from the air. This nitrogen acts as a fertilizer to plant. In return, they take nutrition from the same plant. Another example is Enterobacteria in the intestine. Hence taking too many antibiotics kills the friendly bacteria in out gut causing problems to us.

Saprophytic bacteria: (sapro+phytes = rotten material + plant) These are bacteria survive by eating rotten material. They get their nutrition by consuming dead and decaying material. Thus thereby help in cleaning the environment from the accumulation of waste.

“Imagine the world without them, it would have been a place full of dead bodies all around…:-)”

Pathogenic bacteria: (patho+genisis =disease + causing) These are bacteria responsible for diseases in humans and plants. They grow only in the body of other animals or plants. They get their nutrition from the host. In doing so, they consume the vital living elements in them and induce diseases.

Classification based on the temperature dependence

This is a quite interesting method as they are differentiated based on their preference for surrounding temperature.

Bacteria can at cold temperature and even hot temperatures besides normal room temperatures. So, they are classified based on the temperature they can survive it. Like

Thermophilic (thermo+phyllic=temperature loving); Thermophilic bacteria are those which can survive at a high temperature of 45 to 60-degree temperatures

mesothermic (medium+thermic= medium temperature) can survive at 25 to 45 degrees

Hypothermic (hypo = low ) These bacteria survive at low temperatures like 8 degrees or even less.

In common most bacteria survive in between 25 to 45 degree i.e. they are mesotherms.

Classification based on oxygen requirement

Not all bacteria require oxygen to survive. Some can survive without oxygen. These are the microbes considered to be immortals due to cell division. But this feature adds even more weight to it.

Bacteria are classified based on the oxygen requirement as

Aerobic: Which require oxygen to survive.

Anaerobic bacteria: These bacteria do not require oxygen for survival. They are further as two types

Obligate anaerobes: They survive in absence of oxygen. But when exposed to oxygen they die.

Facultative anaerobes: These also survive in environments without oxygen but when exposed to oxygen they can survive.

Classification of bacteria into different phyla is done as per taxonomy i.e. botanical nomenclature.

Also, see Identification of bacteria


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