Bacteria are unicellular organisms present in the entire biosphere.
They have vital importance on earth and hence studied extensively in microbiology.
You can study them in detail in microbiology courses in medicine, pharmacy, and even basic biology.
These bacteria are unicellular (single-celled) organisms and are of microscopic and invisible to the 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 the identification of bacteria.
Classification of bacteria|by cell wall
The cell wall is a characteristic plant cell. Bacterial cell wall feature has features useful for classification.
You may also like the Differences between Bacterial cell and Human cell.
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 materials like the carbohydrates, proteins and lipids in varying proportions.
The gram staining helps in the differentiation of bacteria as gram-positive and gram-negative.
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 during gram staining are called gram +ve and those which take up an orange are called gram -ve bacteria.
Classification of Bacteria by shape or cell structure
Cohn divided the bacterial into four types based on their shapes in 1872. They have a different cell structure, but most of them come under two basic shapes like bacillus or cocci. Check out the image below for an idea.
a) Bacillus: These are rod-shaped or filament-shaped bacteria. They are of four types like
i) Monobacillus: This is a single rod-shaped bacillus bacteria.
ii) Diplobaciullus: These are a pair of rod-shaped bacteria. Two bacteria cells stick together. They can also be present as four-celled as a tetrad.
iii) Streptobacilli: This is a chain of rod-shaped bacteria. Bacilli bacteria arranged like a long chain.
iv) Palisade: Here two cells of Bacillus are arranged side by side like sticks in a matchbox
b) Coccus: These are spherical shaped bacteria or oval shaped. Based on the number and their arrangement they are divided
i) Monococcus which is a single-celled round-shaped bacteria.
ii) Diplococci is two spherical shaped bacteria existing as pairs.
iii) Streptococci is a chain of many round-shaped bacteria.
iv) Staphylococci is a group of spherical bacteria arranged like a bunch of grapes.
v) Sarcina is a type where 8 round-shaped bacteria are arranged in cubical shape.
c) Comma-shaped bacteria: Here is the bacteria is slightly bent and looks like a comma. Ex: Vibrio cholera bacteria causing cholera.
d) Spirillum bacteria: This is a long spiral-shaped bacteria. They are also called as spirochetes. These are spiral or hair like in shape. Ex: syphilis-causing bacteria.
e) Pleomorphism: Though most bacteria have a specific shape, some do not. They exist in multiple shapes. Examples include Acetobacter.
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 bacterial location classification is done as below.
See the figure above for details.
♦ Atrichous bacteria: With 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 bacteria: Many 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
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 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 our 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 quite an interesting method as they are differentiated based on their preference for the surrounding temperature.
Bacteria can grow at cold temperatures 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 between 25 to 45 degrees, 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.
Based on oxygen dependence, they are either
Aerobic: Which requires oxygen to survive.
Anaerobic bacteria: These bacteria do not require oxygen for survival. They are further as two types
Obligate anaerobes: They survive in the 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., scientific nomenclature.
Also, see→ Identification of bacteria