Ever wondered how many bacteria would a gram of soil contain? Well, 2.5 million is the answer, but you may never be able to see even a single bacterium throughout your life, with the naked eye. And even if you try lining up 10,000 bacteria, side by side, you’ll end up creating only 2.5 centimeters of a short line. Despite the minuscule and microscopic size, bacteria are found in different habitats, such as soil, air, water, hot springs, and acidic environments. Bacteria are single-cell organisms, which belong to neither plants nor animals, and are classified under a group by themselves. Even though they are single-celled organisms, they are found in groups of millions, since they multiply at tremendously fast rates. But bacteria vary largely, based on their shape and size. Go through the following text to explore the unique classification and the hidden world of bacteria. 

Types of Bacteria 

On the Basis of Shape
Categorizing bacteria on the basis of their external appearance is the most basic form of classification. This was the classification used before the advent of DNA sequencing. Bacteria are divided into three main shapes, namely, bacilli, cocci, and spirilla.

  • Rod shaped bacteria are called bacilli, which are both gram positive and negative. E.Coli, Salmonella, Bacillus, and the deadly Clostridium belong to this group.
  • Spherical shaped bacteria are known as cocci, which are mostly gram positive. They are found in chains and bunches, but can also survive individually. This bacteria form includes Staphylococcus and Streptococcus.
  • Spiral shaped bacteria are termed as spirilla. They are gram negative, usually in the shape of a cork screw. Treponema and Borellia are two common spirilla. 

On the Basis of Physiological Activity
Bacteria can be organized on the basis of their requirement of oxygen for their survival. Bacteria that completely depend upon oxygen for their survival are called aerobic bacteria, while bacteria which do not need oxygen for surviving, are known as anaerobic bacteria. Since they cannot tolerate oxygen, anaerobic bacteria are mostly found in low-oxygen zones, such as under the surface of the earth, marshy areas, or underwater. 

On the Basis of Staining Methods
Bacteria can be grouped into gram positive or gram negative, on the basis of their ability of acquiring dye. Bacteria, which takes up crystal violet dye and retains blue or violet color in gram staining protocol, are called gram positive bacteria. Gram negative bacteria, on the other hand, are those bacteria that do not take up the violet dye, instead adopt the safranin stain and appear pink or red. While the thick peptidoglycan allows the gram positive bacteria to retain the dye, the same thick wall in gram negative bacteria prevents the dye from penetrating in. 

On the Basis of Mode of Living
Considering the growth, survival, and reproduction aspects, bacteria are classified into autotrophs and heterotrophs.

  • Autotrophs, or autotrophic bacteria, extract carbon directly from the carbon dioxide present in the atmosphere. While most of the bacteria use sunlight to produce sugar from carbon dioxide, others depend upon different chemical reactions also.
  • Heterotrophs, also called heterotrophic bacteria, do not require natural resources, such as carbon dioxide and sunlight, to fulfill their energy requirements. They obtain their carbon and sugar resources from the surrounding environment or living organisms they are living in. 

On the Basis of Environment
Not all bacteria can survive under all types of conditions. While some need moderate conditions to survive, others are capable of withstanding even extreme conditions. As such, bacteria can be grouped according to the extremity of their surviving conditions.

  • Extremophiles are those bacteria which can resist the most extreme of the conditions. Conversely, bacteria like Mesophiles or Neutrophiles can live in moderate situations.
  • Bacteria like Acidophiles can live in as low as 3pH, or even below pH conditions, whereas Alkaliphiles are capable of tolerating high pH conditions, up to 9 or above.
  • Thermophiles can resist high temperatures between 60 °C and 80 °C, while Psychrophilic bacteria possess the power to bear chilling temperatures of below -15 °C.
  • High saline conditions can be tolerated by Halophiles, whereas high sugar osmotic conditions can be resisted by Osmophiles. 

On the Basis of Phyla
Bacteria are categorized into different phyla, based on DNA sequencing, morphology, biochemistry, and conditions required. These phyla are further grouped into species and genus.

  • Aquificae
  • Xenobacteria
  • Fibrobacter
  • Bacteroids
  • Firmicutes
  • Planctomycetes
  • Chrysogenetic
  • Cyanobacteria
  • Thermomicrobia
  • Chlorobia
  • Proteobacteria
  • Spirochaetes
  • Flavobacteria
  • Fusobacteria
  • Verrucomicrobia 

Besides the above various classifications on which bacteria are segregated into, there exists another unique type of bacteria, called Rickettsia. Since they possess different features of virus and are unable to survive outside living cells, these bacteria stand outside any of the above categorizations. They are usually gram negative cocci and spread through the bite of vectors, or carriers, such as infected fleas, ticks, and lice. They are responsible for typhus fever, Rocky Mountain fever, and Rickettsialpox (a kind of body rash).

Due to the absence of cell wall, Mycoplasma can be bacilli or cocci form of bacteria. As such, they are unaffected by antiobiotics, which work by targeting and damaging the cell wall. Mycoplasma pneumonia causes respiratory disease, whereas Mycoplasma genitalium is responsible for pelvic disease. 

The kingdom of bacteria is extremely diverse, with variation in each species. Their ability to evolve in all kinds of conditions makes them the largest number of living organisms existing on the planet.