Different Types of Bacteria

Bacteria (singular: bacterium) are single-celled organisms which can only be seen through a microscope. They come in different shapes and sizes, and their size is measured in micrometer - which is a millionth part of a meter. There are several different types of bacteria in the world, and they are found everywhere and in all types of environment. There are various groups of bacteria, which belong to the same family and have evolved from the same bacteria (ancestor). However, each of these types possess their own peculiar characteristics - which have evolved after separation from the original species.

Classification of Bacteria

Before the invention of DNA sequencing technique, bacteria were mainly classified based on their shapes - also known as morphology, biochemistry, and staining - i.e. either Gram positive or Gram negative staining. Nowadays, along with the morphology, DNA sequencing is also used in order to classify bacteria. DNA sequencing helps in understanding the relationship between two types of bacteria i.e. if they are related to each other despite their different shapes. Along with the shape and DNA sequence, other things such as their metabolic activities, conditions required for their growth, biochemical reactions (i.e., biochemistry as mentioned above), antigenic properties, and other characteristics are also helpful in classifying the bacteria.

Based on the morphology, DNA sequencing, conditions required and biochemistry, scientists have come up with the following classification with 28 different bacterial phyla:

1. Acidobacteria
2. Actinobacteria
3. Aquificae
4. Bacteroidetes
5. Caldiserica
6. Chlamydiae
7. Chlorobi
8. Chloroflexi
9. Chrysiogenetes
10. Cyanobacteria
11. Deferribacteres
12. Deinococcus-Thermus
13. Dictyoglomi
14. Elusimicrobia
15. Fibrobacteres
16. Firmicutes
17. Fusobacteria
18. Gemmatimonadetes
19. Lentisphaerae
20. Nitrospira
21. Planctomycetes
22. Proteobacteria
23. Spirochaetes
24. Synergistetes
25. Tenericutes
26. Thermodesulfobacteria
27. Thermotogae
28. Verrucomicrobia

Each phylum further corresponds to the number of species and genera of bacteria. In a broad sense, this bacterial classification includes bacteria which are found in various types of environment such as sweet water bacteria, ocean water bacteria, bacteria that can survive extreme temperatures (extreme hot as in sulfur water spring bacteria and extreme cold as in bacteria found in Antarctica ice), bacteria that can survive in highly acidic environment, bacteria that can survive in highly alkaline environment, bacteria that can withstand high radiations, aerobic bacteria, anaerobic bacteria, autotrophic bacteria, heterotrophic bacteria, and so on...

Though bacteria are mainly classified into phylum i.e. the scientific classification of organisms, they can be categorized into the following groups for simplification.

Bacterial Classification Based on Shapes
As already mentioned, before the advent of DNA sequencing, bacteria were classified based on their shapes and biochemical properties. Most of the bacteria belong to three main shapes:

• Rod-shaped bacteria are called bacilli - e.g. E.Coli and Salmonella
• Spherical-shaped bacteria are called cocci - e.g. Staphylococcus and Streptococcus
• Spiral-shaped bacteria are called spirilla - e.g. Treponema and Borellia

Some bacteria belong to different shapes, which are more complex than the shapes mentioned above.

Bacterial Classification Based on Staining Methods
Bacteria are grouped as 'Gram positive' and 'Gram negative' bacteria, based on the results of Gram staining method, wherein an agent is used to bind to the cell wall of the bacteria.

• Gram positive bacteria - take up crystal violet dye and retain their blue or violet color.
• Gram negative bacteria - do not take up crystal violet dye, and thus appear red or pink.

Aerobic and Anaerobic Bacteria
Bacteria are also classified based on the requirement of oxygen for their survival.

• Aerobic bacteria - Bacteria that need oxygen for their survival.
• Anaerobic bacteria - Bacteria that do not require oxygen for survival.

Anaerobic bacteria cannot bear oxygen and may die if kept in an oxygenated environment. Such types of bacteria are usually found in places like, under the surface of the Earth, deep oceans, and bacteria which live in some medium.

Autotrophic and Heterotrophic Bacteria
This is one of the most important classification types as it takes into account the most important aspect of bacterial growth and reproduction.

• Autotrophic bacteria (also known as autotrophs) obtain the carbon it requires from carbon dioxide.
• Heterotrophic bacteria obtain sugar from the environment they are in (for example, the living cells or organisms they are in).

Some autotrophs directly use sunlight in order to produce sugar from carbon dioxide, whereas others depend on various chemical reactions.

Bacterial Classification Based on Environment
As we mentioned earlier, bacteria are found in all types of environment. While some species of bacteria can withstand extreme conditions, others need specific moderate conditions to survive. On the basis on environmental conditions of their habitat, bacteria are classified into:

• Mesophiles - which require moderate conditions to survive.
• Neutrophiles - which require moderate conditions to survive.
• Extremophiles - which can survive in extreme conditions.
• Acidophiles - which can tolerate low pH conditions.
• Alkaliphiles - which can tolerate high pH conditions.
• Thermophiles - which can resist high temperature.
• Psychrophilic bacteria - which can survive extremely cold conditions.
• Halophiles - which can survive in highly saline conditions.
• Osmophiles - which can survive in high sugar osmotic conditions.

The great deal of diversity that you get to see in bacteria species makes it virtually impossible to come up with a classification which will include every single species on the planet. Add to it the fact that these bacteria are also constantly evolving and adapting to the environment wherein survival of living things has been deemed impossible (The best example are the bacteria which inhabit the brine lakes of Mediterranean Sea wherein the salinity levels far exceed the salinity levels of normal sea water.), and the task becomes all the more difficult.