Bacteria – Structure and Classification Study Guide

We encounter thousands and millions of microorganisms without even noticing them in our everyday lives. One such group of microorganisms is bacteria, which varies greatly in their structure.

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Based on the evolutionary data available, it is evident that the first living organisms found on earth were unicellular microorganisms. These unicellular organisms have evolved with time and show a striking resemblance to the current bacteria structure. Bacteria are often associated with several diseases, but, there exist certain good bacteria as well. This study guide mainly discusses the structure of bacteria and also its classifications based on certain governing factors.

What are bacteria?

Bacteria are prokaryotes and unicellular organisms that lack certain cell organelles compared to their eukaryotic counterparts (check out the diagram of a bacterial vs plant cell below).

Prokaryotic cells refer to primitive cells that lack a proper nucleus and other cell organelles than the modern-day cells. Therefore, the cellular structure of bacteria is simpler than other organisms. Like other prokaryotic cells, bacteria cells are also smaller and usually of various types (helical, spheroid, rod, etc.).

Prokaryotic vs Eukaryotic Cell Source

  • As stated earlier, the prokaryotes differ from the eukaryotic cells by their sheer small sizes (0.2-10µm).
  • Apart from size, prokaryotes also lack the presence of mitochondria and showcase the presence of cell walls, which are usually absent for animal cells.
  • Additionally, prokaryotes also exhibit a primate form of propagation through binary fission and not mitosis. In synchrony with these features, a bacterial cell also shows the same characteristics, in addition to a few more which have been listed below.

Capsule One of the main characteristic features of bacteria is the presence of a polysaccharide outer covering outside of the cell wall. This capsule often plays an important role in preventing bacterial cells from phagocytosis.

Cell wall Underneath the capsule lies a peptidoglycan cell wall that offers a definite structure and shape to a bacterial cell. The polysaccharide units present in the cell wall are crisscrossed, with peptide bonds acting as bridges. This peptidoglycan arrangement is often used for antimicrobial study and production.

Cell membrane or plasma membrane Like eukaryotic cells, bacterial cells also possess a cell membrane called the plasma membrane, which shows functions like other cells. The cell membrane of bacteria is also composed of two layers of phospholipid molecules that help in biosynthetic processes.

Nuclear material (DNA) Bacterial cells contain only DNA, which helps propagate new generations. However, unlike in eukaryotic cells, DNA is not enclosed within the special structure, the nucleus or nuclear membrane.

Flagella Flagella are elongated structures that are used for locomotion. The number of flagella varies in number and also in their location.

Pili Certain spiked structures are found inside the bacterial cell and often project onto cell surfaces. Pilli helps bacterial cells to adhere strongly to host tissues. Amino acids form the basic structure of pili that shows great affinity towards special proteins found on the surface of host cells.

Inside bacterial cells, all these structures remain embedded in a gel-like matrix called cytoplasm. Additionally, bacterial cells generally exhibit a fine slime layer surrounding the capsule, facilitating easy colonization.

Classification of Bacteria

Classification of bacteria Source

Bacterial cells are widely classified into different groups based on different features such as shape, mode of respiration, gram stain sensitivity, and mode of nutrition. Details of this bacterial classification are provided below.

Based on shape

Bacteria shape Source

Rod-shaped: Just like the name, bacteria with elongated cell structures in the shape of a rod fall under this category. The bacteria under this category are commonly called bacilli. Examples include Lactobacillus sp., Mycobacterium tuberculosis, etc.

Cocci or spherical shaped: Another group of bacteria based on morphology is the cocci group or spheroid-shaped bacteria. For example, Staphylococcus sp., etc.

Spirilla or spiral-shaped: Bacteria with spiral morphology fall under this group. The common term used to describe this group is spirilla. For example, Leptospira sp., Treponema sp., etc.

Vibrio or comma-shaped: Bacteria resembling a comma (,) shape fall under this category, and the group is also sometimes known as vibrio. For example, Vibrio cholera, etc.

Based on mode of respiration

In living organisms, two forms of respiration usually occur — aerobic and anaerobic. Aerobic respiration refers to the complete breakdown of glucose molecules in the presence of oxygen to yield energy. On the contrary, anaerobic respiration refers to the partial breakdown of glucose in the absence of oxygen to yield reduced energy. Based on these two modes of respiration, bacteria can also be classified into aerobic and anaerobic bacteria. One such example of anaerobic bacteria is Actinomyces, while an example of aerobic bacteria is Mycobacterium.

Based on gram stain sensitivity

In 1884, biologist H. C. Gram proposed this widely used mode of bacterial classification. Gram devised a special stain based on the particular cell wall and peptidoglycan arrangements that shows specific reactions for different bacteria. Classes of bacteria that show positive gram stain are gram-positive bacteria, while those showing no stain are gram-negative bacteria.

Gram-positive bacteria: Bacteria that show positive results on coming in contact with gram stain belong to this category. Both rod and cocci-shaped bacteria elicit gram-positive results. Examples: Bacillus, Clostridium, Staphylococcus, etc. These bacteria have large peptidoglycan structures. They are capable of spores' formation that facilitate colonization.

Gram-negative bacteria: These bacteria show no result on coming in contact with gram stain. Such bacteria have comparatively smaller peptidoglycan structures with an additional cytoplasmic membrane on the outside, and this membrane facilitates transport mechanism across the bacterial body with the external environment. Examples: E. coli, Salmonella, Shigella, etc.


  • Bacteria are almost found throughout every terrain on earth and thus possess the ability to sustain in a variety of temperatures.
  • Bacteria resemble prokaryotic cellular structures, with most cell organelles being absent.
  • Bacteria are usually classified as their shape or sensitivity towards gram stain.

Differences between Gram Positive and Gram Negative Bacteria Source


1. What is bacteria and what are the four structures of bacteria?

Bacteria are unicellular microorganisms that generally resemble primitive cell structures. Based on the shape, bacteria are classified into four structures. They are bacilli, cocci, spirilla, and vibrio.

2. What are the 3 classifications of bacteria?

Bacteria are usually classified based on shape, sensitivity to gram stain, and their mode of respiration.

3. What are the 5 characteristics of bacteria?

  • Unicellular or single-celled organism
  • Absence of various cell organelles, such as mitochondria, specific nucleus, etc.
  • Presence of cell wall
  • The presence of DNA is the only form of genetic material

4. Why do we classify bacteria?

Microbiologists classify bacteria to differentiate one organism from another, and this also helps to identify harmful bacteria from the good ones.

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