Size, Shapes and Arrangements of Bacterial Cells - Complete Guide

By Momina Shehzadi | BS Zoology Graduate, Pakistan

In this article, we will learn Size, Shapes and Arrangements of Bacterial Cells - Cocci, Bacilli, Spirilla - in simple words with diagrams. This topic is very important for BS Zoology and Microbiology students.

Bacterial cells show great variation in their size, shape and arrangement. These characteristics are important for the identification and classification of bacteria. For BS Zoology students, this is one of the most important topics of Chapter 3.

1. SIZE OF BACTERIAL CELLS

The unit of bacterial measurement is the micrometer (µm). One micrometer equals one thousandth (1/1000) of a millimeter, or about 1/25,000 of an inch.

Majority of bacteria range from 0.5 to 1.0 µm in length.

Examples of Bacterial Size:

Staphylococci and Streptococci: Diameters ranging from 0.75 µm to 1.25 µm.

Rod forms such as Clostridium tetani: Ranges from 2-5 µm in length and from 0.3 to 0.8 µm in width.

Mycobacterium tuberculosis hominis: The bacterium responsible for tuberculosis in man, is rod-shaped varying from 0.5 to 5.0 µm in length and 0.3 to 0.6 µm in breadth.

Smallest Bacteria - Dialister pneumosintes: Found in nasopharynx during early stage of influenza. It measures 0.15 µm to 0.2 µm.

Largest Bacterium - Bacillus buschlii: Found in cockroach. It measures 3.0 µm to 6.0 µm in width, and up to 8.0 µm in length.

Largest of all - Sulphur bacteria Thiophysa volutans: Has a diameter of about 18 µm.

2. SHAPES AND ARRANGEMENT OF BACTERIA

The shape of a bacterium is determined by its rigid cell wall.

Typical bacterial cells have 3 main shapes:

Cocci (singular, coccus): Spherical or oval shaped.

Bacilli (singular, bacillus): Straight rods.

Spirilla (singular, spirillum): Helically curved rods.

Some bacteria are pleomorphic, i.e., they can exhibit a variety of shapes.

Other shapes include:

Mycelium: Long, branched, multinucleate filaments found in Streptomyces.

Filament: Long thread-like.

Vibrio: Less than one complete twist (comma shaped).

Spirochetes: Highly flexible and can twist (e.g., Treponema pallidum, Borrelia anserina).

Spirillum: Rigid helical bacteria (e.g., Spirillum volutans).

3. ARRANGEMENT OF BACTERIA

Bacterial cells are usually arranged in a manner characteristic of their particular species.

A) Cocci (Oval Shaped Bacteria) Arrangement:

Cocci are arranged according to planes of division.

i) Division in One Plane:

Diplococcus arrangement: When cocci are present in pairs (e.g., Diplococcus pneumoniae).

Streptococcus arrangement: When cocci are present in long chain of cells.

ii) Division in Two Planes: 

Tetrad arrangement: A square of 4 cocci is formed.

iii) Division in Three Planes :

Sarcina arrangement: A cube of 8 cocci is formed (Octet).

iv) Division in Random Planes:

Staphylococcus arrangement: Cocci are arranged in irregular, grape-like clusters (e.g., Staphylococcus aureus).

B) Bacilli (Rod Shaped Bacteria) Arrangement:

Bacilli all divide in one plane producing three kinds of arrangements:

Bacillus: Single cell of bacteria.

Diplobacillus: When rod shaped bacteria occur in pairs.

Streptobacillus: It is a chain of bacilli.

Coccobacillus: It is in between rod and spherical.

Examples: Escherichia coli, Bacillus subtilis, Pseudomonas.

C) Other Uncommon Shapes:

In addition to common shapes, many others also occur:

1. Pear shaped cells (e.g., Pasteuria).

2. Lobed spheres (e.g., Sulfolobus).

3. Rods with squared ends (e.g., Bacillus anthracis).

4. Disks arranged like stacks of coins (e.g., Caryophanon).

5. Rods with helically sculptured surfaces (e.g., Seliberia).

Conclusion:

 For final year BS Zoology and Microbiology exams, you must remember the size range and all arrangements with examples. This topic is frequently asked in short questions.

 FAQs for Students - Size, Shapes and Arrangements of Bacteria

Q1: What is the unit of bacterial measurement?

Ans: The unit of bacterial measurement is the micrometer (µm). One micrometer is equal to 1/1000 of a millimeter. Majority of bacteria range from 0.5 to 1.0 µm in diameter.

Q2: What are the 3 main shapes of bacteria?

Ans: The three main shapes of bacteria are:

1. Cocci (Coccus): Spherical or oval shaped bacteria.

2. Bacilli (Bacillus): Rod shaped bacteria.

3. Spirilla (Spirillum): Spiral or helically curved bacteria.

Q3: What is the difference between Diplococcus and Streptococcus?

Ans: Both are arrangement of Cocci in one plane.

Diplococcus: Cocci are present in pairs (e.g., Diplococcus pneumoniae).

Streptococcus: Cocci are present in a long chain of cells (e.g., Streptococcus pyogenes).

Q4: What is Tetrad and Sarcina arrangement?

Ans:

Tetrad: When cocci divide in two planes, they form a square of 4 cocci.

Sarcina: When division is in three planes, it produces a cube of 8 cocci, called Sarcina or Octet.

Q5: What is Staphylococcus arrangement?

Ans: When division occurs in random planes, cocci are arranged in irregular, grape-like clusters. This is called Staphylococcus arrangement. Example is Staphylococcus aureus which causes many infections.

Q6: What are the smallest and largest bacteria?

Ans: Smallest Bacteria: Dialister pneumosintes (0.15 to 0.2 µm), found in nasopharynx.

Largest Bacteria: Bacillus buschlii (up to 8.0 µm long), found in cockroach. And Thiophysa volutans (18 µm diameter) is considered the largest amongst all bacteria.

Q7: What is the difference between Spirilla and Spirochetes?

Ans:

Spirilla (e.g., Spirillum volutans): Rigid helical bacteria.

Spirochetes (e.g., Treponema pallidum): Highly flexible bacteria that can twist and contort their shape. Treponema pallidum causes Syphilis.

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Written by Momina Shehzadi, BS Zoology Graduate from Pakistan.