Mycoplasma, Definition, Characteristics, and Cell Structure

Mycoplasmas are considered the smallest living organisms on Earth that completely lack a cell wall. They resemble other microorganisms such as bacteria, chlamydia, rickettsia, and viruses, being the smallest free-living cells known. This topic is important for the NEET exam and the biology syllabus , as it covers key concepts related to microbial structure, pathogenicity, and classification.

Definition of Mycoplasma

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Characteristics of Mycoplasma

• Mycoplasmas are very small, non-motile, prokaryotic organisms, with the exception of Spiroplasma. They can be parasitic or saprophytic.
• They do not have a cell wall. Instead, the cytoplasmic membrane, a three-layered structure, forms the outer boundary of the cell.
• Mycoplasmas need sterols for their growth and are sensitive to environmental factors like desiccation and vibrations.
• Mycoplasmas have a small genome, roughly 1/5 to 1/2 the size of bacterial genomes. Their DNA is a circular, double-stranded chromosome.
• The DNA is fibrillary, with a molecular weight between 44 x 10^6 to 1200 x 10^6 daltons, and the guanine-cytosine ratio ranges from 24-30%.
• They multiply through binary fission or budding.
• Mycoplasma cells contain 4% DNA and 8% RNA , with 70S ribosomes in the cytoplasm, similar to bacteria.
• Mesosomes are absent in mycoplasma cells, but 50-80% of their dry weight consists of proteins, with RNA making up 8-17% and DNA comprising 4-7%.
• Mycoplasmas do not respond to Gram staining, making them Gram-negative. A simpler staining method called Dien’s stain is used to detect them.

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Cell Structure of Mycoplasma

• Ultra-Structure : Mycoplasma cells are prokaryotic, and their outermost boundary is the plasma membrane. This membrane is composed of lipoprotein in three layers, with phospholipids and cholesterol as chemical components. It is selectively permeable and measures about 80-100 Å in thickness.
• Small Size : Mycoplasma cells are very small, ranging from 300 nm to 800 nm in diameter.
• Absence of Rigid Cell Wall : Mycoplasmas lack a rigid cell wall. Instead, they have a triple-layered lipoprotein membrane, about 10 nm thick, that surrounds the cytoplasm.
• Cytoplasm Composition : The cytoplasm contains RNA (ribosomes) and DNA. The ribosomes are of the 70S type and are approximately 14 nm in diameter.
• DNA Structure: Mycoplasma DNA is a double-stranded helix, with lower guanine and cytosine content compared to bacterial DNA.
• Nucleic Acid Content: DNA makes up about 4%, and RNA accounts for about 8% of the total cell content, which is lower than in most other organisms.
• Guanine and Cytosine Content: The guanine and cytosine (G+C) content in the DNA ranges from 23% to 46%.
• Presence of Bleb: In some species like Mycoplasma gallisepticum, polar bodies known as blebs protrude from one or both ends of the cell. These blebs are believed to be the sites of enzymatic activity and help in attachment during infection.
• Reproduction: Mycoplasma reproduces through fragmentation, budding, and the formation of young elementary bodies. The production of elementary bodies is a significant mode of reproduction.

Difference Between Mycoplasma and Bacteria

MCQs of Mycoplasma

Q1. Which one of the following is an incorrect statement regarding mycoplasma?

1. They lack a cell wall.
2. They are the smallest living cells.
3. They cannot survive without oxygen.
4. They are pathogenic in plants and animals.

Q2. The smallest organisms that cause diseases among plants are;

1. Mycoplasma
2. Fungi
3. Bacteria
4. Viruses

Q3. Here are a few statements given below to identify organisms on the basis of the statement:

1. Lack of cell wall.

2. Smallest living cell known.

3. Can survive without oxygen.

4. Pathogenic in animals and plants.

1. Nostoc
2. Anabaena
3. Mycoplasma
4. Chlorella

Answers of MCQs of Mycoplasma