Mycoplasmas represent a distinct group of bacteria, recognized as the smallest self-replicating organisms known. They are classified under the class Mollicutes and the order Mycoplasmatales. These microorganisms are unique among prokaryotes due to several structural differences that set them apart from most other bacteria.
The Missing Cell Wall
A defining feature of mycoplasmas is the absence of a rigid cell wall, unlike most other bacteria. This absence means mycoplasmas do not possess peptidoglycan, the primary component of bacterial cell walls. Unlike walled cells, they are sensitive to osmotic changes in their environment, meaning significant shifts in salt concentration can cause them to burst.
The lack of a rigid cell wall allows mycoplasmas to adopt various shapes, a characteristic known as pleomorphism. They can appear as small spherical forms, but also as elongated, filamentous, or even flask-shaped structures, ranging in diameter from approximately 0.2 to 0.8 micrometers. This structural flexibility allows them to deform and squeeze through filters that would typically trap other bacteria.
Inside the Mycoplasma Cell
Despite lacking a cell wall, mycoplasmas are enclosed by a triple-layered cytoplasmic membrane. This membrane is rich in sterols, such as cholesterol, which are not synthesized by the organism but are acquired from their environment or host. These sterols contribute to the membrane’s durability and provide structural integrity in the absence of a cell wall.
The cytoplasm within the mycoplasma cell contains ribosomes and a nucleoid region. Their genetic material is a single circular, double-stranded DNA molecule, which is not enclosed within a nucleus. Mycoplasmas are notable for having extremely small genomes, typically ranging from less than 600 to over 2,200 kilobase pairs, with some species having fewer than 500 genes. This minimal genetic content is significantly smaller than that of other bacteria like Escherichia coli.
How Structure Shapes Mycoplasma Behavior
The unique structure of mycoplasmas has significant implications for their behavior and survival strategies. Their lack of a cell wall makes them naturally resistant to many common antibiotics, such as beta-lactams (e.g., penicillin), which specifically target cell wall synthesis, rendering these antibiotics ineffective against mycoplasmal infections.
Their parasitic or saprophytic nature means they often depend on host organisms for various growth factors, as their reduced genomes limit their ability to synthesize all necessary components. For example, many mycoplasmas require cholesterol for growth, a unique characteristic among prokaryotes. Some pathogenic species, like Mycoplasma pneumoniae, adhere to host cells, such as those in the respiratory tract, using specialized attachment organelles and adhesin proteins, leading to localized tissue damage through the release of toxic metabolites.