Mycoplasmas are a distinct group of microorganisms, classified as bacteria, yet possessing unusual characteristics. Their most striking feature is their exceptionally small size and complete lack of a rigid cell wall, which significantly influences their biology and interactions with other organisms. These characteristics make them a fascinating subject for understanding the minimal requirements for life.
Defining Mycoplasma Size
Mycoplasmas are the smallest known free-living bacteria capable of growth and reproduction in non-living culture media. Their diameter ranges from 0.2 to 0.8 micrometers (µm), with some species like Mycoplasma genitalium measuring as small as 0.2-0.3 µm. This small size is directly related to their unique structure, specifically the absence of a cell wall.
The lack of a rigid cell wall allows mycoplasmas to be highly flexible and pleomorphic, assuming various shapes from spherical to filamentous. This adaptability enables them to squeeze through tiny spaces, including filters that block other, larger bacteria. This contributes to their designation as “filterable” bacteria.
Implications of Their Smallness
The small size of mycoplasmas has implications for their biology and interaction with hosts. Their small stature allows them to pass through filters used to sterilize laboratory media, which have pore sizes of 0.2 micrometers. This filterability makes them a frequent and challenging contaminant in cell cultures, often going unnoticed due to the absence of visible turbidity.
Mycoplasmas possess small genomes, correlating with their simplified cellular machinery and parasitic or commensal lifestyles. For instance, the genome of Mycoplasma genitalium is approximately 580 kilobase pairs (kb) and contains around 470-480 genes, among the smallest bacterial genomes. This reduced genetic material reflects their reliance on host cells for essential nutrients, as they lack genes for many metabolic pathways. Their unique features also influence their ability to infect hosts, potentially evade immune responses, and sometimes survive intracellularly. Additionally, their small size makes them difficult to observe with standard light microscopes, necessitating specialized detection methods like PCR-based assays or fluorescent DNA staining.
Mycoplasma Size in Perspective
To grasp the scale of mycoplasmas, it helps to compare them with other microscopic entities. While among the smallest bacteria, mycoplasmas are larger than most viruses. For example, Mycoplasma species measure 0.2 to 0.3 micrometers, whereas many viruses are smaller, some only about 10 nanometers in diameter. However, some of the largest viruses, like Pandoraviruses, can reach up to 1 micrometer in length.
Compared to typical bacteria such as Escherichia coli, which can be 1 micrometer in diameter and 1-2 micrometers long, mycoplasmas are considerably smaller. This difference highlights their unique position at the lower end of the bacterial size spectrum. Relative to human cells, mycoplasmas are minuscule. A human red blood cell, for instance, is approximately 6-8 micrometers in diameter. If a human cell were the size of a football field, a mycoplasma would be comparable to the size of a golf ball.