Mycoplasma refers to a genus of bacteria distinguished by its extremely small size and unique cellular structure. This organism is a parasitic or commensal bacterium, meaning it lives in or on a host, such as humans, animals, or plants, to obtain necessary nutrients. Mycoplasma species are responsible for a range of human infections, particularly those affecting the respiratory and urogenital tracts.
Defining the Smallest Bacteria
The organisms belonging to the genus Mycoplasma are among the smallest free-living, self-replicating organisms known. They typically measure between 0.2 and 0.3 micrometers in diameter, significantly smaller than the average bacterium (1 to 10 micrometers). Due to their diminutive size, they were historically difficult to classify and were once informally called Pleuropneumonia-Like Organisms (PPLO).
These bacteria possess one of the smallest genomes among all self-replicating cells, reflecting a minimalistic genetic content adapted for a parasitic lifestyle. Their reliance on a host for many biosynthetic building blocks means they must inhabit environments with a controlled supply of nutrients. Over 100 species of Mycoplasma have been identified; while many are harmless, several are significant pathogens in humans and other animals.
The Crucial Absence of a Cell Wall
The singular feature defining Mycoplasma is the complete absence of a rigid cell wall, a structure composed of peptidoglycan present in nearly all other bacteria. This structural difference places them in a separate class called Mollicutes, derived from the Latin word meaning “soft skin.” This lack of a fixed outer layer has two major biological consequences.
First, without a cell wall, the bacteria are not constrained to a single shape, a characteristic known as pleomorphism. They can contort into various forms, ranging from spherical to filamentous, allowing them to squeeze through filters that trap other bacteria. Second, because the cell wall normally provides protection against osmotic pressure changes, Mycoplasma incorporates sterols, such as cholesterol, into its single plasma membrane for structural strength and rigidity.
Common Human Infections
Several Mycoplasma species cause human disease, primarily targeting the respiratory and urogenital tracts. The most common human pathogen is Mycoplasma pneumoniae, responsible for up to 15 to 20 percent of community-acquired pneumonia cases. This lung infection is often mild and is colloquially known as “walking pneumonia,” highlighting the ability of infected individuals to remain mobile.
Symptoms of M. pneumoniae infection typically begin gradually, appearing one to four weeks after exposure, and include a persistent dry cough, sore throat, headache, and low-grade fever. Transmission occurs through respiratory droplets, often requiring prolonged close contact, which leads to outbreaks in crowded settings like schools or dormitories.
Another significant human pathogen is Mycoplasma genitalium, which is spread through sexual contact and is considered a sexually transmitted infection (STI). M. genitalium often causes few to no symptoms, but it can lead to urethritis—inflammation of the urethra—in both men and women, resulting in pain during urination or abnormal discharge. In women, an untreated infection may progress to cervicitis or pelvic inflammatory disease (PID), increasing the risk of ectopic pregnancy and infertility.
Other Pathogens
Other species, such as Mycoplasma hominis, are sometimes associated with pelvic inflammatory disease and infections in newborns, particularly in individuals with compromised immune systems.
Specialized Diagnostic and Treatment Needs
The unique cell structure of Mycoplasma dictates a specialized approach to treatment. Since the bacteria lack a cell wall, they are naturally resistant to common antibiotics like penicillins and cephalosporins. These drugs interfere with peptidoglycan cell wall synthesis, making them ineffective against Mycoplasma infections.
Effective treatment relies on antibiotics that target other bacterial processes, such as protein synthesis. Macrolides (azithromycin and clarithromycin) are often the first-line choice, along with tetracyclines like doxycycline, which inhibit the bacteria’s ribosomal machinery. Fluoroquinolones may be used as a secondary treatment option, particularly when macrolide resistance is suspected.
Diagnosis of Mycoplasma infections is challenging because the organisms grow very slowly in laboratory cultures, sometimes taking weeks to produce colonies. Healthcare providers frequently rely on advanced molecular techniques, such as Polymerase Chain Reaction (PCR) tests, which detect the bacteria’s specific genetic material. Serological testing, which measures the body’s antibody response, is also used to confirm the infection.