Mycoplasma pneumoniae

Mycoplasma pneumoniae is a bacterium, but an unusual one. It belongs to a class called Mollicutes, and its cells are extremely small and completely lack a cell wall. In humans, it is the cause of Mycoplasma pneumonia, a type of atypical bacterial pneumonia that is also linked to cold agglutinin disease. It ranks among the tiniest organisms capable of replicating on their own.

The story of its discovery began in 1898, when researchers Nocard and Roux isolated a microbe from cattle with pneumonia. This microbe resembled other organisms soon found in various animals, known as pleuropneumonia-like organisms (PPLOs), which were later tied to pneumonias and arthritis in those species. A major step forward came in 1944, when Monroe Eaton grew an agent from human pneumonia cases inside embryonated chicken eggs; this became known as the "Eaton agent." Because it could be cultivated in eggs (a method then used for viruses) and because antibiotics worked against the infection, its true nature was unclear. In 1961, NIH researcher Robert Chanock, who was studying the Eaton agent, visited Leonard Hayflick at the Wistar Institute. Chanock suspected the agent might not be a virus. Hayflick, whose doctoral work had involved PPLOs in animals, suggested it could be a mycoplasma. Chanock sent him egg yolk containing the agent, and Hayflick, using a new culture medium he had developed, isolated a unique mycoplasma. Chanock and Hayflick then proved this organism caused human pneumonia. The leading expert on these organisms, Emmy Klieneberger-Nobel, proposed naming it *Mycoplasma hayflickiae*, but Hayflick declined, and it was instead named *Mycoplasma pneumoniae*.

Taxonomically, *M. pneumoniae* sits within the Mollicutes, a group defined by the absence of a peptidoglycan cell wall. This lack makes them naturally resistant to antibiotics like beta-lactams that target cell wall construction. Mycoplasmas have a stripped-down genome and simple metabolism, making them obligate parasites that depend on their host for many resources. *M. pneumoniae* uses a specialized attachment organelle to stick to cells in the respiratory tract, which also helps it move and invade. Its ability to persist after treatment is partly due to its mimicking of host cell surface molecules.

The bacterium causes disease by adhering to host cells and releasing cytotoxic agents. This leads to loss of cilia and the release of hydrogen peroxide, driving respiratory symptoms and complications like bronchial asthma and chronic obstructive pulmonary disease. It also produces a unique toxin called CARDS, which fuels inflammation and breathing problems. Treatment usually involves macrolides or tetracyclines, which stop protein synthesis. However, resistance—especially in Asia—is on the rise, mostly from mutations in the 23S rRNA gene that block macrolide binding, making management harder and requiring alternative therapies.