Infection is transmitted via aerosols from person to person. Such gene divergences within the P1 adhesin and development of subtype specific antibodies, following initial infection, might account for the frequency of re-infection, which may be due to another subtype. Different P1 adhesin subtypes may be operating in the development and cycling times of M. pneumoniae clinical isolates and grouped them into eight subtypes within two genomic groups based on P1 adhesin subtypes. The long incubation period, relatively low transmission rate, and persistence of organisms in respiratory tract for variable periods following infections might explain the prolonged duration of epidemics. On the basis of serological studies, Lind et al, showed a pattern of endemic disease transmission over a period of 50 years, punctuated with cyclic epidemics every three to five years. Climate, seasonality, and geography are not thought to be of major significance, although most outbreaks in USA tended to occur during late summer and early fall. pneumoniae infections can involve both the upper and lower respiratory tract and occurred worldwide in a endemic fashion with epidemic spurt at four to seven year intervals. Immunity to mycoplasma is transient and recurrences are frequent This concept of immune- mediated lung disease provides a basis for consideration of immuno-modulatory therapeutics in addition to conventional antimicrobial therapies. The more vigorous the cell-mediated immune response and cytokine stimulation, more severe is the clinical illness and pulmonary injury. pneumoniae lung disease has been a subject of considerable interest during past several years.Д¬urrent evidence from human and animal studies suggests that cytokine production and lymphocyte activation may either minimize disease through the enhancement of host defense mechanisms or exacerbate disease through immunological lesion development. Production of multiple arrays of cytokines and other reactive substances in the pathogenesis of M. Mycoplasma is also being incriminated in the development of autoimmunity which can explain the extra pulmonary involvement. Hydrogen peroxide is produced locally, which has a cytopathic effect on airway epithelium and cilia and is responsible for persistent cough. Cytoadhesion protects mycoplasmas from mucociliary clearance. It is an extra cellular pathogen, its survival depends on adherence to the respiratory epithelium, and this fixation to ciliary membranes is primarily by interactive adhesion and accessory proteins. It explains the parasitic and saprophytic existence of the organisms and its fastidious growth requirements which may complicate its detection by culture. The extremely small size (150-200 nm) and its limited metabolic and biosynthetic capabilities are responsible for many of the biological characteristics of the organisms. So phylogenetically, Mycoplasmas are more closely related to gram positive bacteria. Maniloff proposed that the small genome size is due to gradual reduction in genome size from a common gram positive ancestor by the process of degenerative evolution. Mycoplasmas have an extremely small genome (0.58 to 2.20Mb) compared with the 4.64Mb of E. The absence of cell wall structure makes these organisms insensitive to beta-lactam anti microbial agents, prevents them from staining by gram's stain, and is largely responsible for their polymorphism. Mycoplasmas are distinguished from bacteria by the lack of cell wall structure. It belongs to the class Mollicutes and family Mycoplasmataceae. Mycoplasma is the smallest and simplest self limiting bacteria. MOLECULAR TAXONOMY AND BIOLOGICAL CHARACTERISTICS Of the non-pulmonary manifestation, neurological manifestations are thought to be the most common. Apart from respiratory tract infections, this organism is also responsible for producing a wide spectrum of non-pulmonary manifestations including neurological, hepatic, cardiac diseases, hemolytic anemia, polyarthritis and erythema multiforme. pneumoniae is an important cause of respiratory tract infections. Of the many species of Mycoplasma known to infect man, M. Initially it was known as Eaton agent, after Eaton et al, identified this plero-pulmonary like organisms from the sputum of patients with primary atypical pneumonia in 1944. pneumoniae is an important cause of acute respiratory tract infection, especially as a potential etiology of the clinical entity termed 'atypical pneumonia'. Mycoplasma pneumonia is a common respiratory pathogen that produces diseases of varied severity ranging from mild upper respiratory tract infection to severe atypical pneumonia.

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