is an important cause of respiratory tract infections in children as well as adults that can range in severity from mild to life-threatening. Over the past several years there has been much new information published concerning infections caused by this organism. New molecular-based tests for detection are now commercially available in the United States, and advances in molecular typing systems have enhanced understanding of the epidemiology of infections. More strains have had their entire genome sequences published, providing additional insights into pathogenic mechanisms. Clinically significant acquired macrolide resistance has emerged worldwide and is now complicating treatment. susceptibility testing methods have been standardized, and several new drugs that may be effective against this organism are undergoing development. This review focuses on the many new developments that have occurred over the past several years that enhance our understanding of this microbe, which is among the smallest bacterial pathogens but one of great clinical importance.

Since its initial description in the 1940s and eventual elucidation as a highly evolved pathogenic bacterium, Mycoplasma pneumoniae has come to be recognized as a worldwide cause of primary atypical pneumonia. Beyond its ability to cause severe lower respiratory illness and milder upper respiratory symptoms it has become apparent that a wide array of extrapulmonary infectious and postinfectious events may accompany the infections in humans caused by this organism. Autoimmune disorders and chronic diseases such as asthma and arthritis are increasingly being associated with this mycoplasma, which frequently persists in individuals for prolonged periods. The reductive evolutionary process that has led to the minimal genome of M. pneumoniae suggests that it exists as a highly specialized parasitic bacterium capable of residing in an intracellular state within the respiratory tissues, occasionally emerging to produce symptoms. This review includes discussion of some of the newer aspects of our knowledge on this pathogen, characteristics of clinical infections, how it causes disease, the recent emergence of macrolide resistance, and the status of laboratory diagnostic methods.

Mycoplasma pneumoniae is a common cause of upper and lower respiratory tract infections in persons of all ages and may be responsible for up to 40% of community-acquired pneumonias. A wide array of extrapulmonary events may accompany the infections caused by this organism, related to autommunity or direct spread. This review includes a discussion of the latest knowledge concerning the molecular pathological basis of mycoplasmal respiratory disease, how the organism interacts with the host immune system and its association with the development of chronic conditions such as asthma, recent emergence of macrolide resistance and the status of laboratory diagnostic methods. Keywordsantimicrobial resistance; asthma; community-acquired pneumonia; cytadherence; enzyme-linked immunoassay; Mycoplasma pneumoniae; PCR Although more than 200 species in the genus Mycoplasma are now recognized, relatively few are pathogenic in humans. The best known and most intensely studied of these species is Mycoplasma pneumoniae. The initial descriptions of M. pneumoniae as a human pathogen, realization that it was not a virus, characterization of clinical manifestations of mycoplasmal respiratory disease, mode and extent of transmission, and development of serological assays began more than 40 years ago. However, very little was known at that time about how this mycoplasma interacts with and damages host cells, affects the immune system, and the extent to which it may mediate illness outside of the respiratory tract.Progress in understanding the biological properties of M. pneumoniae and its true role as a human pathogen have been hindered significantly over the years by its very slow replication rate (∼6 h), fastidious demands for successful laboratory cultivation and the relatively low sensitivity and specificity of the earliest complement fixation serological tests, which were much better suited for less antigenically complex viral pathogens. Until recent years, as more sophisticated laboratory techniques have become available, dependence on nonstandardized sero-logical tests performed in reference laboratories requiring measurement of antibodies in

SummaryMycoplasma pneumoniae is a minimal microbe with respect to cell envelope composition, biosynthetic and regulatory capabilities and genome size, yet it possesses a remarkably complex, multifunctional terminal organelle. This membrane-bound extension of the mycoplasma cell is defined by the presence of an electron-dense core that appears as paired, parallel bars oriented longitudinally and enlarging at the distal end to form a terminal button. Most non-cytadhering mutants of M. pneumoniae isolated to date exhibit defects in the architecture of the terminal organelle. Detailed characterization of those mutants has revealed the identities of many component proteins of the terminal organelle as well as the likely order in which some of those components are required. Additional questions regarding the composition of the electron-dense core, the means by which the terminal organelle is duplicated during cell division and the manner in which this process is regulated remain to be answered. Thus, it seems that there is much to be learned about cellular engineering and spatial regulation in these 'simple' cell wall-less bacteria.

The surface protein P65 is a constituent of the Mycoplasma pneumoniae cytoskeleton and is present at reduced levels in mutants lacking the cytadherence accessory protein HMW2. Pulse-chase studies demonstrated that P65 is subject to accelerated turnover in the absence of HMW2. P65 was also less abundant in noncytadhering mutants lacking HMW1 or P30 but was present at wild-type levels in mutants lacking proteins A, B, C, and P1. P65 exhibited a polar localization like that in wild-type M. pneumoniae in all mutants having normal levels of HMW1 and HMW2. Partial or complete loss of these proteins, however, correlated with severe reduction in the P65 level and the inability to localize P65 properly.Mycoplasma pneumoniae is a major cause of bronchitis and pneumonia in humans. Adherence of this cell wall-less bacterium to host respiratory epithelium (cytadherence) is pivotal to successful colonization and ensuing pathogenesis (6) and is mediated largely by a differentiated terminal structure, the attachment organelle, which is also believed to function in gliding motility and cell division (reviewed in reference 13). Protein P1 is a major adhesin (12) and localizes primarily to the attachment organelle in wild-type M. pneumoniae cells. Loss of HMW2, whether by frameshift mutation or transposon insertion, results in the inability to cytadhere and reduced levels of the cytadherence-associated proteins HMW1, HMW3, and P65 (8,11,15). These proteins are components of a Triton X-100 (TX)-insoluble network that comprises the mycoplasma cytoskeleton, or Triton shell, and are collectively required for the development of a fully functional attachment organelle, including the proper localization of the adhesin P1 to this structure (2,9,13,14,24).Proteins HMW1 and HMW3 are largely dissimilar but have in common a central acidic and proline-rich (APR) domain which is defined by its amino acid composition but not its sequence (1,7,20). The genes for HMW1 and HMW3 are coexpressed as part of a large transcriptional unit, the hmw operon (7, 30). Both proteins exhibit a distinctive subcellular distribution (25-27); HMW3 is a major component of the attachment organelle, while HMW1 localizes to the filamentous extensions of the mycoplasma cell, including the attachment organelle. The loss of HMW1 and HMW3 in hmw2 mutants occurs posttranslationally, probably a consequence of accelerated turnover by housekeeping protease activity (21). Newly synthesized HMW1 in the cytoplasmic pool associates with the mycoplasma cytoskeleton and is translocated to the mycoplasma surface as a peripheral membrane protein. Translocation and stabilization of HMW1 are much less efficient in the absence of HMW2, resulting in its removal (1).Like HMW1 and HMW3, protein P65 is a component of the M. pneumoniae Triton shell (23) and is found at significantly reduced levels in hmw2 mutants (15). Furthermore, like HMW1, P65 is peripherally associated with the mycoplasma membrane (22; M. F. Balish and D. C. Krause, unpublished data). P65 contains an APR domain (23), whic...