One would anticipate that almost 4 decades of research and clinical experience should leave very little room for ongoing investigation of Mycoplasma pneumoniae. Nevertheless, respiratory infection caused by this pathogen remains common, and advances in science continue as unique attributes of the bacterium and the associated clinical illness unfold.

Microbiologic and subsequently other investigations possibly have been delayed in part by the nature of the bacterium. Mycoplasmas are the smallest bacteria, and M pneumoniae in particular is sufficiently fastidious in vitro that few laboratories have provided culture services. Whereas Escherichia coli from a urine sample or Streptococcus pneumoniae from a cerebrospinal fluid specimen may be cultivated luxuriously in 18 to 24 hours, the isolation of M pneumoniae from sputum may require 7 to 21 days.

These small, cell wall-deficient bacteria are impossible to visualize in respiratory secretions by either Gram stain or other light microscopic methods. In contrast to...

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