In order to determine the possible asymptomatic carriage of Mycoplasma pneumoniae in the upper respiratory tracts of children, Emiel Spuesens and colleagues investigate the prevalence of M. pneumoniae in symptomatic and asymptomatic children at a hospital in The Netherlands. Please see later in the article for the Editors' Summary

The gene encoding major adhesin protein P1 of Mycoplasma pneumoniae, MPN141, contains two DNA sequence stretches, designated RepMP2/3 and RepMP4, which display variation among strains. This variation allows strains to be differentiated into two major P1 genotypes (1 and 2) and several variants. Interestingly, multiple versions of the RepMP2/3 and RepMP4 elements exist at other sites within the bacterial genome. Because these versions are closely related in sequence, but not identical, it has been hypothesized that they have the capacity to recombine with their counterparts within MPN141, and thereby serve as a source of sequence variation of the P1 protein. In order to determine the variation within the RepMP2/3 and RepMP4 elements, both within the bacterial genome and among strains, we analysed the DNA sequences of all RepMP2/3 and RepMP4 elements within the genomes of 23 M. pneumoniae strains. Our data demonstrate that: (i) recombination is likely to have occurred between two RepMP2/3 elements in four of the strains, and (ii) all previously described P1 genotypes can be explained by inter-RepMP recombination events. Moreover, the difference between the two major P1 genotypes was reflected in all RepMP elements, such that subtype 1 and 2 strains can be differentiated on the basis of sequence variation in each RepMP element. This implies that subtype 1 and subtype 2 strains represent evolutionarily diverged strain lineages. Finally, a classification scheme is proposed in which the P1 genotype of M. pneumoniae isolates can be described in a sequence-based, universal fashion.

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