Adaptation to the environment: Streptococcus pneumoniae, a paradigm for recombination-mediated genetic plasticity?

Genetic plasticity plays a central role in the biology of the human pathoge n Streptococcus pneumoniae. This is illustrated by the existence of at leas t 90 different capsular types (the polysaccharide capsule has an essential antiphagocytic function) as well as by the rapid emergence of penicillin-re sistant (Pen(R)) pneumococcal isolates. Natural genetic transformation is b elieved to be essential for this genetic plasticity; capsular types can be switched by intraspecies transformation, whereas interspecies transformatio n is responsible for the appearance, in the PenR isolates, of mosaic pbp ge nes, which encode proteins with reduced affinity for penicillin. Data on th e regulation of competence for transformation in S. pneumoniae, on the cont rol of intra- and interspecies genetic exchange and on the shuffling and ca pture of exogenous sequences during transformation are reviewed, Possible l inks between transformation and changes in environmental conditions are dis cussed, and the adaptive 'strategy' deduced for S. pneumoniae is compared w ith that of Escherichia coli.