Genetic plasticity plays a central role in the biology of the human
pathogen
Streptococcus pneumoniae. This is illustrated by the
existence of at least 90 different capsular types (the
polysaccharide capsule has an essential antiphagocytic function) as
well as by the rapid emergence of penicillin-resistant (PenR)
pneumococcal isolates. Natural genetic transformation is believed
to be essential for this genetic plasticity; capsular types can be
switched by intraspecies transformation, whereas interspecies
transformation is responsible for the appearance, in the PenR
isolates, of mosaic pbp genes, which encode proteins with
reduced affinity for penicillin. Data on the regulation of
competence for transformation in
S. pneumoniae, on the control
of intra- and interspecies genetic exchange and on the shuffling and
capture of exogenous sequences during transformation are
reviewed. Possible links between transformation and changes in
environmental conditions are discussed, and the adaptive
‘strategy’ deduced for
S. pneumoniae is compared with that of
Escherichia coli.
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