Penicillin-resistant strains of Streptococcus
pneumoniae possess altered forms ofpenicillin-binding proteins
(PBPs) with decreased affinity for penicillin. The PBP2B genes
of these strains have a mosaic structure, consisting of regions
that are very similar to those in penicillin-sensitive strains,
alternating with regions that are highly diverged. Penicillinresistant
strains of viridans group streptococci (e.g., S. sanguis
and S. oralis) that produce altered PBPs have also been
reported. The PBP2B genes of two penicillin-resistant clinical
isolates of S. sanguis were identical in sequence to the mosaic
class B PBP2B genes found in penicillin-resistant serotype 23
strains of S. pneumoniae. Emergence of penicillin resistance
appears to have occurred by the horizontal transfer of an
altered PBP2B gene from penicillin-resistant S. pneumoniae
into S. sanguis. The PBP2B genes of three penicillin-resistant
S. oralis strains were similar to the mosaic class B PBP2B gene
of penicillin-resistant strains ofS. pneumoniae but possessed an
additional block of diverged sequence. Penicillin resistance in
S. oralis has also probably arisen by horizontal transfer of this
variant form of the class B mosaic PBP2B gene from a
penicillin-resistant strain of S. pneumoniae.