Penicillin-resistant viridans streptococci have obtained altered penicillin-binding protein genes from penicillin-resistant strains of Streptococcus pneumoniae.

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Abstract
Penicillin-resistant strains of Streptococcus pneumoniae possess altered forms of penicillin-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. Penicillin-reistant strains of viridans group streptococci (e.g., S. sangius and S. oralis) that produce altered PBPs have also been reported. The PBP2B genes of two penicillin-resistant clinical isolates of S. sangius 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. sangius. The PBP2B genes of three penicillin-resistant S.oralis strains were similar to the mosaic class B PBP2B gene of penicillin-resistant strains of S. 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.

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